METSÄNTUTKIMUSLAITOKSEN TIEDONANTOJA 756,2000 FINNISH FOREST RESEARCH INSTITUTE, RESEARCH PAPERS 756, 2000 Phosphorus fertilizer leaching from drained ombrotrophic peatland forests: empirical studies and modelling Mika Nieminen VANTAAN TUTKIMUSKESKUS - VANTAA RESEARCH CENTRE METSÄNTUTKIMUSLAITOKSEN TIEDONANTOJA 756, 2000 FINNISH FOREST RESEARCH INSTITUTE, RESEARCH PAPERS 756, 2000 Phosphorus fertilizer leaching from drained ombrotrophic peatland forests: empirical studies and modelling Mika Nieminen Finnish Forest Research Institute Vantaa Research Centre Academic dissertation To be presented, with the permission of the Faculty of Agriculture and Forestry of the University of Helsinki, for public critisism in Auditorium M II, Metsätalo, Unioninkatu 40 B, Helsinki, on 21 January 2000, at 12 o'clock noon. Supervisor: Dr. Erkki Ahti Finnish Forest Research Institute Vantaa Research Centre Reviewers: Professor Hannu Mannerkoski University of Joensuu, Finland Faculty of Forestry Dr. Pirkko Kortelainen Finnish Environment Institute Opponent: Docent Markku Yli-Halla University of Helsinki, Finland and Agricultural Research Centre of Finland Publisher: Finnish Forest Research Institute, Vantaa Research Centre RO. Box 18, FIN-01301 Vantaa, Finland. Accepted by Matti Kärkkäinen, Research Director, 26.11.1999. ISBN 951-40-1711-0 ISSN 0358-4283 Hakapaino Oy Helsinki 1999 3 CONTENTS ABSTRACT 5 LIST OF ORIGINAL PAPERS 6 DEFINITIONS 7 ACKNOWLEDGEMENTS 8 1 INTRODUCTION 9 1.1 General overview 9 1.2 Mechanisms controlling the leaching of fertilizer-phosphorus from drained peatlands 9 1.2.1 Timing and method of application 9 1.2.2 Fertilizer dissolution 10 1.2.3 Uptake of fertilizer-phosphorus by vegetation 11 1.2.4 Adsorption of fertilizer-phosphorus by peat 12 1.2.5 Climatic conditions and hydrological properties of peat ... 12 1.3 Aims of the study 13 2 SUMMARY OF ORIGINAL PUBLICATIONS 14 3 MODELLING 16 3.1 Model structure 16 3.2 Related data 17 3.3 Parameterization 22 3.3.1 Fertilizer dissolution 22 3.3.2 Accumulation of fertilizer-phosphorus in vegetation biomass and litter 23 3.3.3 Adsorption of fertilizer-phosphorus by peat 24 3.3.4 Other parameters 25 3.4 Calculations 32 3.5 Results 32 3.5.1 Model verification 32 3.5.2 Sensitivity analysis and scenarios 32 4 3.6 Discussion 39 3.6.1 Verification data 39 3.6.2 Model structure 40 3.6.3 Parameter definition 42 3.6.3.1 Fertilizer dissolution 42 3.6.3.2 Accumulation of fertilizer-phosphorus in vegetation biomass and litter 44 3.6.3.3 Adsorption of fertilizer-phosphorus by peat 45 4 CONCLUSIONS 46 REFERENCES 47 PAPERS I-IV 51 5 ABSTRACT Empirical studies and a literature review were performed to understand and estimate the three primary processes controlling phosphorus (P) fertilizer leaching, that is: (1) the rate of dissolution of various P fertilizers, (2) the accumulation of dissolved P in vegetation biomass and litter, and (3) the adsorption of dissolved P by peat. A simulation model of fertilizer-P leaching from drained ombrotrophic peatland forests was developed in which the fertilizer dissolution, biological accumulation and adsorption of P are considered. The daily leaching of fertilizer-P was then modelled for two sites at which ditch outflow P concentrations had been determined in previous studies. The model estimates daily ditch water P concentrations where the fertilizer dose is similar to that used in practical peatland forestry (40-45 kg ha 1 ), and assumes that all the added P enters the soil, i.e. the direct deposition of fertilizer granules to ditches and transport with the overland flow to ditches are negligible. Comparison with measured data indicated that the changes in ditch water P concentrations were satisfactory explained by the proposed model within the ranges of the model parameter values. Running the model for up to about 20 years after fertilization indicated that although the P concentrations in the ditch outflow peaked at different times and at different levels for fertilizers of differing solubility, the total leaching losses over the 20 year period were the same. It was therefore concluded that the rate of fertilizer dissolution is of minor importance in restricting leaching losses. The results indicated that the factors affecting the total leaching of fertilizer are the P adsorption capacity of the peat and the effectiveness of trees and understorey vegetation in accumulating the added P. The risk for high leaching rates of P fertilizer from drained ombrotrophic peat soils is related to their very low P adsorption capacity and incomplete accumulation of fertilizer-P in trees and understorey vegetation. Keywords: fertilizer dissolution, phosphorus adsorption, phosphorus leaching, phosphorus uptake. Author's address: The Finnish Forest Research Institute, Vantaa Research Centre, P. O. Box 18, FIN-01301 Vantaa, Finland. Fax: + 358 9 8570 5569, E-mail: mika.nieminen@metla.fi 6 LIST OF ORIGINAL PAPERS The thesis is based on this summary report that proposes a simple model for fertilizer-induced leaching of phosphorus from drained ombrotrophic peatlands, and the following original papers, which are referred to in the text by their Roman numerals. I Nieminen, M. 1997. Properties of slow release phosphorus fertilizers with special reference to their use on drained peatland forests - A review. Suo 48(4): 115-126. II Nieminen, M. & Jarva, M. 1999. Dissolution of phosphorus fertilizers of differing solubility in peat soil: A field experiment on a drained pine bog. Scandinavian Journal of Forest Research, (accepted). 11l Finer, L. & Nieminen, M. 1997. Dry mass and the amounts of nutrients in understorey vegetation before and after fertilization on a drained pine bog. Silva Fennica 31(4): 391-400. IV Nieminen, M. & Jarva, M. 1996. Phosphorus adsorption by peat from drained mires in southern Finland. Scandinavian Journal of Forest Research 11: 321-326. The authors' contribution Paper II: Mika Nieminen was responsible for planning the study, carrying out the field work, performing the statistical analyses, and interpreting the results. He also wrote the first draft version of the manuscript. Maija Jarva participated in writing the section "Material and methods" and was responsible for carrying out the laboratory analyses. Paper III: Leena Finer was responsible for the field work and the laboratory analyses. Mika Nieminen performed the statistical analyses, and participated in the interpretation of the results and preparation of the manuscript. Paper IV: Mika Nieminen planned the study. He carried out the field work, the statistical analyses, and interpreted the results. He also wrote the first draft version of the manuscript. Maija Jarva was responsible for carrying out the laboratory analyses and participated in writing the section "Laboratory analysis". 7 DEFINITIONS Biomass, as used in this study, includes not only the mass of the living parts of the plants but also the dead tissues fixed to living individuals (e.g. bark, heartwood, dead branches). Biomass has the same meaning as dry mass (dried in 105 "C). Total plant biomass refers to the biomass of all vegetation compartments present in a peatland ecosystem (i.e. the above-ground parts in the tree, bush, field, and bottom layers, and the below-ground parts in root systems). Total leaching of fertilizer refers to the complete loss of the leachable portion of the totally dissolved fertilizer, and total uptake of P-fertilizer to the amount of P taken up (by plants) from the totally dissolved fertilizer. Total leaching and total P uptake of fertilizer therefore depends on the time taken for all the fertilizer to dissolve. 8 ACKNOWLEDGEMENTS This study was carried out at the Vantaa Research Centre of the Finnish Forest Research Institute. For guidance and support at the early stages of my work I am grateful to Prof. Eero Paavilainen, formed head of the Vantaa Research Centre. Numerous discussions with Dr. Erkki Ahti and the advice given by him have been valuable in the completion of this work. I express my warmest thanks to him for his support and constructive criticism and for creating an inspiring atmosphere in which to work. I wish to thank Prof. Juhani Päivänen for his guidance and support, as well as for his comments on my work. I am also grateful to Prof. Hannu Mannerkoski and Dr. Pirkko Kortelainen, the official reviewers of this summary report, for their constructive criticism. Prof. Seppo Kaunisto is acknowledged for his valuable criticism in preparing this summary report and the publications included in this report. I am also grateful to Dr. Maija Jarva and Dr. Leena Finer for their research collaboration. Special thanks are also due to Heikki Takamaa for his contribution in establishing the experiments and collecting the material, Inkeri Suopanki for her assistance in processing the data and preparing the manuscripts, Tuija Hytönen for her conscientious assistance in conducting the laboratory work, and Hannu Nousiainen, Raija Linnainmaa and Hilkka Granlund for helping in preparing the figures and tables in this summary report and original publications. The assistance of numerous other people is also greatly acknowledged. The English language was revised by Dr. Michael Starr, who also gave valuable advice and comment about the content of my work. Finally, I want to thank my relatives and friends for their all-round support throughout. I am especially grateful to my wife, Annukka, my parents, Anja and Pauli, and Sulo, Juha, Mirjami and Auli for being there for me when needed. 9 1 INTRODUCTION 1.1 General overview In the beginning of the 20th century, the area of pristine peatlands in Finland totalled about 10 million hectares, covering one third of the total land area. About half of this area has now been drained for forestry purposes. The present growth of peatland forests is estimated to be 17.7 mill. m\ accounting for about 23 % of the total growth of all Finnish forests (Tomppo 1998). The growth of trees on drained peatlands is often limited by a deficiency of available phosphorus. According to the most recent nutrient status investigations of peatland forests by needle analysis (Moilanen 1992, Veijalainen 1992), about 60 % of the stands would increase their growth after P fertilization. However, the use of P-fertilizers has been very small during the past two decades (Metsätalous j a vesistöt 1996). One reason why there has been so little use of P fertilization has been the concern over the possible leaching of P from fertilized peatlands and the consequent eutrophication of recipient water courses. Indeed, long-term leaching losses of fertilizer-P from drained peatlands have been observed in a number of studies (e.g. Harriman 1978, Kenttämies 1981, Ahti 1983, Malcolm & Cuttle 1983 a, Nieminen & Ahti 1993, Saura et ai. 1999), and the trophic state of a recipient water body was shown to have been altered after the application of P fertilizer to a drained peatland (Lepistö & Saura 1998). However, other investigations have indicated only minor leaching of applied P from drained peatlands (Karsisto 1970, Karsisto & Ravela 1971, Almberger & Salomonsson 1979, Jarva et ai. 1995). To identify those situations where the risk for enhanced leaching is high, information about all the mechanisms controlling the leaching of fertilizer-P is needed. 1.2 Mechanisms controlling the leaching of fertilizer-P from drained peatlands 1.2.1 Timing and method of application When fertilizers were spread on the top of a >SO cm thick snow cover during winter, much of the fertilizer residues still remained in the surface 0-10 cm snow layer at the beginning of snowmelt (Paavilainen 1969). This finding 10 indicated that fertilizer residues might be carried to ditches with the overland flow of the snowmelt and hence caused great concern about possible water quality effects. Adverse results of P fertilizing during the winter period were published by Nieminen and Ahti (1993). They found substantially higher leaching losses from peatland areas where the fertilizers had been applied to the snow compared with those areas where the fertilizers had been spread on the bare ground after snowmelt. However, other investigations indicated insignificant changes in P outflow after spreading P-fertilizers on snow cover (Karsisto 1970, Karsisto & Ravela 1971). The aerial application of fertilizer results in part of the fertilizer granules falling directly into the ditches. This may cause substantial P leaching to water courses. However, the effect of aerial application on P leaching from drained peatlands has not been studied. 1.2.2 Fertilizer dissolution Phosphorus fertilizers that have been commonly used in Finland, either for practical peatland forestry purposes or for research purposes, include: superphosphate, Moroccan phosphate rock, Siilinjärvi apatite, and two commercial PK-fertilizers known under the trade names of "Rakeinen Suo- PK" and "Metsän-PK". Siilinjärvi apatite is a Finnish fluoroapatite of igneous origin while Moroccan phosphate rock is a carbonate apatite of sedimentary origin. "Rakeinen Suo-PK" is manufactured from Moroccan phosphate rock and potassium chloride and "Metsän-PK" from Siilinjärvi apatite and potassium chloride. The P in Siilinjärvi apatite, Moroccan phosphate rock and "Metsän- PK" is water-insoluble while about 16 % of total P in "Rakeinen Suo-PK" and 80 % of that in superphosphate is water-soluble. Water-soluble fertilizer-P is liable to leach to water courses immediately after fertilizer application. Phosphorus in water-insoluble fertilizers has first to be converted into a water soluble form by chemical processes in soil before becoming available to plants or liable to leaching. The effects of different P fertilizers on tree production on drained mires have been studied extensively in Finland (Karsisto 1968,1977, Penttilä & Moilanen 1987, Kaunisto et ai. 1993, Silfverberg & Hartman 1998). However, only one laboratory study has dealt with the dissolution of different P-fertilizers in peat soil (Yli-Halla & Lumme 1987). To fully understand the effects of fertilizers on the growth of peatland forests or the leaching of fertilizer-P from drained 11 peatlands the dissolution of different P-fertilizers under field conditions needs to be known. Phosphate rock dissolution has been studied fairly extensively under field conditions in agricultural soils (Rajan 1987, Rajan et ai. 1991, Rajan & Watkinson 1992). Substantial differences in the rates of dissolution between phosphate rocks of different origin have been shown (Rajan 1987). The rate of dissolution was closely related to soil pH and the fertilizer dose (Rajan et ai. 1991). The release of P from phosphate rocks is also known to depend on the P and Ca sorption capacity of the soil (Mackay & Syers 1986), soil moisture (Bolland & Gilkes 1995), and Ca uptake by different plant species (Flach et al. 1987). In forest soils, mycorrhizal fungi may also play an important role in the release of P from slow-release fertilizers (Wallander et al. 1997). If the peat P adsorption capacity is low, slowly-soluble P fertilizers are recommended in restricting P leaching to water courses. Under conditions of low peat P adsorption, however, the use of slowly soluble fertilizers will significantly restrict the total leaching of fertilizer-P only if the peatland vegetation accumulates more P than from easily soluble fertilizers. Several investigations have shown no significant differences in the total tree growth between sites where fertilizers with differing P solubilities have been used (Karsisto 1968, 1977, Penttilä & Moilanen 1987, Kaunisto et al. 1993, Silfverberg & Hartman 1998). These results imply that the total uptake of P fertilizer by trees is not dependent on fertilizer solubility. It is not known, however, whether the uptake of fertilizer-P by peatland vegetation other than trees depends on the solubility of the fertilizer. 1.2.3 Uptake of fertilizer-phosphorus by vegetation Finer (1991 b) found that the amount of P bound-up in the understorey vegetation plus tree stand on an ombrotrophic pine bog was lower than the amounts of P fertilizer used in practical peatland forestry (40-45 kg ha 1 )- In well-developed unfertilized tree stands on drained mires, the annual uptake of P by the tree stand is only 1-3 kg ha 1 (Finer 1989, 1991 a, 1991b), of which about half is released back to soil as litterfall (Finer 1991b). On this basis, all of the P applied in practical peatland forest fertilization is unlikely to be bound up by the trees and other vegetation biomass. However, the fertilizer-induced changes in biomass P accumulation on drained peatlands are poorly known. Several studies have been carried out to investigate the effect of fertilization on the accumulation 12 of P in one or more vegetation compartments (i.e. in the tree, bush, field and bottom layers, and root systems) present in a drained peatland ecosystem (e.g. Haaveraen 1967, Braekke 1977, 1988, Paavilainen 1980, Malcolm & Cuttle 1983b, Finer 1989, 1991 a). But investigations regarding the changes caused by fertilization in the amounts of P in total plant biomass (including all compartments) are scarce. 1.2.4 Adsorption of fertilizer-phosphorus by peat In mineral soils, phosphate P from fertilizer is tightly bound to the soil by certain compounds, A 1 or Fe oxides and hydrous oxides in particular (Partiff 1978). Because of the low content of these compounds, some peat soils, especially acid Sphagnum peats, have been shown to have a very low P adsorption capacity (Kaila 1959, Rannikko & Hartikainen 1980). Cuttle (1983) estimated the P adsorption capacity of the least sorptive peat soils to be only 0.04 mg P 100 cnr 3 (equivalent to 0.8 kg P ha -1 in the 0-20 cm peat layer). Under conditions of low adsorption capacity and low P uptake by the vegetation, there is a substantial risk for the outflow of the fertilizer-P. Some of the fertilizer-P may also be taken by microbes. Under conditions of high P addition, however, the microorganism phosphate uptake is of minor importance in reducing P leaching from peat soils (Richardson 1985). 1.2.5 Climatic conditions and hydrological properties of peat Harriman (1978) and Ahti (1983) reported an equal increase in ditch outflow P concentrations from drained peatlands after P-fertilization (0.1-0.2 mg P 1'). However, because of the much smaller annual precipitation and consequent runoff in Ahti's (1983) Finnish study, the annual amounts of P leached remained considerably smaller (< 500 g ha 1 ) than the average annual loss of 1-2 kg ha 1 reported by Harriman (1978) for peatlands in Scotland. There is no information about the effects of different temperature conditions on the leaching of fertilizer-P from drained peatlands. However, the outflow of fertilizer-P may be expected to be the highest in areas where unfavourable temperature conditions restrict planth growth and therefore also the uptake of fertilizer. Temperature may also affect P leaching by either increasing or decreasing the rate of P release from slowly soluble fertilizers. However, 13 temperature was not found to significantly affect phosphate rock dissolution in tropical soils (Chien et al. 1980). High P concentrations in ditch outflow after fertilization may be observed in areas subjected to prolonged high temperature droughts. This is because of the concentration effect caused by the loss of water in evapotranspiration. Anaerobiosis in the peat may increase the leaching of phosphate ions for at least two reasons. Firstly, anaerobic conditions are likely to reduce plant growth and thereby also the P uptake by vegetation. Secondly, anaerobiosis inhibits P adsorption due to the reduction and redistribution of Fe (Armstrong 1975). Due to the high water retention capacity of peat (Päivänen 1973), anaerobic conditions tend to prevail in the subsurface peat layers of most peat soils. The changes in the P concentrations in ditch outflow after fertilization are also affected by the rate of water flow through peat. Relatively rapid changes in P concentrations are to be expected from the soils consisting of slightly decomposed Sphagnum peat. This is because of the higher hydraulic conductivity of such soils (Päivänen 1973). Low hydraulic conductivity is typical for highly decomposed peats. In such soils, there may be a substantial delay before any leaching of P fertilizer occurs. 1.3 Aims of the study The objective of this study was to determine the effects of (1) varying dissolution of different P-fertilizers, (2) the accumulation of P in vegetation biomass and litter, and (3) the adsorption of P by peat on the leaching of P from drained ombrotrophic peatland forests. Empirical studies and a literature review were first performed to understand and estimate these three processes. To determine the fate and transport of fertilizer-P via these processes, a simple model of fertilizer-P leaching from drained ombrotrophic peatlands was developed. The model was then applied to the two sites at which fertilizer-P leaching had been determined in previous studies. Finally, the impact of the processes affecting P leaching were investigated using sensitivity analysis and various scenarios. 14 2 SUMMARY OF ORIGINAL PUBLICATIONS In this summary report, the results of the three empirical studies (11-IV) are used to determine model parameter values. The main results of the studies concerning the fate of P fertilizer on drained ombrotrophic peatlands are therefore given in the parameterization section (3.3) and in connection with the interpretation of the model parameter values (3.6.3). However, a brief summary of the studies 11-IV is given below. Detailed descriptions of the material and methods used, the results obtained, and the interpretation of the results are given in the original publications. The first original paper (I) reviews the properties, manufacture, and behaviour in agricultural and peat soils of various P-fertilizers. It was made as background information to the fertilizer dissolution study (II). The fertilizer dissolution study (H) was performed on a drained pine bog in southern Finland (61°51'N; 25°59'E, 131 m a.5.1.). The experiment consisted of five fertilization treatments with three replicates. The treatments were: (1) Super phosphate, (2) Mire-PK (Finnish: Rakeinen Suometsien PK), (3) Moroccan phosphate rock, (4) Siilinjärvi apatite, and (5) Forest-PK (Finnish: Metsän PK). In each of 15 plots, unfertilized and fertilized sampling quadrates were systematically located. Peat samples from the unfertilized quadrates were taken at the beginning of the experiment in August 1996. Peat samples from the fertilized quadrates were taken about 2,12,20, and 24 months after fertilization. The measurements of the amount of residual (undissolved) fertilizer-P were used to investigate the dissolution of the studied fertilizers. Dried (70 °C) and milled samples (1 g) were first pre-extracted with 0.5 M NaCl/ TEA and 1 M NaOH to remove dissolved fertilizer-P. The amount of the residual fertilizer-P was then estimated as the difference in the amount of P dissolved in a mixed acid digestion (HNO3, HCI, H2S04) between fertilized and unfertilized samples. Phosphorus concentrations in the digests were determined by ICP/AES, ARL 3580. The dissolution of the studied fertilizers, calculated as the proportion of the amount of fertilizer-P added, ranged from 44 % (Forest-PK and Moroccan phosphate rock) to 93 % (Superphosphate) over the two years covered in the study (Fig. 3 in publication II). The effects of PK and NPK fertilization on the dry mass and nutrient accumulation in understorey vegetation (III) were studied on a drained low shrub pine bog in eastern Finland (62°14'N; 29°50'E, 81 m a.5.1.). A 3 x 3 Latin square design was used. The treatments were: (1) unfertilized, (2) fertilized with PK(MgB), and (3) fertilized with NPK(MgB). The understorey vegetation was sampled prior to fertilization and at about 2.5 years after fertilization. The understorey vegetation was divided into bush, field and bottom/litter layers. The bush layer vegetation was very sparse and have therefore been ignored in 15 the study. Litter accumulated on and between bryophytes and lichens was not separated from the living bottom layer vegetation and is why the layer was referred to as the bottom/litter layer. Above-ground parts of the layers were harvested from 20 sampling quadrates (0.25 m 2) on each plot (1500 m 2), and the root systems were extracted by hand from the peat samples taken down to a depth of 40 cm. Total nutrient concentrations in the samples were determined according to Halonen et ai. (1983), and nutrient amounts were calculated by multiplying the dry masses with the nutrient concentrations. The results of an earlier study (Finer 1991 a) carried out at the same site on the tree layer were combined with those of study 111 on the understorey vegetation and litter. About 33 %of the added P (53 kg ha 1 ) had accumulated in the total plant biomass (field and bottom/litter layers + tree layer) after 2.5 years on the PK-fertilized plots, and about 25 % on the NPK-fertilized plots (Table 4 in publication III). Peat samples from 20 sites were collected from different parts of southern Finland (between latitudes 60°N and 62°N) to study the adsorption of P by peat and those soil properties likely to influence it (IV). The samples were first used to determine P adsorption isotherms. For this, samples of moist peat (equivalent to 1 g dry weight) were added to bottles containing solutions with increasing amounts of P (from 0.0 mg P l" 1 to 40.0 mg Pl 1). After equilibration (23 h) and filtration (first through a glass fibre paper and then through a membrane filter) the concentration of P remaining in solution was measured (ICP/AES, ARL 3580). Phosphorus that had disappeared from the solution was considered to have been adsorbed, and the amount of P adsorbed was plotted against the equilibrium P concentration to obtain the adsorption isotherms (see Fig. 1 in publication IV). To determine maximum adsorption, the P adsorption index values (PSI) and the total P adsorption capacity (PAC) values were also calculated. The PSI values, determined from one addition of 150 mg P (100 g) 1 dry weight of peat, were calculated as: X/log C, where Xis the quantity of P adsorped (mg P (100 g) 1 dry weight of peat) and Cis the concentration of P in the equilibrium solution (Bache & Williams 1971). For the calculation of the total P adsorption capacity (PAC), the X term in the quotient X/log C was multiplied by the bulk density of the peat sample and the adsorption was expressed as kg P sorbed ha 1. Adsorption was strongly correlated to the Fe content of the peat (Table 2 and Fig. 2 in publication IV). PAC for several of the soils studied was substantially lower than the amount of P fertilizer (40-45 kg ha 1) applied to Finnish peatlands in practical peatland forestry (see Table 3 in publication IV). The following statistical methods were used in the empirical studies: Pearson correlation analysis (IV), regression analysis (11, IV), analysis of variance for repeated measures designs (II), and analysis of variance for Latin square designs and Tukey's test (III). 16 3 MODELLING 3.1 Model structure In this section, a simple model for fertilizer-P leaching from drained ombrotrophic peatlands is presented (Fig. 1, Table 1). In the model, the peat deposit is divided into three layers. In the uppermost active layer, a certain amount of fertilizer-P (DP,) dissolves each day. A fraction (a) of the dissolved fertilizer-P is adsorbed by the peat and another fraction (u) is accumulated in the vegetation biomass and litter. The remaining fraction (l-(a+u)) leaches to the runoff layer and to the ditch from where it is exported from the basin. The bottom passive layer is assumed to have no significant effect on the leaching behaviour of the fertilizer-P. The fraction of P going to leaching is assumed to be completely and evenly mixed with the daily volume (W,) of water present in the runoff layer. The concentration of fertilizer-P in runoff is calculated by dividing the amount of fertilizer-P in the runoff layer by the volume of water in the runoff layer (i.e. RP/W,), and the ditch outflow of fertilizer-P (OP,) is simply given by the product of the concentration of fertilizer-P in the runoff layer and the runoff water flow (i.e. (RP/W,)q,). Fig. 1. Schematic presentation of the fertilizer-P leaching model. 17 Table 1. State variables and equations for fertilizer-P leaching model. The proposed model assumes that all the applied P enters the soil and that the direct deposition of fertilizer granules to the ditches and the transport with the overland flow to ditches are negligible. Therefore, the model should not be applied to sites where the fertilizer has been applied from the air nor where it has been applied on snow. 3.2 Related data In addition to the results of the original publications 11-IV, data from two previous leaching experiments were used for the definition of model parameter values. The two studies had been carried out at Liesineva (Ahti 1983) and Kivisuo (Ahti & Paarlahti 1988, Nieminen & Ahti 1993). The results from these studies were also used for model application and verification. The experiments are briefly described below and more detailed descriptions are to be found in the previously mentioned publications. dRP/d? = DP,-a*DP,-u*DP, -OP, OP, = (RP, / W,)*q, TP,= (RP, / W,)* 1000*d + BP RP,= amount of fertilizer-P in runoff layer at time /, g ha"' DP,= dissolution of fertilizer-P, g ha' 1 d"' a = coefficient for rate of adsorption of fertilizer-P by peat u = coefficient for rate of accumulation of fertilizer-P in vegetation biomass and litter OP,= outflow of fertilizer-P, g ha 1 d" 1 W, = water volume in runoff layer at time t, 1 ha -1 q, = runoff, 1 ha" 1 d' 1 TP, = total P concentration in runoff at time t, mg 1"' d = dilution coefficient BP = background P concentration in runoff, mg l" 1 t = time, d 18 The Kivisuo experiment (61°53'N, 25"58'E, 125 m a.5.1.) was drained and partly fertilized in 1967. The area is divided by ditches into 16 artificial small basins (area 1.2 or 2.0 ha) which differ from each other with respect to ditch spacing and original ditch depth (Fig. 2). The original 40 cm deep ditches were cleaned in 1976. The original site type in basins 1-8 varied from cotton grass pine bog to small sedge pine bog, and in basins 9-16 from small sedge pine bog to Sphagnum fuscum bog (Heikurainen & Pakarinen 1982). The thickness of the peat layer at the time of ditching was 1.5-2.5 m throughout. Some chemical properties of the surface peat at Kivisuo are given in Table 2. Fig. 2. The experimental layout at Kivisuo showing basin areas 1-16. Ditch spacing = 5, 10, 20 or 50 m. Original ditch depth: Areas 1-4 and 13-16 = 0.8 m, and areas 5-12 - 0.4 m. Fertilization: Areas 2, 3, 5 and 7 = application on snow 9.-13.3.1987, areas 1, 4, 6 and 8 = application on bare ground 19.-20.5.1987, and areas 9-16 = control. During the period 9.-13.3.1987, basins 2, 3, 5 and 7 were broad cast fertilized by spreading 500 kg ha -1 of a commercial PK-fertilizer (P 8.7 %, K 16.6 %, the fertilizer is known under the trade name "Rakeinen Suo-PK" in Finland, and is here after referred to as Mire-PK), and 215 kg ha -1 of urea on to the snow pack. During the period 19.-20.5.1987, a similar application was spread on bare 19 Table 2. Some background information about peat properties at Kivisuo and Liesineva (the surface 0-15 cm peat layer). Methods used: "Halonen et ai. (1983) 2) Dry digestion in HCI (Halonen et ai. 1983); ICP/AES ground after snowmelt at basins 1, 4, 6 and 8. Basins 9-16 were used as unfertilized controls. Phosphorus in the Mire-PK fertilizer originates from water-insoluble Moroccan phosphate rock. Due to the addition of nitric acid during granulation while being manufactured, about 16 % of the total P is converted into water-soluble form. The runoff samples were collected from the overflow of a rectangle-notched overfall weir installed in the outlet ditch of each basin area. Results showed that there was a considerable increase in P concentrations in the outflow from the fertilized basins compared to the unfertilized control basins, particularly in the case of the basins where the fertilizer had been applied on to the snow pack (Nieminen & Ahti 1993). Some 9 kg P ha' 1 of the P-fertilizer was lost over 1.5 years after fertilizer application on to the snow pack. The corresponding loss was about 3 kg P ha -1 for the basins where the fertilizer had been applied on bare ground. As the model is not applicable to sites where the fertilizer has been applied on snow, the model verification data comes only from the basins 1, 4, 6 and 8. The tree stand was dominated by Scots pine (Pinus sylvestris), and the volume of the stand measured in 1982 was 28.8, 15.8, 11.7, and 4.3 m 1 ha' 1 in basins 1, 4, 6, and 8, respectively. Kivisuo Liesineva Ash content % 1.39 3.30 P |ot 2) . mg kg" 1 524 1066 K tol 2) , mg kg' 1 388 701 Ca ioi 21 , mg kg' 1 1466 2092 Mg 10I 2) , mg kg- 1 362 379 Al |oI 2) , mg kg" 1 451 1777 Fe ioi 2) , mg kg' 1 805 2120 20 The Liesineva experiment (61°59' N, 23°15' E, 150 m a.5.1.) had been drained already in 1915, but the present ditch network originates from 1955 (Fig. 3). Ditch spacing varies from 5 to 100 meters, and the original ditch depth was 0.8 m. The original site type was cotton grass pine bog with patches of ordinary small sedge bog (Heikurainen & Pakarinen 1982). The original thickness of the peat layer was over 2 m. Some chemical properties of the surface peat at Liesineva are given in Table 2. The tree stand was dominated by Scots pine (.Pinus sylvestris), and the volume of the stand measured in 1979 varied with spacing as shown in Table 3. Fig. 3. The experimental layout at Liesineva showing basin areas 1-6. Ditch spacing 5-100 meters. Size of basin area: 0.15-1.80 ha. N, P and K fertilizations were performed at Liesineva already in 1961 and 1965 (Ahti 1983). However, the model verification data comes from a later fertilization experiment. In 1977, a strip comprising 10 % of the experimental area was fertilized with 500 kg ha -1 of a commercial PK-fertilizer (P 8.3 %, K 15.8 %, P as Moroccan phosphate rock, K as potassium chloride) and 400 kg ha ' of ammonium nitrate fertilizer (N 27.5 %). Compared to the pre-fertilization period, the P concentrations in ditch outflow were 0.1-0.2 mg l 1 higher during the period 1977-1982 (Ahti 1983). 21 Table 3. Volume of the tree stand measured in 1979 in relation to ditch spacing at Liesineva. The Liesineva ditch runoff samples were collected from V-shape gutters installed in the bottom of the ditches. No damming up of runoff water was induced by this sampling method and samples could be collected during somewhat drier periods than would have been possible if sampling had taken place from the overflow of V-notch weirs. Samples from very low flow periods showed higher variation in P concentrations than those from wetter periods (Fig. 4). No explanation was found for the high variation in runoff P concentrations during dry periods. However, very low flows have little effect on the overall leaching of fertilizer-P (calculated as g P ha 1) compared to the influence of very high spring and autumn flows. Samples taken when the runoff was < 0.015 1 s* 1 ha -1 were therefore excluded from the Liesineva verification data. Some of the sampling occassions lacked information about runoff, usually as the result of cessation in runoff flow during the sample collection or immediately after it. The samples with no information about runoff were also excluded from the Liesineva data. Fig. 4. Ditch water P concentrations as a function of runoff flow at Liesineva. Ditch water samples from all sampling occassions during 1977-1982 at each of the six basin areas are included. Ditch spacing, m 5 10 20 40 60 80 100 Stand volume, m 3 ha 1 98.1 93.1 73.4 52.7 35.4 24.2 25.7 22 The location of the water divide between basin areas in experiments such as Liesineva varies over time due to watertable fluctuation (Ahti 1983). The area contributing to the runoff therefore varied between ditch water sampling occassions. This source of variation has not been taken into consideration, and fixed sizes of the basin areas were assumed (Fig. 3). 3.3 Parameterization 3.3.1 Fertilizer dissolution The dissolution (DP,, see Table 1) of the P-fertilizers used at Liesineva (Moroccan phosphate rock) and Kivisuo (Mire-PK) were estimated on the basis of the dissolution curves presented in Fig. 3 in publication 11. The study period in publication II covered the first two years after fertilization. For model application to the Liesineva leaching experiment, however, information was needed about the dissolution of Moroccan phosphate rock for 6 years after fertilizer application. In the model, the daily dissolution rate of Moroccan phosphate rock after the first 2 years is assumed to be the average of the second year (0.0289 % d" 1 of the amount added). It is also assumed in the model that there is no fertilizer dissolution during the period when the soil is frozen (from the beginning of November to the end of April) and that dissolution is enhanced for one month after the winter-period. The rates of dissolution of the most soluble P-compounds in the fertilizers were also assumed to differ from those suggested by the regression models presented in publication 11. The original dissolution curves from publication II and the curves used in the model are illustrated in Fig. 5. Fig. 5. The original dissolution curves from publication II for (a) Mire-PK and (b) Moroccan phosphate rock, and predicted dissolution for Moroccan phosphate rock after the first 2 years after application, and the dissolution curves adopted for the model. W + S = no dissolution during the winter-period and an enhanced dissolution period in spring. SPC = dissolution of the most soluble P-compounds, (see text). 23 In the model, the rate of fertilizer dissolution (D%,) illustrated in Fig. 5 is converted into the amount of fertilizer-P dissolved during time t (DA„ g ha 1 ): where A=dose of application, g ha 1 . The daily dissolution in g ha"' d~' (DP,, see Table 1) is then calculated as the difference between the amount of P dissolved at time t and that at time t- 1: The DP, curves calculated using Equation 2 are presented in Fig. 6 Fig. 6. Dissolution (DP, g ha 1 d1) of (a) Mire-PK and (b) Moroccan phosphate rock calculated using Equation 2, (see text). 3.3.2 Accumulation of fertilizer-phosphorus in vegetation biomass and litter The daily accumulation of fertilizer-P in the vegetation biomass and litter (ACCUM) is given as: where u is the coefficient for the rate of accumulation of fertilizer-P in vegetation biomass and litter and DP, is the dissolution of fertilizer-P from Equation 2. Information about the accumulation of fertilizer-P in the vegetation biomass and litter comes from publication 111. The average biomass accumulation of P (P given as Moroccan phosphate rock with an application dose of 53 kg ha 1 ) DA, = (D%/100) * A, (1) DP,= DA,-DAM (2) ACCUM = u * DP„ (3) 24 over the about 2.5 year study period was 15.3 kg ha -1 (13.3 kg ha-1 fortheNPK fertilized plots, and for the PK-fertilized plots, 17.3 kg ha 1). About 48 %of P in Moroccan phosphate rock was estimated to be released in a plant available form over 2.5 years (Fig. Sb). The coefficient for the rate of accumulation of fertilizer-P in the vegetation biomass and litter (u) is thus calculated as: The accumulation of fertilizer-P in the biomass and litter is therefore given by: 3.3.3 Adsorption of fertilizer-phosphorus by peat The daily adsorption of fertilizer-P by peat (ADSORP) is given as: where a is the coefficient for the rate of adsorption of fertilizer-P by peat and DP, is the dissolution of fertilizer-P from Equation 2. The P adsorption isotherms of the surface 0-15 cm peat layer were used in the estimation of the peat adsorption of fertilizer-P (Fig. 1 in publication IV). Under conditions of low concentrations of Pin equilibrium solution (< 10 mg l 1), the P adsorption isotherms of the surface 0-15 cm peat samples were negative or only slightly positive in each of the 6 ombrotrophic peat soils studied (samples 14-17 and 19-20). Except for sample 19, the adsorption of P by peat was also very low under conditions of high concentrations of P in equilibrium solution (> 30 mg I 1). Phosphorus adsorption isotherms tend to overestimate the real P adsorption in the field (Richardson 1985, IV). The adsorption of P by peat is therefore probably of little consequence to the fate of fertilizer-P in ombrotrophic peat soils. The coefficient for P-adsorption in the model was thus set zero. The adsorption of fertilizer-P by peat is thus given as: In the particularly Fe-rich peat soils, however, the adsorption of P is likely to significantly affect the fate of fertilizer-P (IV). u= 15.3/(48/100) *53 (4) ACCUM = 0.6 * DP, (5) ADSORP = a*DP„ (6) ADSORP = 0.0 * DP, (7) 25 3.3.4 Other parameters The runoff data (q, in Table 1) for the frost-free period comes from manual measurements performed usually 3-5 times weekly, but sometimes only once a week. Both the Kivisuo experimental area and the Liesineva area lack winter runoff measurements. Runoff for winter period was estimated using the nearby small basin areas of the former National Board of Waters (NBW). Estimation was done by first calculating linear regression equations for the relationship between monthly means of frost-free period runoff values in different basin areas at Liesineva and Kivisuo and respective values in the nearby NBW areas. The missing runoff values for Liesineva and Kivisuo were then estimated using these equations and measured winter runoff values in the NBW areas. The correlations between the frost-free period runoff values in different basin areas at Liesineva and Kivisuo and respective values in the NBW areas were high (r=o.BB-0.96) and statistically significant (pcO.OOl). Runoff from different basins at Kivisuo and Liesineva during the study periods used in the model are given in Fig. 7. In the model, the runoff values are in 1 ha 1 d 1 (Table 1). The background P concentrations (BP in Table 1) were calculated as the average of P concentrations in runoff samples taken during the pre-fertilization periods (4.6.-20.11.1976 at Liesineva; 27.5.-2.11.1986 at Kivisuo). The background P concentrations are assumed to represent the concentrations that had existed in each basin area if the fertilizations had not been performed (Table 4). At the Kivisuo basin 8, however, the P concentrations of the runoff samples collected in 27.5.-2.11.1986 were about 0.25 mg l 1 lower than those of the samples collected just prior to fertilizer application in 1.-18.5.1987. The very high runoff P concentrations prior to fertilizer application may be because some fertilizer- P was accidentally spread on basin area 8 in connection with fertilization of the basin areas 2, 3, 5 and 7 in 9.-13.3.1987 (Fig. 2). It is also possible that some fertilizer-P was leached to basin 8 from basin 7. Whatever the reason for this "excess input" of P, it was thought important enough to be accounted for in the model. The Kivisuo basin 8 background P concentration for the period 19.5.-2.11.1987 was therefore set at 0.30 mg 1 If only a part of the basin area is fertilized, as was the case at Liesineva, the outflow waters from the fertilized part of the basin are mixed with waters from the unfertilized part. In the model, the fertilizer-induced changes in runoff P concentrations are therefore multiplied by a dilution coefficient (d in Table 1), which is the ratio of the fertilizer-treated area to the whole basin area (1.0 for Kivisuo, and 0.1 for Liesineva). 26 Fig. 7. Daily runoff from each Kivisuo basin 1.5 years and Liesineva basin 6 years since fertilization. 27 Fig. 7. continues 28 Table 4. Background P concentrations in ditch runoff from Kivisuo and Liesineva. P, mg r 1 Experiment Basin Kivisuo 1 0.051 4 6 8 0.041 0.046 0.055 a) Liesineva 1 0.023 2 3 4 0.034 0.062 0.017 5 6 0.012 0.009 a) 0.30 mg P l"' in 19.5.—2.11.1987 In the estimation of the water volume in the runoff layer (W, in Table 1), two basic assumptions were made: (1) the location of the divider between the active layer and the runoff layer varies due to fluctuations in the watertable level, and (2) the divider between the runoff layer and the passive layer is located at the same depth as the bottom of the ditches (Fig. 1). The thickness of runoff layer (RL„ mm) at time t is therefore calculated as: (8) where D=ditch depth, mm; GW=watertable level at time t, mm (= the vertical distance between soil surface and watertable). The water volume (W„ mm) in the runoff layer at time t is then calculated as: (9) where PW=water content of the peat at saturation, %. RL=D-GW„ W,= RL ,*(PW/100), 29 Ditch cleaning in 1976 at Kivisuo basins 6 and 8 (original ditch depth=4oo mm) somewhat increased the depth of the ditches. Ditch depth (D) for these basins was therefore set at 600 mm. No ditch cleanings were performed at the other basins however, and the ditch depth was set at the same depth as the original ditch depth (800 mm). In the case of the Kivisuo experimental area, the data for watertable level (GW,) during the frost-free period comes from manual measurements made at 1-2 weeks intervals. However, no watertable measurements were made at Liesineva. The level of watertable for the snowless period (1.6.-30.9) in the different basins at Liesineva were estimated using the measured runoff data and curves derived by Ahti (1987) showing the relationship between runoff and watertable level (Fig. 8). For the period in winter when the soil was frozen, the watertable level was set at 100 mm for those basins with an average ditch spacing of <3O m, 50 mm for those basins with an average ditch spacing of >3O-50 m, and 10 mm for those basins with an average ditch spacing of >5O m. The water content of the peat at saturation (PW) was set at 92 % (based on measurements for undecomposed Sphagnum peat by Päivänen, 1973). The water volumes in the runoff layer during the study periods are given in Fig. 9. For use in the model, these values were converted to 1 ha" 1 (Table 1). Fig. 8. The relationship between runoff and watertable level over the period 1977- 1981 (redrawn from Ahti, 1987). The first number in the treatments gives ditch spacing (m) and the second number ditch depth (cm). The curve from the 20/60 treatment was used to estimate the watertable level for basin 1, 2 and 3 at Liesineva, and that from the 40/60 treatment for the basin 4. The level of watertable at the Liesineva basins 5 and 6 was estimated using the curve from the 100/60 treatment. 30 Fig. 9. Daily water content of the runoff layer at each Kivisuo basin 1.5 years and Liesineva basin 6 years since fertilization. 31 Fig. 9. continues 32 3.4 Calculations Model verification was done by checking the model's output with P concentrations in ditch outflow from each of the 4 basin areas at Kivisuo (basins 1, 4, 6, and 8, see Fig. 2), and 6 basin areas at Liesineva (Fig. 3). Sensitivity analysis was done by studying the response of the model's output to changes in the values of one parameter at a time or the simultaneous changes in the values of two parameters. For the sensitivity analysis and various scenarios, the model was run for up to about 20 years after fertilization. 3.5 Results 3.5.1 Model verification No systematic deviations between the modelled and measured ditch water P concentrations were found (Fig. 10). The general structure and parameterization of the model were therefore judged satisfactory. The fit of the model's output with the measured data from Liesineva, particularly in the case of the basins 1 and 6, was the worst. At Liesineva basin 1, the measured runoff P concentrations were lower than the modelled concentrations during the third and the fourth year after fertilizer application, and during almost the whole study period at Liesineva basin 6. For Liesineva basin 3, the measured runoff P concentrations were often higher than the modelled concentrations during the last two years after application. 3.5.2 Sensitivity analysis and scenarios To determine the sensitivity of the model's output to changes in parameter values and to calculate alternative scenarios of P leaching, a number of simulations were performed by altering parameter values. Table 5 gives a brief description of these simulations. Total amounts (kg ha 1 ) of P leaching from the fertilization area are also given for the different simulations in Table 5. The model's output was rather sensitive to changes in the values of each of the four parameters tested (Figs. 11-15). The maximum P concentrations in ditch outflow and the duration of P leaching were significantly affected by the fertilizer dissolution rate (Fig. 11). The maximum P concentrations for the dissolution rates 0.0274*f, 0.0584*f, 0.0822*?, and 0.1096*f were 0.44, 0.87, 33 Table 5. Simulations for sensitivity analysis and scenarios. The dilution coefficient (d) was set to 1.0, the background P concentration (BP) to 0.00 mg M, and the fertilizer dose to 40 kg P ha 1 . For each simulation, the model was run for 8 000 days after fertilizer application. a| D%, = 100 % for ? > 3650 days b) D%, = 100 % for ? > 1825 days C) D%, = 100 % for ? > 1217 days d) D%,= 100% for ? > 913 days Number Percent Adsorption Water content Runoff Total dissolution Plus in runoff (q„ mm d ') leaching (D%,) Accumulation layer (kg P ha 1) (a+u) (W„ mm) Ia =0.0274*/ a) 0.60 400 1.0 16.0 Ib =0.0548*/ b) 0.60 400 1.0 16.0 Ic =0.0822*/ 0.60 400 1.0 16.0 Id =0.1096*/ d) 0.60 400 1.0 16.0 Ha =0.0274*/ a) 0.00 400 1.0 40.0 Hb =0.0274*/ a) 0.30 400 1.0 28.0 Ile =0.0274*/ a) 0.60 400 1.0 16.0 Ild =0.0274*/ a> 0.90 400 1.0 4.0 Ula =0.0274*/ a) 0.60 200 1.0 16.0 Illb =0.0274*/ a) 0.60 400 1.0 16.0 IIIc =0.0274*/ a) 0.60 600 1.0 16.0 Illd =0.0274*/ a) 0.60 800 1.0 16.0 IVa =0.0274*/ a) 0.60 400 0.5 16.0 IVb =0.0274*/ a) 0.60 400 1.0 16.0 IVc =0.0274*/ a) 0.60 400 1.5 16.0 IVd =0.0274*/ a) 0.60 400 2.0 16.0 Va =0.0274*/ a) 0.60 200 0.6 16.0 Vb =0.0274*/ a) 0.60 400 0.8 16.0 Vc =0.0274*/ a > 0.60 600 1.0 16.0 Vd =0.0274*/ a) 0.60 800 1.2 16.0 34 Fig. 10. Measured (circles) and modelled total P (TP, in Table 1) concentrations in ditch runoff from each Kivisuo basin 1.5 years and Liesineva basin 6 years since fertilizer application. 35 Fig. 10. continues 36 1.25, and 1.57 mg I~\ respectively. A 0.3 unit increase in the coefficient for biomass P accumulation and P adsorption by peat (a+u) caused a 0.33 mg l 1 decrease in the maximum P concentration in ditch outflow (Fig. 12). The P concentrations in ditch outflow were also significantly affected by the volume of water in the runoff layer (Fig. 13), and the runoff (Fig. 14). The maximum P concentrations for the runoff values 0.5, 1.0, 1.5, and 2.0 mm d~' were 0.87, 0.44, 0.29, and 0.22 mg I'. For the very dry conditions (low runoff and small water volume in the runoff layer) the maximum P concentrations in ditch outflow were significantly higher than for the wet conditions (Fig. 15). The maximum P concentrations for the simulations Va, Vb, Vc, and Vd were 0.73, 0.55,0.44, and 0.36 mg I" 1 , respectively. Although the P concentrations in ditch outflow and the duration of leaching were sensitive to changes in the rate of fertilizer dissolution (Fig. 11), and the water volume of the runoff layer and the runoff (Figs. 13-15), the total amounts of P leached over the period of about 20 years were the same (Table 5). The only factors affecting the total P leaching would be the peat P adsorption capacity and the effectiveness of the trees and understorey vegetation in accumulating added P (Table 5). Fig. 11. Modelled ditch water P concentrations in response to changes in the dissolution offertilizer-P (D% t ). For details of simulation parameters, see Table 5. 37 Fig. 12. Modelled ditch water P concentrations in response to changes in the adsorption of fertilizer-P by peat (a) plus accumulation in the vegetation biomass and litter ( u). For details of simulation parameters, see Table 5. Fig. 13. Modelled ditch water P concentrations in response to changes in the water volume of the runoff layer (W,). For details of simulation parameters, see Table 5. 38 Fig. 14. Modelled ditch water P concentrations in response to changes in the runoff flow (q t ). For details of simulation parameters, see Table 5. Fig.15. Modelled ditch water P concentrations in response to changes in the water volume of the runoff layer (W1) and the runoff flow (q1). For details of simulation parameters, see Table 5. 39 3.6 Discussion 3.6.1 Verification data The Liesineva data are not completely satisfactory for verification purposes. One reason for this is that the locations of the water divide between basin areas varies over time due to watertable fluctuation (Ahti 1983). Exact sizes of the basin areas can therefore not be determined. In the case of the basin 6, for example, a significant part of the fertilizer-P may have leached to southern ditch outside the basin area boundary assumed in Fig. 3, and not to the ditch with the sampling point. Another weakness in the Liesineva data for verification purposes is that most of the runoff samples were taken during low runoff flows in the summer-time. Due to the lack of the runoff samples from maximum spring and autumn flows, the model's behaviour could not be tested during the most critical P outflow events, i.e. when the daily outflow of fertilizer-P is likely to be the greatest. The runoff samples from the very low flows (< 0.015 1 s" 1 ha ') were excluded from the Liesineva verification data. Even though the high variation in runoff P concentrations (Fig. 4) could not be explained, it should be noted that very little P is leached during these very low flow periods. For example, if the daily average runoff flow is 0.0101 s~' ha -1 and the runoff P concentration is 0.3 mg l" 1 , the amount of P leaching is < 0.3 gha 1 d During a maximum spring flow (e.g. 2.01 s" 1 ha 1 ), a similar runoff P concentration would correspond to >5O g ha 1 d' 1 in the amount of P leaching. It should also be noted that during dry periods the Liesineva runoff samples were often taken from almost stagnant water rather than from clearly flowing water, as is normal practice in nutrient outflow experiments. Sometimes the runoff flow ceased totally during or after sample collection. With fixed basin sizes and frequent sampling of runoff also during high flow periods, the Kivisuo experimental area is more suitable for the verification of the P-outflow model than the Liesineva experiment. However, due to some unexplained excess input of P, the Kivisuo basin 8 is not as satisfactory as the data from the other basins. The ditch water samples were analysed for the total dissolved P (ICP/AES, ARL 3580) at Kivisuo, and for the dissolved reactive P (ascorbic acid molybdenum blue method) at Liesineva (for the definition "reactive P", see 40 Ekholm, 1998). Both the total dissolved P and the dissolved reactive P can contain a number of different forms of inorganic and organic P. Water-soluble phosphate is likely to be the main fertilizer-derived P component in ditch outflow from drained ombrotrophic peatlands however. Increases in P leaching from drained ombrotrophic peatlands following clear-cutting have been shown to be mainly due to increased concentrations of water-soluble phosphate (Nieminen 1999). The fit of the model to Liesineva data was not as good as that to Kivisuo data. In addition to the reasons mentioned above, this may also be because the fertilizer-induced changes in runoff P concentrations were significantly lower at Liesineva than at Kivisuo. The runoff P concentrations therefore remained lower at Liesineva. Factors not considered in the model (e.g. atmospheric P deposition) are thus likely to have relatively more effect on runoff P concentrations at Liesineva than at Kivisuo. 3.6.2 Model structure In the model, the peat deposit is divided into three layers, (1) the active layer, from where dissolved phosphate ions are adsorbed by the peat and taken up by plant roots, (2) the runoff layer, which controls the leaching of dissolved phosphate ions not sorbed by peat and not taken up by the vegetation, and (3) the passive layer, the remainder of the peat deposit which does not affect the fate of fertilizer-P (Fig. 1). This division of the peat deposit with respect to the behaviour of fertilizer-P closely resembles the division of peat into the acrotelm and catotelm (Ivanov 1981, Ingram 1983, Damman & French 1987). The acrotelm is the surface peat layer above the lowest watertable level, and the catotelm is the permanently anaerobic peat layer below this level (Fig. 16). The acrotelm is further divided into two layers, an upper aerobic layer above the highest watertable level, and a lower acrotelm layer between the lowest and highest watertable levels. Thus, as in the model presented here, three hydrologically and ecologically different layers are defined. In the model, the uptake and adsorption of fertilizer-P occurs in the uppermost active peat layer, where there is sufficient aeration and where most of the biological activity takes place (would correspond to the upper level of the acrotelm). The leaching of phosphate ions to the ditch occurs from the runoff layer where the water level fluctuates (would correspond to the lower level of the acrotelm). The permanently anaeorobic passive layer, where water movement is negligible, corresponds to the catotelm. 41 Fig. 16. Properties of the acrotelm and catotelm of a bog (redrawn from Damman & French , 1987). The boundary between the runoff layer and the passive layer (i.e. the acrotelm and the catotelm) was assumed to be at the same level as the bottom of the ditches. It is reasonable to assume that ditching increases water movement in the peat layer above the level of the bottom of the ditches. In drained peatlands, the division between the acrotelm and the catotelm may therefore be expected to be approximately at the same level as the bottom of the ditches. In areas with very shallow or highly decomposed peat deposits however, there may be a nearly water-impermeable soil layer above the level of the bottom of the ditches. In such areas, the division between the acrotelm and catotelm is likely to be above the level of the bottom of the ditches. In the model, the fraction of fertilizer-P going to leaching (l-(a+u)) was assumed to be mixed with the total volume of water in the saturated layer above the level of the bottom of the ditches (i.e. in the runoff layer). No empirical data are available to support this assumption. The model's output was very sensitive to changes in the water volume (W,) in the runoff layer however (Fig. 13). Different water volume estimates in the model would therefore have given significantly different model results. 42 No significant delay was assumed between the dissolution of P-fertilizer in the active layer and the leaching of dissolved phosphate ions to the runoff layer. This is probably true for sites like Kivisuo and Liesineva, where the surface peat layers consist of slightly decomposed Sphagnum peat. In such sites, the rate of water flow through the surface peat is rapid. The water permeability of highly decomposed peat is low. A substantial delay may therefore occur before the dissolved phosphate ions move through the surface peat layers and appear in the runoff layer. In a laboratory study by Sarasto (1963), the vertical water flow in slightly decomposed Sphagnum peat was about 64 cm h" 1 . For highly decomposed peat, the corresponding value was only 0.64 cm h l . Soil and the soil water freezing are likely to change water flow pathways during the winter and upon snowmelt. These changes may be expected to influence the leaching of fertilizer-P. However, frozen soil parameters were not included in the model due to a lack of information about their significance and because the number of runoff samples collected during the frozen soil period was very limited. There is no information about the effects of different application doses on fertilizer-P leaching from drained peatlands. As the model was based on application doses similar to those used in practical peatland forestry (i.e. 40- 45 kg ha 1 ), its results should not be compared to sites where the application dose markedly differs from that used in practice. The fertilizer-P leaching model was based on empirical data collected from nutrient-poor ombrotrophic peatland sites. The leaching of fertilizer-P from the nutrient-rich minerotrophic sites is likely to differ from that of the nutrient poor ombrotrophic sites. This is because such sites differ from each other with respect to the factors (e.g. the peat P adsorption capacity, IV) affecting leaching. Although the results of this study are applicable to a limited range of peatland types, over one million hectares of such ombrotrophic peatlands have been drained for forestry purposes in Finland (Tomppo 1998). The model was based on data from only a few sites, however, and further investigations are needed. 3.6.3 Parameter definition 3.6.3.1 Fertilizer dissolution The estimation of fertilizer dissolution (DP,) for the first two years after fertilization was based on publication 11. For model application to the Liesineva 43 leaching experiment, however, information was needed about the dissolution of Moroccan phosphate rock for 6 years after fertilization. In the model, the daily dissolution of Moroccan phosphate rock in years 3-6 after application was assumed to be the average of the second year. Tree growth tends to steadily increase in years 2-6 after the application of Moroccan phosphate rock (Karsisto 1968, 1977, Penttilä & Moilanen 1987, Kaunisto et ai. 1993). This steady increase in tree growth supports the assumption of a uniform dissolution rate for the second to sixth year after the application of Moroccan phosphate rock. Sudden high increases or decreases in the rate of dissolution of Moroccan phosphate rock in years 2-6 after application would most probably be seen in tree growth. The original dissolution curves from publication II were not directly adopted for use in the model. It is assumed in the model that there is no dissolution during the winter-period when the soil is frozen and there is no free water for dissolution reactions. An enhanced dissolution period in spring is assumed in the model. This was because the fertilizer granules are likely to be broken down into finer particles by freeze-thaw processes during the winter, and this disintegration of the granules may be expected to increase their dissolution. However, it is still unknown if the rates of dissolution of the most soluble P compounds in Moroccan phosphate rock and Mire-PK are as assumed in Fig. 5. To describe the seasonal and other short-term changes in the rates of dissolution of different P-fertilizers, shorter sampling intervals than those in the publication II (2-10 months) should be used. In the leaching experiments used for model application and verification, urea was added together with Mire-PK at Kivisuo, and potassium chloride and ammonium nitrate together with Moroccan phosphate rock at Liesineva. However, in publication 11, which was used to estimate fertilizer dissolution for the model, only Mire-PK or Moroccan phosphate rock were applied. Nitrogen and potassium fertilizer salts have been shown to increase phosphate rock dissolution (I and the references therein). The actual dissolution rates at Kivisuo and Liesineva may therefore have been higher than those used in the model. The proportion of phosphate rock that dissolves correlates negatively with the dose of phosphate rock (Rajan et ai. 1991). The doses of fertilizer given in the dissolution experiment (II) were higher than those used in the Kivisuo and Liesineva leaching experiments. The dissolution rates used in the model may therefore have underestimated the real dissolution of the fertilizers at Kivisuo and Liesineva. 44 3.6.3.2 Accumulation of fertilizer-phosphorus in vegetation biomass and litter Using the results from publication 111 to determine the value of u (coefficient for the accumulation of fertilizer-P in the vegetation biomass and litter) is associated with at least two possible errors. Firstly, if the addition of fertilizer- P and other fertilizer nutrients increases the mineralization of peat P, part of the fertilizer-induced accumulation may be due to the uptake of peat P. It was assumed, however, that the increases in P accumulation in vegetation biomass and litter presented in publication 111 were solely due to accumulation of fertilizer-P. Secondly, as was speculated in publication 111, the vegetation samples may still have contained residual unreactive phosphate rock at the time of sampling. Thus, the results of publication 111 may somewhat overestimate the real P accumulation in vegetation biomass. Isotopic labelling methods could give more reliable information about the rate of accumulation fertilizer-P in vegetation biomass and litter. The accumulation of fertilizer-P in the vegetation biomass and litter was assumed not to depend on the rate at which the fertilizer dissolves, i.e. the coefficient u was assumed to be the same for all fertilizers regardless of solubility. This assumption is supported by the fact that a number of investigations have shown no significant differences in the total tree growth between P sources of differing solubility (I and the references therein). This indicates that the total uptake of P-fertilizer by trees does not depend on fertilizer solubility, which is in agreement with the assumption. It is not known, however, whether the uptake of fertilizer-P by peatland vegetation other than trees depends on the solubility of the fertilizer. In the model, the accumulation of P in the total plant biomass was also assumed not to depend on the volume of the tree stand, i.e. though different amounts of fertilizer-P may be expected to allocate to different vegetation compartments (tree, bush, field, and bottom layers, and root systems) in areas with differing tree stand volume, the accumulation in the total plant biomass (sum of all compartments) is assumed to be the same. In a study by Päivänen (1970) carried out on a treeless low-sedge pine bog, about 7 % of P in a phosphate rock fertilizer had accumulated in the above-ground vegetation biomass (field layer vegetation) after three years. On a treeless upland bog in England, the corresponding value was 9.9 % (Malcolm & Cuttle 1983b). Calculations with the data presented by Paavilainen (1980) showed that the average accumulation of P in the above-ground vegetation biomass (field layer + tree layer) after three years had been about 6 % of the amount applied with a phosphate rock 45 fertilizer (dwarf shrub pine bog with tree stand volume »95 m 3 ha '). Calculations with the results presented by Finer (1991 a), and those in the publication 111 showed that the fertilizer-induced (Moroccan phosphate rock) accumulation in the above-ground biomass (field layer + tree layer) was 6-8 % after three years (low-shrub pine bog with tree stand volume «80 m 3 ha 1 )- These results indicate that the accumulation of fertilizer-P in the total plant biomass is of the similar magnitude for peatland areas with significantly different tree stand volumes. In addition to the results in publication 111, only one study was found concerning the accumulation of fertilizer-P in the total plant biomass and litter of ombrotrophic peatlands (Vasander 1981). However, there was no replication of the fertilization treatments in Vasander's study and the treatments differed not only with respect to fertilization but also with respect to drainage (the control site was a pristine bog, whereas the fertilized site had been drained 4 years before fertilization). Vasander's (1981) estimates for the accumulation of fertilizer-P in vegetation and litter must therefore be regarded with caution. 3.6.3.3 Adsorption of fertilizer-phosphorus by peat In the estimation of the adsorption of fertilizer-P by peat, P adsorption isotherms of the surface 0-15 cm peat layer were used (IV). These isotherms indicated that ombrotrophic peat soils have very little ability to retain P against leaching. The adsorption coefficient (a) in the model was therefore set to zero. However, results of publication IV showed rather high P adsorption for the subsurface 15-30 cm peat layer in most of the studied ombrotrophic soils. The P adsorption isotherms were determined under aerobic conditions in the laboratory. Under field conditions, however, anaerobic conditions tend to prevail below the uppermost peat layers during most of the year. Anaerobiosis is known to inhibit P adsorption due to reduction and redistribution of Fe (Armstrong 1975). High P adsorption values as determined in the laboratory for the 15-30 cm peat samples may therefore not be realised under field conditions. Phosphorus adsorption values may also be overestimated because the samples are thoroughly mixed in the laboratory. This mixing results in the close contact with the peat and the phosphate ions. Under field conditions, however, the movement of water and phosphate ions is mainly confined to the large pores and channels and there is little contact with a large portion of the soil matrix (Richardson 1985). Laboratory studies with soil samples placed in leaching columns probably better estimate P adsorption under field conditions. Results from such leaching column 46 Studies (e.g. Fox & Kamprath 1971) indicate very low P adsorption capacity for organic soils with low amounts of inorganic colloids. The low P adsorption capacity of ombrotrophic peat soils is related to their low Fe content (IV). Applying compounds rich in Fe can therefore be used to increase the P adsorption capacity of peat and thereby reduce P leaching losses (Scheffer & Kuntze 1989). According to Scheffer et al. (1986), much of the phosphate sorbed by Fe remains available to plants. Thus, tree production would necessarily not be decreased by an application of Fe. The effect of P+Fe fertilization on the growth of peatland forests and the leaching of P from drained peatlands should therefore be investigated. 4 CONCLUSIONS The aim of the study was to determine the effects of P-fertilizers of varying dissolution rate, the adsorption of P by peat, and the accumulation of P in vegetation biomass and litter on the leaching of P from drained ombrotrophic peatland forests. The results indicated that although the P concentrations in ditch outflow and the duration of leaching varied between the P-fertilizers of differing solubility, the total leaching losses over the period of about 20 years were the same. The results suggest that the factors restricting the total leaching of fertilizer are the P adsorption capacity of the peat and the effectiveness of trees and understorey vegetation in accumulating the added P. Ombrotrophic peat soils have very low capacity to adsorb P however. This is related to low contents in peat of P-sorbing compounds, Fe in particular. Compared with amounts of P fertilizer used in practical peatland forestry (40-45 kg ha 1 ), the accumulation of P in trees and understorey vegetation is also low. Due to low peat P adsorption capacity and P accumulation in trees and understorey vegetation, the risk for high leaching rates of applied P from drained ombrotrophic peatlands is consequently substantial. The use of application doses of less than 40-45 kg ha 1 and P-fertilizers including P-sorbing compounds (especially Fe) in reducing phosphate leaching to water courses should therefore be investigated. 47 REFERENCES Ahti, E. 1983. Fertilizer-induced leaching of phosphorus and potassium from peatlands drained for forestry. Communicationes Instituti Forestalls Fenniae 111: 1-20. Ahti, E. 1987. Water balance of drained peatlands on the basis of water table simulation during the snowless period. Communicationes Instituti Forestalls Fenniae 141: 1-64. Ahti, E. & Paarlahti, K. 1988. Ravinteiden huuhtoutuminen talvella lannoitetulta metsäojitusalueelta. (Summary: Leaching of nutrients from a peatland area after fertilizer application on snow). Suo 39: 19-25. Almberger, P. & Salomonsson, L-Ä. 1979. Domänverkets gödlingsförsök pä torvmarker. Mätningar av fosforutlakning efter gödsling med räfosfat. Sveriges Skogsvärd förbunds Tidskrift 5-6: 1-7. Armstrong, W. 1975. Waterlogged soils. In: Etherington, J. R. (ed.). Environment and Plant Ecology, pp. 184-216. John Wiley, London. Bache, B. W. & Williams, E. G. 1971. A phosphate sorption index for soils. Journal of Soil Science 22: 289-301. Bolland, M. D. A. & Gilkes, R. J. 1995. Long-term residual value of North Carolina and Queensland rock phosphates compared with triple superphosphate. Fertilizer Research 41: 151-158. Braekke, F. H. 1977. Growth and chemical composition of Scots pine on nutrient deficient peat after drainage and fertilization. Meddelelser fra Norsk institutt for skogforskning 33(8): 285-305. Braekke, F. H. 1988. Nutrient relationships in forest stands: field vegetation and bottom litter layer on peatland. Meddelelser fra Norsk institutt for skogforskning 40(7): 1-20. Chien, S. H., Clayton, W. R. & McClellan, G. H. 1980. Kinetics of dissolution of phosphate rocks in soils. Soil Science Society of American Journal 44: 260- 264. Cuttle, S. P. 1983. Chemical properties of upland peats influencing the retention of phosphate and potassium ions. Journal of Soil Science 34: 75-82. Damman, A. W. H. & French, T. W. 1987. The ecology of peat bogs of the glaciated northeastern United States: a community profile. United States Department of the Interior Fish and Wildlife Service. Biological Report 85 (7.16): 1-100. Ekholm, P. 1998. Algal-available phosphorus originating from agriculture and municipalities. Monographs of the Boreal Environment Research 11: 1-60. Finer, L. 1989. Biomass and nutrient cycle in fertilized and unfertilized pine, mixed birch and pine and spruce stands on a drained mire. Acta Forestalia Fennica 208: 1-63. Finer, L. 1991 a. Effect of fertilization on dry mass accumulation and nutrient cycling in Scots pine on an ombrotrophic bog. Acta Forestalia Fennica 223: 1-42. Finer, L. 1991b. Turvemaiden ravinnetaseet. In: Mäkkeli, P. & Hotanen, J-P. (eds.). Metsänkasvatuksen perusteet turve- ja kivennäismailla. Metsäntutkimuspäivä Joensuussa 1991. Metsäntutkimuslaitoksen tiedonantoja 383: 11-22. Flach, E. N., Quak, W. & Van Deist, A. 1987. A comparison of rock phosphate- 48 mobilizing capacities of various crop species. Tropical Agriculture (Trinidad) 64: 347-352. Fox, R. L. & Kamprath, E. J. 1971. Adsorption and leaching of P in acid organic soils and high organic matter sand. Soil Science Society of American Proceedings 35: 154-156. Halonen, 0., Tulkki, H. & Derome, J. 1983. Nutrient analysis methods. Metsäntutkimus laitoksen tiedonantoja 121: 1-28. Harriman, R. 1978. Nutrient leaching from fertilized forest watersheds in Scotland. Journal of Applied Ecology 15: 933-942. Haveraaen, O. 1967. Vekst- og naeringsstudier i et gjodlings forsok med svartgran, Picea mariana (Mill.), pä myr. (Summary: Growth and nutrient studies in a fertilized experiment with Black spruce, Picea mariana (Mill.), on peatland). Meddelelser fra det Norske Skogforsoksvesen 23: 137-175. Heikurainen, L. & Pakarinen, P. 1982. Peatland classification. In: Laine, J. (ed.). Peatlands and their utilization in Finland. Finnish Peatland Society, Finnish National Committee of the International Peat Society, pp. 14-23. Ingram, H. A. P. 1983. Hydrology. In: Gore, A. J. P. (ed.). Mires: swamp, bog, fen and moor. Ecosystems of the world, vol. 4A, pp. 67-158, Elsevier, Amsterdam. Ivanov, K. E. 1981. Water movement in mirelands. Academic Press, London. 276 p. Jarva, M., Kaunisto, S., Nieminen, M., Sallantaus, T. & Saura, M. 1995. Metsänlannoit teen huuhtoutuminen Liesinevan sarkaleveyskoekentältä - alustavia tuloksia. In: Saukkonen, S. & Kenttämies, K. (eds.). Metsätalouden vesistövaikutukset ja niiden torjunta. METVE-projektin loppuraportti, pp. 121-128. Suomen ympäristö 2 - ympäristönsuojelu. Helsinki. Kaila, A. 1959. Retention of phosphate by peat samples. Journal of the scientific agricultural society of Finland 31(3): 215-225. Karsisto, K. 1968. Eri fosforilajien soveltuvuus suometsien lannoitukseen. (Summary: Using various phosphatic fertilizers in peatland forests). Suo 19: 104-111. Karsisto, K. 1970. Lannoituksessa annettujen ravinteiden huuhtoutumisesta turvemailta. (Summary: On the washing of fertilizers from peaty soils). Suo 21: 60-66. Karsisto, K. 1977. Kotimaisten fosforirikasteiden käyttökelpoisuus suometsien lannoituksessa. (Summary: Possibilities of native phosphate concentrates in fertilizing peatland forests). Suo: 28: 43-46. Karsisto, K. & Ravela, H. 1971. Eri ajankohtina annettujen fosfori-ja kalilannoitteiden huuhtoutumisesta metsäojitusalueilta. (Summary: Washing away of phosphorus and potassium from areas drained for forestry and topdressed at different times of the year). Suo 22: 39-46. Kaunisto, K., Moilanen, M. & Issakainen, J. 1993. Apatiitti ja flogopiitti fosfori- ja kaliumlannoitteina suomänniköissä. (Summary: Apatite and phlogopite as phosphorus and potassium fertilizers in peatland pine forests). Folia Forestalia 810: 1-30. Kenttämies, K. 1981. The effects on water quality of forest drainage and fertilization in peatlands. Publications of Water Research Institute, National Board of Waters, Finland 43: 24-31. Lepistö, L. & Saura, M. 1998. Effects of forest fertilization on phytoplankton in a boreal brown-water lake. Boreal Environment Research 3: 33-43. Mackay, A. D. & Syers, J. K. 1986. Effect of phosphate, calcium and pH on the 49 dissolution of phosphate rock in soil. Fertilizer Research 10: 175-184. Malcolm, D. C. & Cuttle, S. P. 1983 a. The application of fertilizers to drained peat. 1. Nutrient losses in drainage. Forestry 56: 155-174. Malcolm, D. C. & Cuttle, S. P. 1983b. The application of fertilizers to drained peat. 2. Uptake by vegetation and residual distribution in peat. Forestry 56: 175-183. Metsätalous ja vesistöt 1996. Yhteistutkimusprojektin "Metsätalouden vesistöhaitat ja niiden torjunta" (METVE) yhteenveto. Maa-ja metsätalousministeriön julkaisuja 4/1996. 102 p. Moilanen, M. 1992. Suopuustojen ravinnetila Pohjois-Suomen vanhoilla ojitusalueilla. In: Valtanen, J., Murtovaara, I. & Moilanen, M. (eds.). Metsäntutkimuspäivä Taivalkoskella 1991. Metsäntutkimuslaitoksen tiedonantoja 419: 58-65. Nieminen, M. 1999. Päätehakkuun ja maanmuokkauksen vaikutus valumaveden laatuun vanhoilla ojitusalueilla. In: Ahti, E., Granlund, H. & Puranen, E. (eds.). Metsätalouden ympäristökuormitus. Seminaari Nurmeksessa 23.-24.9.1998. Tutkimusohjelman väliraportti. Metsäntutkimuslaitoksen tiedonantoja 745:103- 107. (in press). Nieminen, M. & Ahti, E. 1993. Talvilannoituksen vaikutus ravinteiden huuhtoutumiseen karulta suolta. (Summary: Leaching of nutrients from an ombrotrophic peatland area after fertilizer application on snow). Folia Forestalia 814: 1-22. Paavilainen, E. 1969. Tutkimuksia levitysajankohdan vaikutuksesta nopealiukoisten lannoitteiden aiheuttamiin kasvureaktioihin suometsissä. (Summary: Influence of the time of application of fast-dissolving fertilizers on the response of trees growing on peat). Folia Forestalia 75: 1-24. Paavilainen, E. 1980. Effect of fertilization on plant biomass and nutrient cycle on a drained dwarf shrub pine swamp. Communicationes Instituti Forestalls Fenniae 98(5): 1-77. Päivänen, J. 1970. Hajalannoituksen vaikutus lyhytkortisen nevan pintakasvillisuuden kenttäkerrokseen. (Summary: On the influence of broadcast fertilization on the field layer of the vegetation of open low-sedge bog). Suo 21:18-24. Päivänen, J. 1973. Hydraulic conductivity and water retention in peat soil. Acta Forestalia Fennica 129: 1-70. Partiff, R. L. 1978. Anion adsorption by soils and soil materials. Advances in Agronomy: 30: 1-50. Penttilä, T. & Moilanen, M. 1987. Fosforilannoitteet suometsien lannoituksessa Pohjois suomessa. In: Saarenmaa, H. & Poikajärvi, H. (eds.). Korkeiden maiden metsien uudistaminen. Ajankohtaista tutkimuksesta. Metsäntutkimuspäivät Rovaniemel lä 1987. Metsäntutkimuslaitoksen tiedonantoja 278: 136-148. Rajan, S. S. S. 1987. Partially acidulated phosphate rock as fertilizer and dissolution in soil of the residual rock phosphate. New Zealand Journal of Experimental Agriculture 15: 177-184. Rajan, S. S. S. & Watkinson, J. H. 1992. Unacidulated and partially acidulated phosphate rock: Agronomic effectiveness and the rates of dissolution of phosphate rock. Fertilizer Research 33: 267-277. Rajan, S. S. S., Fox, R. L. & Saunders, W. M. H. 1991. Influence of pH, time and rate of application on phosphate rock dissolution and availability to pastures. 11. Soil chemical studies. Fertilizer Research 28: 95-101. Rannikko, M. & Hartikainen, H. 1980. Retention of applied phosphorus in Sphagnum 50 peat. Proceedings of the 6th International Peat Congress, Duluth, Minnesota, August 17-23, 1980, pp. 666-669. Richardson, C. J. 1985. Mechanisms controlling phosphorus retention capacity in freshwater wetlands. Science 22: 1424-1427. Sarasto, J. 1963. Tutkimuksia rahka-ja saraturpeiden vedenläpäisevyydestä. (Summary: A study on the permeability to water of different kinds of peat). Suo 3: 32-36. Saura, M., Sallantaus, T., Bilaletdin, Ä. & Frisk, T. 1999. Leaching of nitrogen and phosphorus fertilizers from forested basins. Boreal Environment Research, (in press). Scheffer, B. & Kuntze, H. 1989. Phosphate leaching from high moor soils. International Peat Journal 3: 107-115. Scheffer, 8., Kuntze, H. & Bartels, R. 1986. Anwendung von Rotschlamm und Griinsalz auf sauren Hochmoorböden zur Reduzierung des Phosphataustrages. Z. f. Kulturtechnik und Flurbereinigung 27: 76-82. Silfverberg, K. & Hartman, M. 1998. Long-term effects of different phosphorus fertilizers in Finnish pine mires. In: Sopo. R. (ed.). Proceedings of the International Peat Symposium, The Spirit of Peatlands, Jyväskylä, Finland 7-9 September, 1998, pp. 73-75. Tomppo, E. 1998. Peatland forests of Finland 1951-1994. In: Sopo. R. (ed.). Proceedings of the International Peat Symposium, The Spirit of Peatlands, Jyväskylä, Finland 7-9 September, 1998, pp. 84-86. Vasander, H. 1981. Luonnontilaisen keidasrämeen sekä lannoitetun ojikon ja muuttu man ravinnevarat. (Summary: Nutrients in an ombrotrophic bog ecosystem in the virgin state and after forest-improvements). Suo 32(4-5): 137-141. Veijalainen, H. 1992. Neulasanalyysituloksia suometsistä talvella 1987-88. (Summary: Nutritional diagnosis of peatland forests by needle analysis in winter 1987-88). Metsäntutkimuslaitoksen tiedonantoja 408: 1-28. Wallander, H., Wickman, T. & Jacks, G. 1997. Apatite as P source in mycorrhizal and non-mycorrhizal Pinus sylvestris seedlings. Plant and Soil 196: 123-131. Yli-Halla, M. & Lumme, I. 1987. Behaviour of certain phosphorus and potassium compounds in a sedge peat soil. Silva Fennica 21(3): 251-257. I ISSN 0039-5471 Suo 48(4): 115-126 © Suoseura Finnish Peatland Society Helsinki 1997 Properties of slow-release phosphorus fertilizers with special reference to their use on drained peatland forests. A review Hidasliukoisten fosforilannoitteiden ominaisuudet ja käyttökelpoisuus suometsien lannoituksessa. Kirjallisuuteen perustuva tarkastelu Mika Nieminen Mika Nieminen, The Finnish Forest Research Institute, Vantaa Research Centre, P.O. Box 18, FIN-01301 Vantaa, Finland (email: mika.nieminen@metla.fi) Slow-release phosphorus fertilizers have long been considered as a primary fertiliza tion option on drained peatland forests in Finland. There has also been growing interest in using slow-release P-fertilizers as a better alternative to water-soluble fertilizers on agricultural land. Manufacture of different P-fertilizers, and those fertilizer and site properties which affect the rate of P release, are reviewed. The effects of slow-release P-fertilizers on plant growth and the liability to be leached into waterways are also discussed. In particular, the behaviour of slow-release P-fertilizers on drained peatland forests is considered. Key words: fertilizer dissolution, peatland forestry, phosphate minerals, phosphorus outflow INTRODUCTION The use of slow-release phosphorus (P)-fertiliz ers instead of water-soluble forms is attractive from the viewpoint of both economy and envi ronmental protection (slow-release P-fertilizer is a generic term and refers here to products which contain higher amounts of sparingly soluble com ponents than traditional water-soluble fertilizers). Slow-release P-fertilizers are much cheaper than totally water-soluble ones (Sanches & Salinas 1981), costing as little as one-fifth the price of a single superphosphate. The concentration of P in slow-release P-fertilizers is also often greater than in water-soluble fertilizers, thus leading to sav ings in transport and application. Many develop- ing countries have indigenous phosphorus depos its, the use of which significantly decreases the cost of food production compared with the use of imported water-soluble fertilizers (Sale & Mok wunye 1993). Water-soluble P-fertilizers are most often produced by the acidulation of phosphate rocks with phosphoric acid. Each ton of phos phoric acid used results in 2.2 tons of phosphogyp sym, a byproduct the disposal of which is a great environmental problem (Rajan et ai. 1994). Use of totally water-insoluble P-fertilizers eliminates this problem. For some soils and in some envi ronments, slow-release P-fertilizers have also been shown to better match plant demand than water soluble ones (Rajan & Watkinson 1992, Dahana yake et al. 1995). This should both increase eco- 116 Mika Nieminen nomical efficiency and reduce leaching of ferti lizer-P to watercourses. Slow-release P-fertilizers have long been con sidered as a primary option on drained peatland forests in Finland, and the effects on tree produc tion of P-fertilizers of differing solubility have been studied fairly extensively (Karsisto 1968, 1977, Penttilä & Moilanen 1987, Kaunisto et ai. 1993, K. Silfverberg & M. Hartman unpubl.). There has also been a growing interest in the use of slow-release P-fertilizers as a better alterna tive to water-soluble fertilizers on agricultural land (Bolan et al. 1993, Hagin & Harrison 1993, Sale & Mokwunye 1993). The purpose of this paper is to summarize the information known about the properties of slow-release P-fertilizers in order to be able to answer the following questions: What are the different types and characteristics of slow-release P-fertilizers? How do fertilizer properties affect the release of P from fertilizers? How do site characteristics affect the release of P from fertilizers? Are the slow-release P-fertilizers likely to better match plant demand than water-soluble ones? Are the slow-release P-fertilizers likely to be less liable to be leached into watercourses than water-soluble ones? The last three questions are discussed with spe cial reference to the use of slow-release P-ferti lizers on drained peatland forests. ORIGIN AND MANUFACTURE OF P-FERTI LIZERS All P-fertilizers are produced from various min eral deposits of phosphorus. These include: Fe- Al phosphates (e.g. variscite and strengtite), Ca- Al-Fe phosphates (e.g. crandallite and millisite), and Ca phosphates (apatites) (McClellan & Gre million 1980). Apatite is the tenth most abundant mineral in the earth's crust, and the major source of commercial phosphate. On the basis of their geological formation, phosphate minerals are classified to igneous, sedi mentary and metamorphic (Khasawneh & Doll 1978, McClellan & Gremillion 1980). Igneous rocks are formed by crystallization of primary minerals from hot molten magma. Sedimentary rocks contain significant amounts of organic phos phorus, and metamorphic rocks are transitional in chemical nature between igneous and sedimen tary rocks. Sedimentary apatites of commercial interest are collectively known as phosphate rock (PR), and account for about 80% of the world's phosphate production. PRs have also received the greatest research interest and most of the infor mation reviewed here comes from these studies. The typical phosphate mineral in igneous and metamorphic depositions is fluoroapatite, while sedimentary deposits are composed of carbonate apatite (francolite) (McClellan & Lehr 1969, McKelvey 1973). Moroccan phosphate rock used earlier on drained peatland forests in Finland is an example of a carbonate apatite-type PR, while the present peatland forest fertilizer, Siilinjärvi apatite, is a fluoroapatite of igneous origin. There are several possible ways to use phos phate minerals in agriculture and forestry. They can be applied directly, as is done in the case of Siilinjärvi apatite on drained peatland forests in Finland, for example. Direct application has been extensively used for tree crops also in Asia (Chien & Menon 1995). Total and citric acid-soluble P contents of some commercial PR materials are given in Table 1. The materials with citric P > 30% are usually regarded as possible alternatives to water-soluble P-fertilizers for direct applica tion to agricultural land, while materials with cit ric P < 30% are considered too unreactive to be directly applied. In order to increase their solubility, P miner als can be heated (thermally treated phosphates) or acidulated with mineral acids such as sulphu ric acid, phosphoric acid, or nitric acid (Fig. 1). The high energy consumption in the manufacture of thermally treated phosphates makes them ex pensive forms of P-fertilizer. Nevertheless, a large number of experiments have been carried out on agricultural land in Australia and New Zealand evaluating the suitability of thermal phosphates as slow-release fertilizers (Bolland & Gilkes 1987, 1991, 1995). Traditionally. P has been applied to agricul tural land in the form of completely acidulated water-soluble fertilizers, such as single superphos phate, triple superphosphate and diammonium- SUO 48(4), 1997 117 Fig. 1. Phosphate mineral is the primary source of phos phorus fertilizers. Reproduced from Bolan et al. (1993). Kuva 1. Fosforilannoitteiden valmistus fosforimineraa leista. Muunnettu julkaisusta Bolan ym. (1993). phosphate. However, partially acidulated phos phorus fertilizers have recently become of great interest as low-cost fertilizers on agricultural land. Partially acidulated P-fertilizers are produced by either direct acidulation of phosphorus minerals with less acid than that required for complete acidulation, or indirectly, by mixing different por tions of fully acidulated and unacidulated P-ferti lizers (Bolan et al. 1993). The wide variation in the chemical composi tion of different phosphate minerals, and the number of alternative methods in the manufac- Ture of P-fertilizers, cause significant difficulties in predicting the behaviour of P-fertilizers under field conditions, unless the mechanisms control ling the release of P are known. FERTLIZER PROPERTIES CONTROLLING THE RELEASE OF P The most informative fertilizer property control ling the rate of dissolution of fertilizer-P is, of course, the amount of water-soluble P. Water-solu- Table 1. Total P and 2% citric acid-soluble P contents of some commercially available phosphate rock materials. Source: Bolan et al. (1993). Taulukko 1. Eräiden raakafosfaattien kokonaisfosforipitoisuus ja 2% sitruunahappoon liukeneva fosfori. Lähde: Bolan ym. (1993). Phosphate rock Raakafosfaattilaji Origin Alkuperä Total P (%) Kokonais-P Citric P (% of total P) Sitruunahappoliukoinen P (% kokonais-P:stä) Christmas Island A Christmas Island 16.4 23 Christmas Island C Christmas Island 11.6 19 Duchess Australia 13.5 6 Nauru Nauru 15.6 22 Central Florida USA 14.6 20 Khouribga Morocco 14.4 26 lordan Jordan 15.0 26 Youssafia Morocco 13.8 27 Egyptian Egypt 13.0 33 Arad Israel 14.4 30 Moroccan Morocco 14.0 32 Chatham Rise New Zealand 9.0 23 Sechura Peru 13.2 43 Gafsa North Africa 13.0 33 North Carolina USA 12.8 32 118 Mika Nieminen Fig. 2. Relationship between citrate solubility of apatite and its composition as indi cated by the C0 3/P04 mole ratio. Source: Lehrand McClel lan (1972). Kuva 2. Apatiittimineraalien sitruunahappoliukoisuuden ja kiderakenteen CO/POj moolisuhteen välinen riippu vuus. Lähde: Lehrja McClel lan (1972). ble P is not only readily available to plants, but can also increase the release of fertilizer-P from water-insoluble PR residues (Rajan & Watkinson 1992). However, this effect has not been shown in all experiments (Rajan 1987). High leaching rates of P were observed following the applica tion of a PK-fertilizer with 20% of total P in a water-soluble form (Nieminen & Ahti 1993, Saura et al. 1995). This is why fertilizers with high amounts of water-soluble P are nowadays con sidered inappropriate for use on drained peatland forests in Finland. For carbonate apatite-type PRs, the key-fac tor determining the reactivity of water-insoluble PR is the degree to which isomorphic substitu tion of phosphate by carbonate has occurred within the apatite crystal structure (Bolan et al. 1993, Chien 1993, Hagin & Harrison 1993, Sale & Mok wunye 1993). Because P04 2~ions have a tetrahe dron structure and CO, 2 " a planar structure, CO, 2 ~ substitution for P0 4 2 ~ results in structural insta bility making PRs more reactive (Fig. 2). Because the C03 2VP0 4 2 " molar ratio in an apatite molecule is difficult to measure, the reactivity of different PRs is normally measured with neutral ammonium citrate, 2% citric acid or 2% formic acid (Chien 1993). Great differences in reactivity between PRs of different origin are normal (Table 1). Moroc can phosphate rock used earlier on drained peat- land forests in Finland is one of the PRs consid ered as 'reactive' (citric P > 30% of total P) by agricultural standards (e.g. Bolan et al. 1993). The citric acid solubility of the present peatland forest fertilizer, Siilinjärvi apatite, is only 1-2%. Phosphorus deposits can contain other miner als in addition to apatite. Some of these (e.g. sili ceous minerals) may depress P release from ferti lizers, whereas some (e.g. calcite) have no effect on P reactivity in the soil (Hammond et al. 1986). Perhaps surprisingly, the particle size of phos phorus fertilizers has only a limited influence on P dissolution. Reducing particle size firstly in creases reactivity, but fine grinding to a size of less than 100 mesh (150 (im) is not adviseable, since finer particles do not increase reactivity greatly (Khasawneh & Doll 1978). In the case of long intervals between sequential fertilizer appli cations (e.g. forest fertilization), as plant root sys tems develop and fertilizer granules physically break into finer particles, the effect of granules on plant growth may be similar to powdered fer tilizers (Gillion et al. 1979. Penttilä & Moilanen 1987). The granulation technique also affects the re activity of the fertilizer. In granulation, small amounts (2-5%) of binding agents, e.g. mineral acids (Stangel 1978), are added to improve the physical characteristics of the granules. It is to be noted that use of mineral acids as binding agents SUO 48(4), 1997 119 Fig. 3. Effect of soil pH on the proportion of added phos phate rock dissolved 3,4 and 6 years after application. Source: Rajan et ai. (1991). Kuva 3. Raakafosfaalista 3:ssa, 4:ssd ja 6:ssa vuodes sa liuennut fosfori ja sen riip puvuus maan happamuu desta. Lähde: Rajan ym. (1991). not only produce granules with good physical characteristics but also produce small amounts of water-soluble P. Nitric acid was added in the granulation of "Suo-PK", a commercial PK-ferti lizer used earlier on drained peatlands in Finland. This is why Suo-PK also contained water-soluble P and nitrogen (2%). SITE CHARACTERISTICS CONTROLLING THE RELEASE OF P The dissolution of a slow-release phosphorus fer tilizer, using fluoroapatite as an example, can be described by the following equation: Ca, O(PO4 )6F2 + 12H + —>10Ca :+ + 6H2P04~+ 2F~ (1) From the equation it is seen that the driving force for the dissolution of a slow-release P-ferti lizer is the ability to maintain high concentrations of H + ions but low concentrations of Ca 2+ and H,P04~ ions in the soil solution. Several studies have shown that low soil pH enhances PR disso lution (Fig. 3). Drained peatlands, which have highly acidic surface peat layers (pH < 4-5), cer tainly have the ability to provide H + ions to pro mote dissolution. As low levels of Ca 2+ ions in soil solution pro mote fertilizer-P dissolution, plant species hav ing a high Ca 2+ uptake benefit the most from ferti lizations with slow-release fertilizers (Flach et al. 1987). High concentrations of Ca 2 * ions in a soil solution are unlikely to significantly depress fer tilizer-P dissolution on drained peatlands. Owing to the high cation exchange capacity (CEC) of peat (Pätilä & Nieminen 1990), and the ability of organic matter to provide chelating substances that reduce Ca 2+ ion activity in the soil solution (Chien 1979), drained peat soils most probably have the ability to maintain low concentrations of Ca 2t ions in the solution. High concentrations of H 2 P04 ~ ions in a soil solution have less effect on PR dissolution than those of Ca 2+ ions (Mackay & Syers 1986). How ever, high soil P-sorption capacities have been shown to promote PR dissolution (Smyth & Sanchez 1982, Syers & Mackay 1986). The P sorption capacities of Fe- and Al-poor peat soils can be extremely low (Cuttle 1983, Nieminen & Jarva 1996), which might depress the rate of P release. However, the high water content of peat and water movement through the surface soil are likely to rapidly remove the released phosphate ions away from the surfaces of fertilizer particles, and so promote additional fertilizer-P dissolution. Furthermore, peatland forests are often severely P deficient (Moilanen 1992, Veijalainen 1992), which is likely to result in high fertilizer-P uptake from the soil solution. Application of non-P-fertilizers together with slow-release P sources may change the activity of H +, H 2 PO.f or Ca2+ ions in the soil solution, and 120 Mika Nieminen thus alter fertilizer-P dissolution. Thus, N and K fertilizer salts were shown to increase PR avail ability to plants (Volk 1944, Jones 1948). This effect was probably because the N and K fertiliz ers increased the plant growth and thereby also the uptake of P from the soil solution (Chien 1979). The effect of other fertilizer nutrients on PR dis solution may, however, be also purely chemical. For example, the addition of urea hydrolyzes or ganic matter in the soil and the products of this hydrolysis increase the dissolution of PR by re ducing the Ca 2+ activity in the solution (Chien 1979). Climatic factors are also important in affect ing the dissolution of slow-release P-fertilizers. In areas with < 600 mm annual average rainfall in south-western Australia, the limited moisture sup ply severely depressed the dissolution of PR (Bol land & Gilkes 1995). However, due to the high water retention capacity of peat, the limited mois ture supply probably seldom depresses P release on drained peatlands. Since flooding has not been shown to substantially affect dissolution in flooded rice soils (Hammond et al. 1986), excess mois ture is also not likely to restrict P release from fertilizers. Temperature has not been found to signifi cantly affect PR dissolution in tropical soils (Chien et al. 1980). In regions with a distinct winter-pe riod, however, below-zero temperatures and the frozen surface soil during the winter-period are unlikely to enable any fertilizer-P dissolution. Considerable dissolution during the winter-period, when there is no nutrient uptake by the vegeta tion, would be extremely harmful to recipient watercourses. PLANT RESPONSE TO SLOW-RELEASE P FERTILIZERS The effects on plant yield of slow-release P-ferti lizers have been studied extensively on agricul tural land (e.g. Rajan & Marwaha 1993, Sale & Mokwunye 1993, Bolland & Gilkes 1995, Daha nayake et al. 1995). Due to variety in crop spe cies, management practises, fertilizer and soil properties, it is difficult to generalise about the effectiveness of slow-release P-fertilizers. How ever, water-soluble and slow-release P-fertilizers have most often been shown to be equally effec tive (Rajan & Marwaha 1993). Partially acidu lated phosphate rocks (water-soluble P 40-50% of total P) were as effective as superphosphate in 86% of 109 field experiments conducted in Ger many over the period 1961-1981 (Hagin 1985). McLean and Logan (1970) found that in 66% of 53 field trials partially acidulated Florida rock was as effective as superphosphate. It is generally ac cepted that unacidulated and partially acidulated phosphate rocks can be as effective as superphos phate for long-term or perennial crops, but not always for short-term and annual crops (Chien et al. 1990). The crop experiments where slow-re lease P-fertilizers have given better growth re sponse than water-soluble fertilizers are scarce compared with those where slow-release fertiliz ers were equally or less effective (see, however, Hammond 1978, Khasawneh & Doll 1978, Gar bouchev 1981, Hagin 1985, Rajan & Watkinson 1992, Dahanayake et al. 1995). Fertilizations are usually repeated at 1-4-year intervals on agricultural land. For many fertiliz ers and environments, this time interval is prob ably far too short for all the applied slow-release P-fertilizer to be released in a plant-available form. Despite this, the agronomic effectiveness of wa ter-soluble and slow-release P-fertilizers is often similar (see above). The time interval between sequential fertilizations is considerably longer in forestry (15-25 years), thus enabling all water insoluble P to be released in a plant-available form. It is therefore not surprising that, in the long-term studies, different sources of P have affected tree production similarly on drained peatland forests (Karsisto 1968, 1977, Penttilä & Moilanen 1987, Kaunisto et al. 1993, K. Silfverberg & M. Hartman unpubl.). There is usually a more rapid response after fertilization with superphosphate (Fig. 4). However, the overall growth response is not af fected by the solubility of the fertilizer, since the duration of response is longer for slow-release fertilizers. LEACHING OF FERTILIZER-P INTO WA TERWAYS Agricultural land on acid peaty sands in high rain fall (> 800 mm annual average) areas of south- 121 SUO 48(4), 1997 Kuva 4. Increase in the an nual height growth of Scots pine after fertilization in 1961 with different sources of P. Source: Karsisto (1977). Kuva 4. Männyn vuotuisen pituuskasvun lisääntyminen eri fosforilannoitteilla vuon na 1961 tehdyn lannoituksen jälkeen. Lähde: Karsisto (1977). western Australia have a high risk for fertilizer-P to be leached into waterways (Bolland et al. 1995). Peatlands drained for forestry in Finland pose a similar risk (Nieminen & Ahti 1993, Saura et al. 1995, Nieminen & Jarva 1996). Slow-release P fertilizers have been considered as environmen tally better alternatives to water-soluble P-ferti lizers both in south-western Australia and in Fin land. Because the yield effects of PRs and super phosphates were similar, Bolland et al. (1995) concluded that different sources of P are unlikely to reduce leaching of P into waterways from Aus tralian acid peaty sands. As mentioned in the pre vious section, slow-release and water-soluble P fertilizers have equal effects on the growth of trees on drained peatland forests, indicating that the uptake of fertilizer-P by trees is not related to the solubility of the fertilizer. However, it is not known whether the uptake of fertilizer-P by peat land vegetation other than trees depends on the source of P, but it is known that the adsorption of fertilizer-P by peat is not related to the solubility of the fertilizer (Kaunisto et al. 1993, K. Silfver berg & M. Hartman unpubl.). Thus, because slow release P-fertilizers have not been shown to bet ter match tree stand demand or to be more effi ciently adsorbed by peat than water-soluble ferti lizers, the uptake of fertilizer-P by understorey vegetation is the only mechanism that can cause significant differences between water-soluble and slow-release P-fertilizers in overall P outflow to watercourses. If the fertilizer-P uptake by under- storey vegetation does not depend on the source of P, the question arises, why have slow-release P-fertilizers been regarded as significantly less liable to be leached to watercourses than water soluble fertilizers from drained peatland forests (Karsisto 1970, Karsisto & Ravela 1971, Alm berger & Salomonsson 1979)? The obvious answer is that the slow release of P from water-insoluble fertilizers has not been suf ficiently accounted for in all P-outflow experi ments. In the studies of Karsisto (1970), Karsisto and Ravela (1971), and Almberger and Salomons son (1979), the monitoring of P outflow afterfer tilization with PR lasted only a few months. How ever, considerable changes in the leaching behav iour of slow-release P-fertilizers are unlikely to occur immediately after fertilization (Bolland et al. 1995, Jarva et ai. 1995). For some fertilizers and in some environments, it may take even a few years before P outflow is significantly increased (Fig. 5). However, once slow-release P-fertiliz ers start to be lost to watercourses, P outflow con tinues for several years (Harriman 1978, Kenttä mies 1981, Ahti 1983, Malcolm & Cuttle 1983). CONCLUSIONS The properties of phosphate minerals used for fertilizer manufacture differ significantly. Meth ods used to manufacture commercial P-fertilizers also vary significantly. It is thus difficult to un- 122 Mika Nieminen Fig. 5. Phosphorus concentrations of ditch water before (1975 and 1976) and after (1977-1982) fertilization with 500 kg ha -1 of a commercial PK-fertilizer, including 8.3% of Pas water-insoluble Moroccan phosphate rock. Only 10% of the area was fertilized. The values are the means of those presented by Ahti (1983) in appendices 1-3. Kuva 5. Ojaveden fosforipitoisuudet ennen (1975 ja 1976) ja jälkeen (1977-1982) lannoituksen. Lannoitteena annettiin 500 kg ha~' kaupallista PK-lannoitetta, joka sisälsi 8,3% fosforia veteen liukenemattomassa muodossa (Marokkolaista raakafosfaattia). Vain 10% valuma-alueen pinta-alasta lannoitettiin. Kuvan arvot ovat keskiarvoja Ahdin (1983) liitteissä 1-3 esittämistä arvoista. derstand or predict the behaviour of a particular P-fertilizer unless the fertilizer and site proper ties controlling the release of P are known. The most important fertilizer properties con trolling the release of P from fertilizers include: the amount of water-soluble P, the C03 2 7P0 4 :~ mole ratio of the apatite crystal structure, particle size, and granulation technique. There is no experimental data about the site properties controlling the release of P from slow release fertilizers on drained peatlands. However, because of high acidity and sufficient moisture supply, drained peatlands are likely to exhibit con siderable fertilizer-P dissolution. Frozen soil dur ing the winter-period is probably the factor most severely depressing P release in northern peat lands. Slow-release P-fertilizers have not been shown to better match tree stand demand or to be more efficiently adsorbed by peat than water-soluble fertilizers on drained peatland forests. If the ferti lizer-P uptake by peatland vegetation other than trees is also not related to the solubility of the fertilizer, there should be no major differences between water-soluble and slow-release P-ferti lizers in overall P outflow to watercourses. ACKNOWLEDGEMENTS I thank Prof. Seppo Kaunisto, Dr. Erkki Ahti and Dr. Leena Finer for valuable comments on the manuscript and Dr. Michael Starr for revising the English. REFERENCES Ahti, E. 1983. Fertilizer-induced leaching of phosphorus and potassium from peatlands drained for forestry. Communicationes InstitutiForestallsFenniae 111: 1-20. Almberger, P. & Salomonsson, L-Ä. 1979. Domänverkets gödlingsförsök pä torvmarker. Mätningar av fosforut lakning efter gödsling med räfosfat. Sveriges Skogs värdsförbunds Tidskrift 5-6: 1-7. Bolan, N. S., Hedley, M. J. & Loganathan, P. 1993. Prepa ration, forms and properties of controlled-release phos phate fertilizers. Fertilizer Research 35: 13-24. Bolland, M. D. A. & Gilkes, R. J. 1987. How effective are calciphos and phosphal. Fertilizer Research 12:229-239. Bolland, M. D. A. & Gilkes, R. J. 1991. Evaluation of two rock phosphates and a calcined rock phosphate as main tenance fertilizer for crop-pasture rotations in Western Australia. Fertilizer Research 28: 11-24. Bolland, M. D. A. & Gilkes, R. J. 1995. Long-term residual value of North Carolina and Queensland rock phos phates compared with triple superphosphate. Fertilizer Research 41: 151-158. SUO 48(4), 1997 123 Bolland, M. D. A., Clarke, M. F. & Yeates, J. S. 1995. Ef fectiveness of rock phosphate, coastal superphosphate and single superphosphate for pasture on deep sandy soils. Fertilizer Research 41: 129-143. Chien, S. H. 1979. Dissolution of phosphate rocks in acid soils as influenced by nitrogen and potassium fertiliz ers. Soil Science 127: 371-376. Chien, S. H. 1993. Solubility assessment for fertilizer con taining phosphate rock. Fertilizer Research 35: 93-99. Chien, S. H. & Menon, R. G. 1995. Factors affecting the agronomic effectiveness of phosphate rock for direct application. Fertilizer Reserach 41: 227-234. Chien, S. H„ Clayton, W. R. & McClellan, G. H. 1980. Kinetics of dissolution of phosphate rocks in soils. Soil Science Society of American Journal 44: 260-264. Chien, S. H„ Sale, P. W. G. & Hammond, L. L. 1990. Com parison of the effectiveness of phosphorus fertilizer products. In: Proc. Symposium on Phosphorus Require ments for Sustainable Agriculture in Asia and Oceania, pp. 143-156. International Rice Research Institute, Manila, Philippines. Cuttle, S. P. 1983. Chemical properties of upland peats in fluencing the retention of phosphate and potassium ions. Journal of Soil Science 34: 75-82. Dahanayake, K., Ratnayake, M. P. K. & Sunil, P. A. 1995. Potential of Eppawala Apatite as a directly applied low cost fertilizer for rice production in Sri Lanka. Ferti lizer Research 41: 145-150. Flach, E. N., Quak, W. & Van Deist, A. 1987. A comparison of rock phosphate-mobilizing capacities of various crop species. Tropical Agriculture (Trinidad) 64: 347-352. Garbouchev, I. P. 1981. The manufacture and agronomic efficiency of a partially acidulated phosphate rock fer tilizer. Soil Science Society of American Journal 45: 970-974. Gillion, 1., Reinhorm, T., Semiat, R. & Hagin, J. 1979. Evalu ation of granulated phosphate rock for direct applica tion. In: Seminar on phosphate rock for direct applica tion, pp. 387-394. International Fertilizer Development Centre. Muscle Shoals, Alabama. Hagin, J. 1985. Partially acidulated phosphate rock-A re view. Technion-Israel Institute of Technology, Haifa, Israel, pp. 14-56. Hagin, J. & Harrison, R. 1993. Phosphate rocks and par tially-acidulated phosphate rocks as controlled release P fertilizers. Fertilizer Research 35: 25-31. Hammond. L. L. 1978. Agronomic measurements of phos phate rock effectiveness. In: Seminar on phosphate rock for direct application, pp. 147-173. International Ferti lizer Development Centre, Muscle Shoals, Alabama. Hammond, L. L., Chien, S. H. & Mokwunye, A. U. 1986. Agronomic value of unacidulated and partially acidu lated phosphate rocks indigenous to tropics. Advances in Agronomy 40: 89-140. Harriman, R. 1978. Nutrient leaching from fertilized forest watersheds in Scotland. Journal of Applied Ecology 15: 933-942. Jarva, M., Kaunisto, S., Nieminen, M., Sallantaus, T. & Saura, M. 1995. Metsänlannoitteen huuhtoutuminen Liesinevan sarkalevey skoekentältä - alustavia tuloksia. In: Saukkonen, S. & Kenttämies, K. (eds.). Metsätalou den vesistövaikutukset ja niiden torjunta. METVE projektin loppuraportti, pp. 121-128. Suomen ympä ristö 2 - ympäristönsuojelu. Helsinki. Jones, U. S. 1948. Availability of phosphorus in rock phos phate as influenced by potassium and nitrogen salts, lime, and organic matter. Agron. J. 40: 765-770. Karsisto, K. 1968. Eri fosforilajien soveltuvuus suometsien lannoitukseen (Summary: Using various phosphatic fertilizers in petland forests). Suo 19: 104-111. Karsisto, K. 1970. Lannoituksessa annettujen ravinteiden huuhtoutumisesta turvemailta (Summary: On the wash ing of fertilizers from peaty soils). Suo 21: 60-66. Karsisto, K. 1977. Kotimaisten fosforirikasteiden käyttökel poisuus suometsien lannoituksessa (Summary: Possi bilities of native phosphate concentrates in fertilizing peatland forests). Suo 28: 43-46. Karsisto, K. & Ravela, H. 1971. Eri ajankohtina annettujen fosfori-ja kalilannoitteiden huuhtoutumisesta metsäoji tusalueilta (Summary: Washing away of phosphorus and potassium from areas drained for forestry and top dressed at different times of the year). Suo 22: 39-46. Kaunisto, S., Moilanen, M. & Issakainen, J. 1993. Apatiitti ja flogopiitti fosfori- ja kaliumlannoitteina suomänni köissä (Summary: Apatite and phlogopite as phospho rus and potassium fertilizers in peatland pine forests). Folia Forestalia 810: 1-30. Kenttämies, K. 1981. The effects on water quality of forest drainage and fertilization in peatlands. Publications of Water Research Institute, National Board of Waters, Finland 43: 24—31. Khasawneh, F. E. & Doll, E. C. 1978. The use of phosphate rock for direct application to soils. Advances in Agro nomy 30: 159-206. Mackay, A. D. & Syers, J. K. 1986. Effect of phosphate, calcium and pH on the dissolution of phosphate rock in soil. Fertilizer Research 10: 175-184. Malcolm. D. C. & Cuttle, S. P. 1983. The application of fertilizers to drained peat. 1. Nutrient losses in drain age. Forestry 56: 155-174. McClellan, G. H. & Gremillion, L. C. 1980. Evaluation of phosphatic raw materials. In: Khasawneh. F. E., Sam ple. E. C. & Kamprath, E. J. (Eds). The role of phos phorus in agriculture, pp. 45-85. ASA-CSSA-SSA Madison, Wisconsin, USA. McClellan, G. H. & Lehr, J. R. 1969. Crystal chemical in vestigation of natural apatites. The American Miner alogist 54: 1374-1391. McKelvey, V. E. 1973. Abundance and distribution of phos phorus in the lithosphere. In: Griffith, E. J., Beeton, A., Spensor, J. M. & Mitchell, D. T. (Eds). Environmental phosphorus handbook. John Wiley and Sons, New York. pp. 13-31. McLean, E. O. & Logan, T. J. 1970. Sources of phosphorus for plants grown in soils with differing P fixation ten dencies. Soil Science Society of American Proceed- 124 Mika Nieminen ings 34: 907-911. Moilanen, M. 1992. Suopuustojen ravinnetila Pohjois-Suo men vanhoilla ojitusalueilla. In: Valtanen, J., Murto vaara, I. & Moilanen, M. (eds.). Metsäntutkimuspäivä Taivalkoskella 1991. Metsäntutkimuslaitoksen tiedon antoja 419: 58-65. Nieminen, M. & Ahti, E. 1993. Talvilannoituksen vaikutus ravinteiden huuhtoutumiseen karulta suolta (Summary: Leaching of nutrients from an ombrotrophic peatland area after fertilizer application on snow). Folia Forestalia 814: 1-22. Nieminen, M. & Jarva, M. 1996. Phosphorus adsorption by peat from drained mires in southern Finland. Scandinavian Journal of Forest Research 11: 321-326. Pätilä, A. & Nieminen, M. 1990. Effects of atmospheric deposition on the nutrient status of oligotrophic pine mires. In: Peat 90, Versatile peat. International confer ence on peat production and use. June 11.-15., 1990. Jyväskylä, Finland. Volume 2, posters. Association of Finnish Peat Industries, University of Jyväskylä, Con tinuing Education Centre, pp. 115-129. Penttilä, T. & Moilanen, M. 1987. Fosforilannoitteet suometsien lannoituksessa Pohjois-Suomessa. In: Saarenmaa, H. & Poikajärvi, H. (eds.). Korkeiden maiden metsien uudistaminen. Ajankohtaista tutkimuk sesta. Metsäntutkimuspäivät Rovaniemellä 1987. Metsäntutkimuslaitoksen tiedonantoja 278: 136-148. Rajan, S. S. S. 1987. Partially acidulated phosphate rock as fertilizer and dissolution in soil of the residual rock phosphate. New Zealand Journal of Experimental Ag riculture 15: 177-184. Rajan, S. S. S. & Marwaha, B. C. 1993. Use of partially acidulated phosphate rocks as phosphate fertilizers. Fertilizer Research 35: 47-59. Rajan, S. S. S. & Watkinson, J. H. 1992. Unacidulated and partially acidulated phosphate rock: Agronomic effec tiveness and the rates of dissolution of phosphate rock. Fertilizer Research 33: 267-277. Rajan, S. S. S., Fox, R. L. & Saunders, W. M. H. 1991. Influence of pH, time and rate of application on phos phate rock dissolution and availability to pastures. 11. Soil chemical studies. Fertilizer Research 28: 95-101. Rajan, S. S. S., O'Connor, M. B. & Sinclair, A. G. 1994. Partially acidulated phosphate rocks: Controlled release phosphorus fertilizers for more sustainable agriculture. Fertilizer Research 37: 69-78. Sale, P. W. G. & Mokwunye, A. U. 1993. Use of phosphate rocks in the tropics. Fertilizer Research 35: 33-45. Sanches, P. A. & Salinas, J. G. 1981. Low-input technol ogy for managing Oxisols and Utisols in tropical Ameri ca. Advances in Agronomy 34: 279^06. Saura, M., Sallantaus, T., Bilaletdin, Ä. & Frisk, T. 1995. Metsälannoitteiden huuhtoutuminen Kalliojärven valuma-alueelta. In: Saukkonen, S. & Kenttämies, K. (eds.). Metsätalouden vesistövaikutukset ja niiden torjunta-METVE-projektin loppuraportti, pp. 87-104. Suomen ympäristö 2 - ympäristönsuojelu. Helsinki 1995. Smyth, T. J. & & Sanches, P. A. 1982. Phosphate rock dis solution and availability in Cerrado soils as affected by phosphorus sorption capacity. Soil Science Society of American Journal 46: 339-345. Stangel, P. J. 1979. The IFDC phosphate program. In: Semi nar on phosphate rock for direct application, pp. 3-35. International Fertilizer Development Centre, Muscle Shoals, Alabama. Syers, J. K. & Mackay, A. D. 1986. Reactions of Sechura phosphate rock and single superphosphate in soil. Soil Science Society of American Journal 50: 480-485. Veijalainen, H. 1992. Neulasanalyysituloksia suometsistä talvella 1987-88 (Summary: Nutritional diagnosis of peatland forests by needle analysis in winter 1987-88). Metsäntutkimuslaitoksen tiedonantoja 408: 1-28. Volk, G. W. 1944. Availability of rock and other phosphate fertilizers as influenced by lime and form of nitrogen fertilizer. Agron. J. 36: 46-56. TIIVISTELMÄ: Hidasliukoisten fosforilannoitteiden ominaisuudet ja käyttökelpoisuus suometsien lannoituksessa. Kirjallisuuteen perustuva tarkastelu Hidasliukoiset fosforilannoitteet (hidasliukoisuus tarkoittaa tässä yhteydessä sitä, että lannoitteessa on enemmän veteen liukenematonta fosforia kuin perinteisissä vesiliukoisissa fosforilannoitteissa) ovat kiinnostava vaihtoehto vesiliukoisille lannoit teille sekä taloudelliselta että ympäristönsuojelul liselta kannalta. Hidasliukoiset lannoitteet ovat selvästi vesiliukoisia fosforilannoitteita halvempia (Sanches & Salinas 1981). Hidasliukoisten lan- noitteiden fosforipitoisuus on myös yleensä vesi liukoisia lannoitteita suurempi, mikä alentaa lan noitteiden kuljetus-ja levityskustannuksia. Vesi liukoiset fosforilannoitteet valmistetaan yleisim min käsittelemällä erilaisia raakafosfaatteja fos forihapolla. Fosforihapon valmistuksessa syntyy sivutuotteena kipsijauhetta (Rajan ym. 1994), jon ka varastointi ja hävittäminen on huomattava ym päristöongelma. Veteen liukenematonta fosforia 125 SUO 48(4), 1997 sisältävien lannoitteiden käytössä ei tätä ongelmaa ole. Joissakin tapauksissa hidasliukoisilla fosfori lannoitteilla on myös saatu suurempia kasvun lisäyksiä kuin vesiliukoisilla lannoitteilla (Rajan &Watkinson 1982, Dahanayake ym. 1995). Tämä sekä lisää lannoituksen taloudellisuutta että toden näköisesti vähentää lannoitefosforin huuhtoutu mista vesistöihin. Tässä kirjallisuuskatsauksessa kootaan yhteen tutkimustuloksia fosforilannoitteiden alkuperästä ja ominaisuuksista sekä lannoitefosforin liukene miseen vaikuttavista tekijöistä. Edelleen on tarkoi tus verrata hidasliukoisilla ja vesiliukoisilla lan noitteilla saatuja kasvutuloksiaja liukoisuudeltaan erilaisten fosforilannoitteiden huuhtoutumista. Kirjallisuuskatsauksessa pyritään erityisesti arvioimaan hidasliukoisten fosforilannoitteiden käyttäytymistä ojitetuilla turvemailla. Kaikki fosforilannoitteet valmistetaan erilai sista fosforimineraaliesiintymistä. Näistä yleisim piä ja tavallisimmin lannoitefosforin valmistuk sessa käytettyjä ovat Ca-fosfaatit eli apatiitit. Muita fosforimineraaliesiintymiä ovat Ca-Al-Fe fosfaatit ja Fe-Al-fosfaatit (McClellan & Gremil lion 1980). Geologisesti fosforimineraalit luokitellaan elo peräisiin, magmaattisiin ja metamorfisiin esiinty miin (Khasawneh & Doll 1978, McClellan & Gre million 1980). Noin 80% kaikista fosforilannoit teista valmistetaan eloperäisistä esiintymistä. Elo peräistä alkuperää olevia, kaupallisesti kiinnosta via apatiittiesiintymiä kutsutaan yleisesti raaka fosfaateiksi (= phosphate rocks). Eri alkuperää olevat fosforiesiintymät ovat yleensä huomattavan erilaisia kemialliselta koostumukseltaan. Taulu kosta 1 nähdään, että esimerkiksi fosforin koko naismäärät ja sitruunahappoliukoisuus vaihtelevat huomattavasti eri raakafosfaattiesiintymien vä lillä. Fosforimineraaleja voidaan louhimisen jajau hamisen jälkeen käyttää sellaisenaan kasvien fos forinlähteenä. Useimmiten fosforin liukoisuutta kuitenkin lisätään joko kuuma- tai happokäsittelyl lä (Kuva 1). Osittain happokäsitellyt fosforilan noitteet ovat viime vuosina olleet tiiviin tutkimuk sen kohteena maataloudessa, koska ne ovat perin teisiä vesiliukoisia fosforilannoitteita edullisempia. Huomattava vaihtelu eri fosforimineraaliesiin tymien kemiallisessa koostumuksessa ja lannoit teiden valmistusmenetelmissä aiheuttaa sen, että fosforilannoitteiden käyttäytymistä kenttäolosuh teissa on vaikea ymmärtää tai ennustaa. Käyttäy tymisen selvittämiseksi lannoitefosforin liukene miseen vaikuttavat tekijät on kunnolla tunnettava. Tärkein lannoitefosforin liukenemiseen vai kuttava lannoitelajista riippuva tekijä on lannoit teen sisältämän vesiliukoisen fosforin määrä. Vesiliukoista fosforia sisältäviä lannoitteita ei kui tenkaan viime vuosina ole suometsissä juurikaan käytetty. Eloperäistä alkuperää olevien apatiittiesiinty mien osalta veteenliukenemattoman fosforin liu kenemisnopeus määräytyy suurelta osin apatiitin kiderakenteen C03 2 7 P04 2 "-moolisuhteen perus teella (Kuva 2). Fosfaatin korvautuminen karbo naatilla aiheuttaa kiderakenteessa muutoksia, jotka lisäävät fosforin liukoisuutta (Hammond ym. 1986). Koska apatiitin kiderakenteen C03 27 P04 2~- moolisuhteen mittaaminen on vaikeaa, veteenliu kenemattoman fosforin liukoisuus määritetään yleensä neutraalilla ammonium nitraatilla, 2% sitruunahapolla tai 2% muurahaishapolla (Chien 1993). Raekoolla on myös vaikutusta fosforin liuke nemiseen (Khasawneh & Doll 1978). Pitkäaikai sissa kokeissa (esim. suometsien lannoituksessa) raekoko ei kuitenkaan merkittävästi vaikuttane lannoituksen tehokkuuteen, koska murentuessaan rakeet alkavat ennenpitkää käyttäytyä kuten hienorakeisimmat lannoitteet (Gillion ym. 1979, Penttilä & Moilanen 1987). Myös rakeistamisessa käytetty tekniikka vai kuttaa lannoitteen liukenemiseen. Rakeistamises sa lannoitteeseen lisätään 2-5% sitovia ainesosia (esim. mineraalihappoja) rakeiden fysikaalisten ominaisuuksien paranratamiseksi (Stangel 1978). Aiemmin suometsissä käytetyn PK-lannoitteen (Suo-PK) rakeistamisessa lannoiteseokseen oli lisätty typpihappoa, joka muuttaa osan fosforista vesiliukoiseen muotoon. Tästä syystä Suo-PK sisälsi jonkin verran vesiliukoista fosforia ja typ peä. Tärkeimmät lannoitefosforin liukenemiseen vaikuttavat kasvupaikkatekijät selviävät yhtälöstä 1, joka kuvaa fluoroapatiitin liukenemista. Yhtälöstä voidaan päätellä, että kasvupaikan kyky tuotaa maaveteen vetyioneja ja toisaalta pidättää maavedestä kalsiumia ja fosforia ovat rat kaisevia tekijöitä hidasliukoisten fosforilannoittei- 126 Mika Nieminen Den liukenemisessa. Happamuuden voimakas vai kutus veteenliukenemattoman fosforin liukenemi seen näkyy Kuvasta 3. Ojitetuilla turvemailla maa veden vetyionipitoisuus (pH yleensä < 4-5) ei useinkaan rajoittane lannoitefosforin liukenemis ta. Turpeen voimakas kationinvaihtokyky (Pätilä & Nieminen 1990) aiheuttaa sen, että myöskään korkeat maaveden Ca-konsentraatiot eivät yleensä liene esteenä lannoitefosforin liukenemiselle. Turvemaiden kyky pidättää maavedestä fosfaattia on sen sijaan heikko (Cuttle 1983, Nieminen & Jarva 1996). Veden suotautuminen pintaturpeen läpi kuitenkin todennäköisesti huuhtoo lannoitteis ta vapautuneet fosfaatti-ionit lannoiterakeiden pinnoilta syvempiin turvekerroksiin, jolloin maa veden fosfaattikonsentraatio lannoiterakeiden lähellä ei enää rajoittane fosforin liukenemista. Muita lannoitefosforin liukenemiseen vaikutta via kasvupaikkatekijöitä ovat kasvupaikan hydro logia ja maan lämpötila. Kuivuus ei rajoittane lan noitefosforin liukenemista suometsissä. Myös kään liiallinen kosteus ei estäne liukenemista, koska riisipelloilla tulvittamisen ei ole havaittu vähentävän hidasliukoisen fosforin liukenemista (Hammond ym. 1986). Sen sijaan maaveden jääty minen talvella estää todennäköisesti kokonaan veteen liukenemattoman fosforin liukenemisen. Fosforilannoitteiden liukoisuuden vaikutuk sesta kasvien kasvuun on tehty useita tutkimuksia sekä maatalousmaalla että suometsissä. Maa talousmaalla tehtyjen tutkimusten perusteella on vaikea yksiselitteisesti päätellä hidasliukoisten fosforilannoitteiden tehokkuutta vesiliukoisiin lannoitteisiin verrattuna, koska viljelymenetelmät, kasvilajit ja lannoitteiden sekä maan ominaisuudet vaihtelevat eri tutkimuksissa. Useimmiten vesiliu koiset ja hidasliukoiset lannoitteet ovat kuitenkin olleet yhtä tehokkaita (McLean & Logan 1970, Hagin 1985, Rajan & Marhawa 1993). Maatalou dessa lannoitukset toistetaan yleensä 1-4 vuoden välein. Monissa tapauksissa tämä ajanjakso on todennäköisesti liian lyhyt, jotta kaikki hidasliu koinen fosfori ehtisi muuttua vesiliukoiseen muo toon. Siksi ei ole yllättävää, että joissakin kokeissa vesiliukoiset fosforilannoitteet voivat olla hidas- liukoisia tehokkaampia. Suometsätaloudessa lan noitusten aikaväli on sen sijaan huomattavasti pi dempi (15-25 vuotta), joten kaikki hidasliukoinen fosfori todennäköisesti ehtii muuttua vesiliukoi seksi ennen lannoituksen uusimista. Siten ei ole yllättävää, että pitkäaikaisissa kokeissa lannoitus vaikutus on ollut samaa suuruusluokkaa liukoisuu deltaan erilaisilla lannoitteilla (Karsisto 1968, 1977, Penttilä & Moilanen 1987, Kaunisto ym. 1993, K. Silfverberg & M. Hartman julkaisema ton). Yleensä vesiliukoiset lannoitteet kylläkin an tavat nopeamman kasvunlisäyksen, mutta lannoit teen ominaisuudet eivät vaikuta kokonaistuok seen, koska hidasliukoisilla lannoitteilla lannoitus vaikutuksen kesto on pidempi (Kuva 4). Lannoitefosforin huuhtoutuminen ojitusalueil ta riippuu siitä, kuinka suuri osa levitetystä fosfo rista sitoutuu puustoon, pintakasvillisuuteen ja tur peeseen. Ojitusalueilla tehdyt tutkimukset ovat osoittaneet, että samoin kuin puuston kykyyn hyö dyntää lannoitefosforia, lannoitteen liukoisuus ei vaikuta turpeeseen pidättyneen fosforin määriin (Kaunisto ym. 1993, K. Silfverberg & M. Hartman julkaisematon). Jos lannoitteen liukoisuus ei puus ton ja turpeen lisäksi vaikuta myöskään aluskas villisuuteen pidättyvän fosforin määriin, niin lan noitefosforin huuhtoutumisessa ei pitäisi olla mer kittäviä eroja liukoisuudeltaan erilaisten fosfori lannoitteiden välillä. Hidasliukoisia fosforilan noitteita on kuitenkin pidetty selvästi vähemmän huuhtoutumisalttiina kuin vesiliukoisia lannoit teita (Karsisto 1970, Karsisto & Ravela 1971, Almberger & Salomonsson 1979). Tämä käsitys voi hyvinkin aiheutua siitä, että em. huuhtoutuma tutkimukset olivat hyvin lyhytaikaisia (seurantaa vain muutama kuukausi lannoituksen jälkeen). Siten hidasliukoinen lannoitefosfori oli todennä köisesti vielä lähes kokonaan veteen liukenemat tomassa muodossa kokeiden päättyessä. Pitkäai kaiset kokeet ovat osoittaneet, että myös hidasliu koiset fosforilannoitteet huuhtoutuvat ojitusalueil ta (Harriman 1978, Kenttämies 1981, Ahti 1983, Malcolm & Cuttle 1983). Huuhtoutuminen alkaa usein kuitenkin lisääntyä merkittävästi vasta joitakin vuosia lannoituksen jälkeen (Kuva 5). Received 9.10.1997, accepted 18.11.1997 II 1 Mika Nieminen and Maija Jarva DISSOLUTION OF PHOSPHORUS FERTILIZERS OF DIFFERING SOLUBILITY IN PEAT SOIL: A FIELD EXPERIMENT ON A DRAINED PINE BOG. Mika Nieminen and Maija Jarva Finnish Forest Research Institute, Vantaa Research Centre, P. O. Box 18, FIN-01301 Vantaa, Finland Running headline: Dissolution of phosphorus fertilizers in peat soil Mika Nieminen and Maija Jarva 2 Nieminen, M. & Jarva, M. (Finnish Forest Research Institute, Vantaa Reserch Centre, P. O. Box 18, FIN-01301 Vantaa, Finland). Dissolution of phosphorus fertilizers of differing solubility in peat soil: a field experiment on a drained pine bog. Received May 25, 1999. Accepted October 16, 1999. Scand. J. For. Res. 00: 00-00, 1999. A 2-year field trial was conducted on a drained pine bog to measure the dissolution of five phosphorus (P) fertilizers of differing solubility. The rate of fertilizer dissolution was measured using a sequential fractionation which involved pre-extraction with 0.5 M NaCl/TEA followed by 1 M NaOH and a mixed acid (HNO 3+HCI+H:SO 4) digestion. About 56 % of Siilinjärvi apatite, a hard magmatic rock phosphate, dissolved over the two years. For a commercial phosphorus+potassium fertilizer manufactured from Siilinjärvi apatite and potassium chloride (water-soluble P10Ca :+ + 6H;PO 4 ' + 2F (1) The methods proposed to measure the rate and extent of dissolution of slow release P fertilizers are based on this reaction formula. In most studies, the extent of dissolution has been dertermined directly by measuring the amount of dissolved P (Mackay et al. 1986, Bolan & Hedley 1989, Mahimairaja et al. 1995) orCa (Hughes & Gilkes 1984, 1994) in the soil. Under field conditions, however, a fraction of the dissolved P and Ca are likely to be removed by plant uptake and leaching and the amounts remaining in the soil would thus underestimate the extent of dissolution. Under field conditions, the decrease in the amount of residual fertilizer-P has provided an accurate estimate of phosphate rock dissolution (Rajan 1983, Rajan et al. 1991, Rajan & Watkinson 1992, Tambunan et al 1993). The undissolved (residual) fertilizer-P is usually determined by pre-washing the soil samples from fertilized and respective unfertilized plots with either NaCl (Rajan 1983, McKay et al. 1986) or NaCI/TEA (Tambunan et al. 1993, Mahimairaja et al. 1995), and sequentially extracting with NaOH and either HCI (Bolan & Hedley 1989, Mahimairaja et al. 1995) or H:SO.j (Rajan 1987, Tambunan et al. 1993). NaCl and NaCI/TEA remove water-soluble and loosely bound P, and NaOH is assumed to remove inorganic and organic P associated with amorphous hydrous oxides of Al and Fe (Hedley et al. 1982). Extraction with HCI and H;SO4 dissolves calcium phosphate (apatite) and hence an increase in acid extractable P from fertilizer-P treated soil compared to untreated is a measure of the residual fertilizer-P. The assumptions 5 Mika Nieminen and Maija Jarva included are: (1) the Ca-P fraction is apatite phosphate because the NaCl or NaCl/TEA and NaOH solutions do not dissolve apatite, (2) the dissolution of apatite does not form any Ca-P compounds which are less soluble than apatite, and (3) the undissolved fertilizer residues do not move outside the soil sampling zone (Rajan 1987). This paper reports a study to investigate the extent of P release from fertilizers of different solubility in peat soil under field conditions over a 2 year period. 6 Mika Nieminen and Maija Jarva MATERIAL AND METHODS Site description and field work The study site is located in southern Finland (61°51'N; 25°59'E, 131 m a.5.1.). The mean annual precipitation at the nearby meteorological station is 550 mm, and the mean annual temperature 3 °C. The peat layer was over 3 m thick and consisted almost entirely of Sphagnum peat. Some chemical properties of the surface peat layer are presented in Table 1. According to the peatland site type classification used in Finland (Heikurainen & Pakarinen 1982), the site was a Small-sedge pine bog with patches of Sphagnum fuscum bog. The site was drained in 1945-1947 with a 60 m ditch spacing. A naturally regenerated Scots pine (Pinits sylvestris L.) stand, with an average volume of about 10 m ? ha" 1 , is growing on the site. The field experiment was established during August 1996, and consisted of five fertilization treatments with three replicates. The treatments were as follows: (1) Superphosphate, (2) Mire-PK (Finnish: Rakeinen Suometsien PK), (3) Moroccan phosphate rock, (4) Siilinjärvi apatite, and (5) Forest-PK (Finnish: Metsän PK). Some chemical and physical properties of the fertilizers are given in Tables 2 and 3. Siilinjärvi apatite is a Finnish fluoroapatite of igneous origin, while Moroccan phosphate rock is a carbonate apatite of sedimentary origin. Mire-PK and Forest-PK are commercial P+K fertilizers used mainly on drained peatlands in Finland. Mire-PK is manufactured from Moroccan phosphate rock and potassium chloride, and Forest-PK from Siilinjärvi apatite and potassium chloride. In the manufacture of Mire-PK, nitric acid is added to Mika Nieminen and Maija Jarva 7 improve the physical properties of the granules and is why Mire-PK contains water soluble P (15.8 % of total P) and N (2 %). In each of 15 plots (10 mxlo m) 9 circular patches (3.14 m : ) were systematically located. In each of these patches there were 5 systematically located sampling quadrates (20 cm x 20 cm). Four of the quadrates were fertilized by evenly spreading the fertilizer (0.2 g P 100 cm" 2 ) on the bottom layer vegetation of mosses and lichens, and one quadrate was left unfertilized. Peat samples from the unfertilized quadrates were taken at the beginning of the experiment in August 1996, and the samples from the fertilized quadrates about 2, 12, 20, and 24 months after fertilization. On each sampling occasion, a 10 cm x 10 cm core was taken from the middle of one systematically chosen quadrate from each of the 9 circular patches. The cores were divided into the 0-5 and 5-10 cm depth layers. The samples from each plot were combined by layers. The shrub vegetation was cut away at a height of 2-3 cm from the soil surface prior to the sample collection. The bottom layer vegetation was, however, included in the peat sample. This was to ensure that the fertilizer residues would remain in the sampling zone. Laboratory analyses and calculations The samples were dried at 70 °C to constant mass and homogenized in a stainless steel mill (sieve mesh diameter 2.0 mm). The dissolution of the studied fertilizers was Mika Nieminen and Maija Jarva 8 investigated using a modification of the sequential P fractionation procedure described by Tambunan et al. (1993). The peat samples (1 g) were prewashed with 40 ml of 0.5 M NaCl/TEA (pH 7.0) to remove water-soluble and loosely bound P and exchangeable Ca. Removal of exchangeable Ca was carried in order to prevent the formation of Ca(OH); or CaCOj in the NaOH extracts and readsorption or co-precipitation of the dissolved P by these precipitates (Mackay et al. 1986). The samples were then extracted with 40 ml of 1 M NaOH to remove Al- and Fe-bound inorganic and organic P. The extraction times were 2 h and 16 h for the NaCl/TEA and NaOH extracts, respectively. Tambunan et al. (1993) used 0.5 M H;So 4 to subsequently extract the fertilizer-P residue. However, Siilinjärvi apatite has been shown not to completely dissolve in 0.5 M H:SO 4 (Yli-Halla & Lumme 1987). Thus, a mixed acid digestion procedure was used in this study (Anon. 1990). This procedure, referred to as the HNO3-HCI-H2SO4 digestion, first involved the boiling dry of the residue remaining after the NaCl/TEA and NaOH extractions in a solution of concentrated HNO3 (9 ml) and concentrated HCI (18 ml). After the sample had cooled, 20 ml of concentrated H;SO 4 and 10 ml of concentrated HN03 were added, and the suspension was then boiled until it became clear (about 30 min). Distilled water (100 ml) was then added and the suspension boiled again for about 10 min. The suspension was then transferred to a 250 ml volumetric flask and made up to mark with distilled water. Vacuum filtration was used to remove the solutions in the NaCl/TEA and NaOH extractions (Schleicher and Schuell Rundfilter 589 : ). Centrifugation could not be used, since peat being light did not settle tightly in the bottom of the centrifuge tube (see also Rannikko & Hartikainen 1980). The HNO3-HCI-H;SO4-digestions were not filtered. 9 Mika Nieminen and Maija Jarva Phosphorus concentrations in all the solutions were determined by ICP/AES, ARL 3580. The extent of fertilizer-P dissolution was calculated using the following equation: Percent = (fHN(VHCI-H,50. 1 -Pn + Fertilizer-P added) - fHNCh.HCI-H.SOj-P,)) x 100 (1) dissolution (%) Fertilizer-Padded The subscript 0 refers to the amount of P in the control quadrates, and t to that in the fertilized quadrates at the different times of sampling. To convert mg P kg' 1 soil to kg P ha' 1 , bulk density values of the soil samples obtained at each sampling time were used. In the P fractionation method used in this study, the following assumptions have been made: (1) the NaCl/TEA and NaOH extractions do not dissolve apatite; (2) the dissolution of apatite in acid Ca-poor Sphagnum peat is not expected to form any acid extractable Ca-P compounds (this is confirmed in the study made by Rannikko & Hartikainen (1980)), or the insoluble residual P fractions (Hedley et al. 1982); and (3) the fertilizer residues did not move outside the soil sampling zone. The assumptions made in this study somewhat differ from those of the earlier studies (Rajan 1983, 1987). This is because the HN03-HCI-H2S04-digestion of the final residue is expected to dissolve not only apatite phosphate but also the chemically stable residual P compounds. 10 Mika Nieminen and Maija Jarva Recovery experiments were made to test the fractionation procedure. Siilinjärvi apatite and Moroccan phosphate rock, about 0.05 g each, were mixed with 1 g of unfertilized air-dried Sphagnum peat and analyzed using the above described P fractionation procedure. Both Siilinjärvi apatite and Moroccan phosphate rock contain insignificant amounts of water-soluble P (Table 2) and were dry-blended with the peat. The sequential extraction procedure was judged suitable if it recorded nil recovery during the extraction with NaCl/TEA and NaOH and 100 % recovery during digestion with HNO3-HCI-H2SO4. Recovery of P from the fertilizers in different extracts was calculated using the following equation: Recovery (%) = P (treated minus control) x 100 (2) added P During the course of the field sample analysis it was realized that the milling of the samples with a mesh diameter of 2.0 mm did not give satisfactory results for all samples. Duplicate analysis was carried out for a few samples. The results showed very high variation in HNO3-HCI-H:SO4-P concentrations for 4 samples with high amounts of residual fertilizer-P (> 3000 mg kg" 1 ). The coefficients of variation (CV) between duplicate analyses for HNO3-HCI-H:SO 4-P were >25 %. The samples with high amounts (> 2000 mg kg" 1 ) of HNO3-HCI-H;SO4-P (i.e. all the surface 0-5 cm peat samples containing undissolved residues of Siilinjärvi apatite, Moroccan phosphate rock, Mire-PK, and Forest-PK) were therefore re-milled to pass a mesh diameter of 0.2 mm and the P fractionations and the analyses of extracts repeated. The same previously mentioned four samples were then re-analysed in duplicate. The results showed that decreasing the diameter of the milled sample from about 2.0 mm to about 11 Mika Nieminen and Maija Jarva 0.2 mm considerably decreased the within-sample variation. After re-milling, the CVs for HN03-HCI-H;S04-P were < 6 %. The effect of different fertilizers on the concentrations of NaCl/TEA-P, NaOH-P, and HNO3-HCI-H2SO4-P in the soil was tested using analysis of variance for repeated measures designs. The grouping factor was fertilizer treatment, with five levels, and the repeated or within factor was sampling time, also with five levels. To illustrate the trends in the dissolution of different P-fertilizers, the data were also analysed with regression analysis using sampling time as the independent variable and the percent fertilizer dissolution as the dependent variable. The statistical analyses were made using the BMDP (1990) software package. RESULTS Testing of the fractionation procedure The NaCl/TEA extraction recovered about 0.5 % of the added Siilinjärvi apatite and Moroccan phosphate rock, and the NaOH extraction recovered a further 0.5 % (Table 4). The recovery of the added P in the HN03-HCI-H;SO4 digestion was 96 % for Siilinjärvi apatite, and about 94 % for Moroccan phosphate rock. Sequential extraction of the field samples Mika Nieminen and Maija Jarva 12 Prior to the fertilizer application the values for ranged from 190 to 350 mg kg" 1 in the surface 0-5 cm peat samples, and after application from 600 to 10 237 mg kg" 1 . In the 5-10 cm peat samples, the values prior to application ranged from 53 to 252 mg kg" 1 and after application from 67 to 344 mg kg" 1 . There were significant differences between fertilizer treatments (pcO.001) and sampling times (p<0.001) in the HNO3-HCI-H2SO4-P of surface 0-5 cm peat samples, and the interaction between fertilizer treatment and sampling time was also significant (p<0.001). However, in the 5-10 cm peat samples there was no difference in HNO3- HCI-Hr-SOj-P among fertilizer treatments (p=0.431) and the differences between the samples collected prior to the fertilizer application and after application were insignificant. These result indicate that there was negligible movement of fertilizer residues below 5 cm depth. The dissolution of different P-fertilizers was therefore calculated using only the values obtained from the surface (0-5 cm) peat samples. The differences in NaCl/TEA-P between the fertilizer treatments (p=0.024) and sampling times (pcO.001) in the surface 0-5 cm peat samples were statistically significant. There was also a significant interaction between the fertilizer treatment and sampling time (p<0.001). The concentrations of NaCl/TEA-P were higher for the plots fertilized with superphosphate and Mire-PK at the first sampling time after fertilization (Fig. 1). No substantial differences between the different fertilizer treatments were found at the later sampling times. The concentrations of NaOH-P in the surface peat samples at the first sampling occassion after fertilizer application were also higher for the plots fertilized with superphosphate and Mire-PK than for the other fertilizers (Fig. 2). The differences between fertilizer treatments (p=0.002) and 13 Mika Nieminen and Maija Jarva sampling times (p<0.001) were statistically significant, as was also the interaction between fertilizer treatment and sampling time (p<0.001). The rate of dissolution of superphosphate was particularly high during the first few months after fertilizer application compared with the other fertilizers (Fig. 3). Some of the added Mire-PK also dissolved immediately after fertilization, whereas the dissolution of Moroccan phosphate rock and Siilinjärvi apatite was only significant about three months after fertilizer application. About 93 % of the added superphosphate had dissolved after two years. For the other fertilizer treatments, the corresponding values were: Forest-PK and Moroccan phosphate rock 44 %, Mire-PK 58 %, and Siilinjärvi apatite 56 %. DISCUSSION Since the recovery of phosphate rock residue has been shown to be incomplete in acid soils using HCI (Tambunan et al. 1993) and Siilinjärvi apatite was not completely dissolved in H:S04 (Yli-Halla & Lumme 1987), neither of these acids was judged suitable for extracting the fertilizer-P residue in our study. We therefore used a HN03 - HCI-H:SO4 mixture to digest the fertilizer-P residue. Near complete (94-96 %) recovery of added fertilizer-P was achieved in the HNO3-HCI-H2SO4 digestion, and given the uncertainties involved in P fractionation, the method was considered reliable. In practical peatland forest fertilization, the dissolution of fertilizer-P may be somewhat higher than that suggested by the results presented here however. This is because the application dose in the experiment (200 kg P ha" 1 ) was about five times higher than that used in practical peatland forestry (35-45 kg P ha' 1 ), and the Mika Nieminen and Maija Jarva 14 proportion of phosphate rock that dissolves has been shown to decrease with increasing amounts of phosphate rock application (Rajan et ai. 1991). In the dissolution experiments performed under field conditions, high additions of P fertilizer are usually used. This is because small additions make the P fractionation method inaccurate for soils with high levels of native P. The rates and extents of dissolution of the fertilizers were related to their reactivity as measured using citric and formic acids (Table 2), except in the case of Siilinjärvi apatite. The unexpected high dissolution of Siilinjärvi apatite may be due to the small particle size of the fertilizer (Khasawneh & Doll 1978). In studies of fertilization and tree growth (Karsisto 1977, Kaunisto et al. 1993), the effectiveness of Siilinjärvi apatite has usually been less than that of Moroccan phosphate rock during the first few years after fertilization however. These results imply that Siilinjärvi apatite would dissolve more slowly than Moroccan phosphate rock. Earlier studies have indicated rather small increases in tree growth during the first 1-2 years after the application of Moroccan phosphate rock and Siilinjärvi apatite (Karsisto 1977, Kaunisto et al. 1993), and also rather small changes in the leaching of P to water courses during the first few months after the application of Moroccan phosphate rock and Forest-PK (Karsisto 1970, Karsisto & Ravela 1971, Almberger & Salomonsson 1979, Ahti 1983, Jarva et al. 1995). Applications of superphosphate have, in contrast, been shown to increase tree production significantly during the first year after application (Karsisto 1977), and fertilization with Mire-PK resulted in the leaching of P to water courses immediately after application (Nieminen & Ahti 1993). 15 Mika Nieminen and Maija Jarva These results can be explained by the differences in the rates of dissolution of the fertilizers as shown in this study. It is therefore concluded that information about the dissolution rates of P-fertilizers of different solubility is very important in understanding the behaviour of fertilizer-P on drained peatlands. 16 Mika Nieminen and Maija Jarva ACKNOWLEDGEMENTS We are grateful to Heikki Takamaa for helping in establishing the experiment and collecting the samples and to Tuija Hytönen for conducting the laboratory work. Raija Linnainmaa and Hilkka Granlund are acknowleged for helping in preparing the manuscript. We also wish to thank Professor Seppo Kaunisto and Dr. Erkki Ahti for reading the manuscript and giving valuable advice. The English language was revised by Dr. Michael Starr, who also gave valuable advice concerning other aspects of the manuscript. The study was financially supported by Kemira Ltd. Mika Nieminen and Maija Jarva 17 REFERENCES Ahti, E. 1983. Fertilizer-induced leaching of phosphorus and potassium from peatlands drained for forestry. Comm. Inst. For. Fenn. Ill: 1-20. Almberger, P. & Salomonsson, L-A. 1979. Domänverkets gödlingsförsök pä torvmarker. Mätningar av fosforutlakning efter gödsling med räfosfat. Sveriges Skogsvärdsförbunds Tidskrift 5-6: 1-7. (In Swedish.) Anon. 1990. Kokonaisfosforin määrittäminen apatiitista ja apatiittia sisältävistä seosjauheista autoanalysaattorilla. Kemira Agro Oy, Espoo, 2 pp. (In Finnish.) Bolan, N. S. & Hedley, M. J. 1989. Dissolution of phosphate rocks in soils. 1. Evaluation of extraction methods for the measurement of phopshate rock dissolution. Fert. Res. 19: 65-75. Chien, S. H. 1993. Solubility assessment for fertilizer containing phosphate rock. Fert. Res. 35: 93-99. Halonen, 0., Tulkki, H. & Derome, J. 1983. Nutrient analysis methods. Metsäntutkimuslaitoksen tiedonantoja 121: 1-28. Hedley, M. J., Stewart, J. W. B. & Chauhan, B. S. 1982. Changes in inorganic and organic soil phopshorus fractions induced by cultivation practises and laboratory incubations. Soil Sci. Soc. Am. J. 46: 970-976. Heikurainen, L. & Pakarinen, P. 1982. Peatland classification. In Laine, J. (ed.). Peatlands and their utilization in Finland. Finnish Peatland Society, Finnish National Committee of the International Peat Society, pp. 53-62. ISBN 951-99402-9-4. Mika Nieminen and Maija Jarva 18 Hughes, J. C. & Gilkes, R. J. 1984. The effect of chemical extractant on the estimation of rock phosphate fertilizer dissolution. Aust. J. Soil. Res. 22: 475-481. Hughes, J. C. & Gilkes, R. J. 1994. The dissolution of North Carolina phosphate rock in some south-western Australian soils. Fert. Res. 38: 249-253. Jarva, M., Kaunisto, S., Nieminen, M., Sallantaus, T. & Saura, M. 1995. Metsänlannoitteen huuhtoutuminen Liesinevan sarkaleveyskoekentältä - alustavia tuloksia. In Saukkonen, S & Kenttämies, K. (eds.). Metsätalouden vesistövaikutukset ja niiden torjunta. METVE-projektin loppuraportti, pp. 121-128. Suomen ympäristö 2 - ympäristönsuojelu. Helsinki. (In Finnish.) Karsisto, K. 1968. Using various phosphatic fertilizers in peatland forests. Suo 21: 60- 66. (In Finnish with English summary.) Karsisto, K. 1970. On the washing of fertilizers from peaty soils. Suo 21: 60-66. (In Finnish with English summary.) Karsisto, K. 1977. Possibilities of native phosphate concentrates in fertilizing peatland forests. Suo 28: 43-46. (In Finnish with English summary.) Karsisto, K. & Ravela, H. 1971. Washing away of phosphorus and potassium from areas drained for forestry and topdressed at different times of the year. Suo 22: 39-46. (In Finnish with English summary.) Kaunisto, S., Moilanen, M. & Issakainen, J. 1993. Apatite and phlogopite as phosphorus and potassium fertilizers in peatland pine forests. Folia For. 810: 1-30. (In Finnish with English summary.) Khasawneh, F. E. & Doll, E. C. 1978. The use of phosphate rock for direct application to soils. Adv. Agron. 30: 159-206. Mika Nieminen and Maija Jarva 19 Mackay A. D., Syers, J. K., Tillman, R. W. & Gregg, P. E. H. 1986. A simple model to describe the dissolution of phosphate rock in soils. Soil Sci. Soc. Am. J. 50: 291- 296. Mahimairaja, S., Bolan, N. S. & Hedley, M. J. 1995. Dissolution of phosphate rock during the composting of poultry manure: an incubation experiment. Fert. Res. 40: 93- 104. Moilanen, M. 1992. Suopuustojen ravinnetila Pohjois-Suomen vanhoilla ojitusalueilla. In Valtanen, J., Murtovaara, I. & Moilanen, M. (eds„). Metsäntutkimuspäivä Taivalkoskella 1991. Metsäntutkimuslaitoksen tiedonantoja 419, pp. 58-65. (In Finnish.) Nieminen, M. & Ahti, E. 1993. Leaching of nutrients from an ombrotrophic peatland area after fertilizer application on snow. Folia For. 814: 1-22. (In Finnish with English summary.) Nieminen, M. & Jarva, M. 1996. Phosphorus adsorption by peat from drained mires in southern Finland. Scand. J. For. Res. 11: 321-326. Penttilä, T. & Moilanen, M. 1987. Fosforilannoitteet suometsien lannoituksessa Pohjois-Suomessa. In Saarenmaa, H. & Poikajärvi, H. (eds.). Korkeiden maiden uudistaminen. Ajankohtaista tutkimuksesta. Metsäntutkimuspäivät Rovaniemellä 1987. Metsäntutkimuslaitoksen tiedonantoja 278: 136-148. (In Finnish.) Rajan, S. S. S. 1983. Effect of sulphur content of phosphate rock/sulphur granules on the availability of phosphate to plants. Fert. Res. 4: 287-296. Rajan, S. S. S. 1987. Partially acidulated phosphate rock as fertilizer and dissolution in soil of the residual rock phosphate. New Zealand Journal of Experimental Agriculture 15: 177-184. Mika Nieminen and Maija Jarva 20 Rajan, S. S. S., Fox, R. L. & Saunders, W. M. H. 1991. Influence of pH, time and rate of application on phosphate rock dissolution and availability to pastures. 11. Soil chemical studies. Fert. Res. 28: 95-101. Rajan, S. S. S. & Watkinson, J. H. 1992. Unacidulated and partially acidulated phosphate rocks: Agronomic effectiveness and the rates of dissolution of phosphate rocks. Fert. Res. 33: 267-277. Rannikko, M. & Hartikainen, H. 1980. Retention of applied phosphorus in Sphagnum peat. Proceedings of the 6th International Peat Congress, Duluth, Minnesota, August 17-23, 1980, pp. 666-669. Silfverberg, K. & Hartman, M. 1998. Long-term effects of different phosphorus fertilizers in Finnish pine mires. In Sopo, R. (eds.). Proceedings of the International Peat Symposium, The Spirit of Peatlands, Jyväskylä, Finland 7-9 September, 1998, pp. 73-75. Tambunan, D., Hedley, M. J., Bolan, N. S. & Turner, M. A. 1993. A comparison of sequential extraction procedures for measuring phosphate rock reidues in soils. Fert. Res. 35: 183-191. Tomppo, E. 1998. Peatland forests of Finland in 1951-1994. In Sopo, R. (eds.). Proceedings of the International Peat Symposium, The Spirit of Peatlands, Jyväskylä, Finland 7-9 September, 1998, pp. 84-86. Veijalainen, H. 1992. Nutritional diagnosis of peatland forests by needle analysis in winter 1987-88. Metsäntutkimuslaitoksen tiedonantoja 408: 1-28. (In Finnish with English summary.) Yli-Halla, M. & Lumme, I. 1987. Behaviour of certain phosphorus and potassium compounds in a sedge peat soil. Silva Fenn. 21: 251-257. Table 1. Some chemical properties of the surface peat layer (0-15 cm) at the study site. Methods used: et. ai (1983) 2> Soil: water (1 g:25 ml) 3> Dry digestion in HCI (Halonen et. ai 1983); ICP/AES 4) Nieminen & Jarva (1996) Ash content 0 , % 1.39 P H 2) 3.5 P tot 3) , mg kg" 1 524 K, ot 3) , mg kg' 1 388 Ca10t 3) , mg kg" 1 1466 Mg10t 3) , mg kg" 1 362 Al lot 3) , mg kg" 1 451 Fe l0i 3) , mg kg" 1 805 P adsorption capacity 4 ', kg ha' 1 3.0 Table 2. Some chemical characteristics of the fertilizers used in the experiment Methods used: "According to Anon. (1990). 21 Analyzed by boiling (10 min) in deionized water using an initial fertilizer: solution ratio of 1:25. 3) According to Chien (1993). Fertilizer Total P", % of Total P % Water- soluble 2' 2 % acid soluble j) 2 % Formic acid soluble J> Superphosphate 9.2 79.0 79.1 79.5 Mire-PK 9.1 15.8 46.0 40.6 Forest-PK 9.1 <0.1 20.7 14.3 Moroccan phosphate rock 14.8 <0.1 15.7 10.5 Siilinjärvi apatite 16.0 <0.1 6.6 9.5 Table 3. Particle size distribution (weight fraction, %) of the fertilizers used in the experiment. Fertilizer Size range (mm) < 0.045 0.045- 0.14 0.14- 0.4 0.4- 2.0 >2.0 Superphosphate 0 0 0 4 96 Mire-PK 0 0 1 21 78 Forest-PK 0 0 1 3 96 Moroccan phosphate rock 6 59 28 7 0 Siilinjärvi apatite 32 52 14 2 0 Table 4. Fertilizer-P recovered by sequential fractionation following an immediate addition of Siilinjärvi apatite (n=l3) and Moroccan phosphate rock (n=ls) to Sphagnum peat at a fertilizer: soil ratio of 1:20. !) SD = Standard deviation Fertilizer Extract Fertilizer-P recovery (%) ± SD 1 ' Siilinjärvi NaCI / TEA 0.5710.06 apatite NaOH 0.32 ±0.38 HNO3-HCI-H2SO4 96.00 ±5.03 Moroccan NaCI/TEA 0.50 ±0.27 phosphate rock NaOH 0.65 ±0.36 HNO3-HCI- H2SO4 93.74 ±4.18 21 Mika Nieminen and Maija Jarva Figure legends Fig. 1. Changes in NaCl/TEA extractable P with time in the surface 0-5 cm peat samples. Values are standard deviations. Fig. 2. Changes in NaOH extractable P with time in the surface 0-5 cm peat samples. Values are standard deviations. Fig. 3. Rates of dissolution of the studied fertilizers. Fitted lines are the regression equations as shown in the figure. Fig. 1. Fig. 2. Fig. 3. III Silva Fennica 31(4) research articles 391 Dry Mass and the Amounts of Nutrients in Understorey Vegetation before and after Fertilization on a Drained Pine Bog Leena Finer and Mika Nieminen Finer, L. & Nieminen, M. 1997. Dry mass and the amounts of nutrients in understorey vegetation before and after fertilization on a drained pine bog. Silva Fennica 31(4): 391 400. Dry mass and nutrient (N, P, K, Ca, Mg, B) contents of field layer vegetation and a combination of bottom layer vegetation and litter (referred to as bottom/litter layer in the text) were studied one year before and three years after fertilization (NPK and PK) on a drained low-shrub pine bog in eastern Finland. The results of an earlier study on the tree layer were combined with those of this study in order to estimate the changes caused by fertilization in the total plant biomass and litter. Before fertilization the average dry mass of the field and bottom/litter layers was 8400 kg ha -1 and 7650 kg ha -1 , respectively. The above-ground parts accounted for 25 % of the total field layer biomass. The dry mass of the field and bottom/litter layers together was < 20 % of the dry mass accumu lated in the total plant biomass and litter. The corresponding figures for N, P, K, Ca, Mg and B were 44 %, 38 %, 30 %, 38 %, 31 % and 17 %, respectively. Fertilization did not significantly affect the dry mass of either the field layer vegetation or the bottom/litter layer. 33 % of the applied P was accumulated in the total plant biomass and litter on the PK-fertilized plots, and 25 % on the NPK-fertilized plots. For the other elements, the proportions on the PK-fertilized plots were K31%,Ca6%, Mg 11 % and 813%. On the NPK-fertilized plots, the corresponding figures were N 62 %, K 32 %, Ca 6 %, Mg 9 % and B 13 %. Except for B and K, the accumulation of fertilizer nutrients in the understorey vegetation and litter was of the same magnitude or greater than the uptake by the tree layer. Keywords biomass, litter, peatland, root systems, Scots pine Authors' addresses Finer, The Finnish Forest Research Institute, Joensuu Research Sta tion, P.O. Box 68, FIN-80101 Joensuu, Finland; Nieminen, The Finnish Forest Research Institute, Vantaa Research Centre, P.O. Box 18, FIN-01301 Vantaa, Finland Fax to Finer +358 13 251 4111 E-mail leena.finer@metla.fi Received 26 September 1996 Accepted 29 October 1997 research articles Silva Fennica 31(4) 392 1 Introduction The understorey vegetation and litter layer ac count for a considerable proportion of the total nutrient storage in forested ombrotrophic peat lands (Haveraaen 1967, Paavilainen 1980, Braekke 1988). Consequently, the processes which bind and/or release nutrients in these lay ers contribute significantly to the overall nutri ent cycling in such ecosystems (Paavilainen 1980, Braekke 1988). However, the proportion of nu trients bound by the understorey vegetation and litter layer varies according to the structure and density of the tree layer. When the tree layer reach a certain density, the shrub, field and moss layers are shaded out and the litter layer builds up (Reinikainen et ai. 1984, Braekke 1988). Thus, nutrient cycling becomes gradually more and more dominated by the tree layer growing on the site. Scots pine stands usually respond to fertiliza tion on drained ombrotrophic bogs (e.g. Meshe chok 1968, Braekke 1979, 1983, Paavilainen 1979). The composition of the understorey vege tation is also affected, i.e. some species suffer while others are better able to compete and ex pand due to the change in nutrient availability (Heikurainen and Laine 1976, Finer and Braekke 1991). Increased total biomasses and nutrient concentrations of the above-ground parts of field layer species have been reported (Haveraaen 1967, Päivänen 1970, Vasander 1982), while the biomass of the moss layer decreases during the first years after fertilization (Vasander 1982, Jäp pinen and Hotanen 1990, Vasander et al. 1993). Several studies have been carried out on the dry mass and the amounts of nutrients in differ ent vegetation layers on drained and fertilized peatlands (e.g. Haaveraen 1967, Paavilainen 1980, Braekke 1988, Finer 1991 a). However, only one study was found about the accumulation of fertilizer nutrients in the total plant biomass (in cluding tree, field and bottom layers and root systems) (Vasander 1981). These results are im portant for achieving efficient and economical use of fertilizers, and for assessing the risk of nutrients leaching into water bodies. Informa tion about the rates of accumulation can also be used for modelling purposes. The aim of the study was to estimate the ef- fects of PK and NPK fertilization on the dry mass and nutrient accumulation in 1) field layer vegetation, 2) bottom/litter layer, and 3) total plant biomass and litter on a drained ombro trophic pine bog in eastern Finland. In order to determine the effects of fertilization on the total plant biomass and litter, the results of an earlier study (Finer 1991 a) from the same site were used as an estimate of the fertilizer-induced changes in tree layer. 2 Material and Methods 2.1 Site Description and Experimental Design The study site was located in eastern Finland (62°14'N; 29°50'E, 81 m a.5.1.). The peat layer was > 1 m thick and consisted of Sphagnum peat with remnants of wood and Carex species below the 20 cm surface peat layer. The chemical prop erties of the peat have been presented by Braekke and Finer (1991). A naturally regenerated Scots pine (Pinus sylvestris L.) stand, with an average volume of 81 m 3 ha^ 1 in 1984, was growing on the site. According to the peatland site type clas sification used in Finland (Heikurainen and Pa karinen 1982), the site was a low-shrub pine bog. A detailed description of the vegetation is presented by Finer and Braekke (1991). The site was drained in 1967 with a 50 m ditch spacing. The climatic data and fluctuation in the water table during 1984-1987 were reported by Finer (1991b). A 3 x 3 Latin square design with a 1500 m : plot size was used. The treatments were: unferti lized (0), fertilized with PK(MgB), and with NPK(MgB). The amounts of the elements (kg ha" 1 ) applied were: N 150, Ca 135, P 53, K 100, Cl 95, Mg 25, S 28 and B 2.4. The fertilizers were applied in spring 1985 as ammonium ni trate, Moroccan phosphate rock, potassium chlo ride, magnesium sulphate, and sodium borate. 2.2 Field Work Sampling was carried out twice: prior to fertili Finer and Nieminen Dry Mass and the Amounts of Nutrients in Understorey Vegetation ... 393 zation during 12.-27.9.1984 and after fertiliza tion during 18.8.-3.9.1987. The above-ground parts of the understorey vegetation were harvest ed from 20 systematically located quadrates (0.25 m 2) on each plot. The understorey vegeta tion was divided into bush, field and bottom/ litter layers. Zero level for sampling of all layers was taken as the lower level of the living moss layer. The bush layer consisted of trees (Pinus sylvestris L., Picea abies (L.) Karsten, Betula pitbescens Ehrh.) with height > 0.5 m and breast height diameter < 2.5 cm. However, the bush layer was very sparse and even missing from several of the plots. It was therefore excluded from the study. The sampled field layer consist ed of the above-ground parts of dwarf shrubs, sedge-like species, juvenile trees (height < 0.5 m) and herbs. The coverage of the field layer was 42 % prior to fertilization in 1984 (Finer and Braekke 1991). The bog dwarf shrubs (Betula nana L., Ledum palustre L., Calluna vulgaris L. Hull, Chamaedaphne calyculata L. Moench, Vac cinium uliginosum L.) and the forest dwarf shrubs ( Vaccinium myrtillus L„ Vaccinium vitis-idaea L.) both had a coverage of 19 %. Eriophorum vaginatum L. and Carex globularis L., the only sedge-like species, had a total coverage of 3 % in the field layer. The total coverage of the field layer was not affected by fertilization (Finer and Braekke 1991). Dryopteris spp. and Epilobium spp. became established after fertilization, but occurred only sporadically. The bottom layer consisted of bryophytes and lichens. Their total coverage before fertilization was 90 %, of which the coverage of Sphagnum species (Sphagnum angustifolium (Russow) C. Jens, Sphagnum russowii Warnst., Sphagnum nemoreum Scop.) was 31 % and that of Pleuro zium schreberi (Brid.) Mitt. 52 % (Finer and Brsekke 1991). Fertilization did not affect the total coverage of the bottom layer. The Sphag num species suffered from fertilization and were partly replaced by Pleurozium schreberi. A de tailed description of the vegetation composition in the field and bottom layers before and after fertilization is presented by Finer and Braekke (1991). In the early stages of the field work it was evident that the accurate separation of the living bottom layer vegetation from the litter accumu lated on and between living bryophytes was tech nically very difficult. Thus, a composite sample including both the living bottom layer vegeta tion and all the litter (tree litter, field layer vege tation litter and bottom layer vegetation litter) that had accumulated in it was collected. After collecting the above-ground parts of the understorey vegetation and the litter, a square peat core (24.7 cm 2 ) was taken down to a depth of 40 cm in the middle of each vegetation har vesting quadrate. The living roots, rhizomes and buried stems (diameter < 10 mm) of the field layer vegetation were extracted by hand from each subsample. The changes in the biomass of the field layer roots after fertilization have al ready been presented by Finer (1991b). The changes in the dry mass and nutrient con tents of the tree layer were presented by Finer (1991 a). These results were combined with our data to calculate the changes in the total plant biomass and litter. 2.3 Laboratory Analyses and Calculations All the samples were dried to constant mass at 60 °C. Subsamples were taken for dry mass de termination at 105 °C. The samples were ho mogenized in a stainless steel mill (sieve mesh diameter 2 mm). Total N was determined by the Kjeldahl method, K, Ca and Mg by atomic ab sorption spectrophotometry after HCI digestion, P spectrophotometrically by the molybdate-hy drazine method, and B by the azomethine meth od (Halonen et ai. 1983). The change from 1984 to 1987 (i.e. the differ ence between three years after fertilization and prior to fertilization) was chosen as the parame ter for determining the effect of fertilization on dry mass and nutrient accumulation in different vegetation compartments. Analysis of variance and Tukey's test were used to test whether the changes in dry mass and the amounts of nutri ents from 1984 to 1987 differed between the treatments (0, PK, NPK). The statistical tests were done using the BMDP (1990) software pack age. research articles Silva Fennica 31(4) 394 3 Results 3.1 Understorey Vegetation and Litter Before fertilization the average above-ground dry mass of the field layer was 2100 kg haand that of the bottom/litter layer 7650 kg ha -1 (Tables 1 and 2). Neither the dry mass of the above-ground field layer nor the bottom/litter layer was affect ed by fertilization (Fig. 1). Before fertilization the average amount of N in the above-ground field layer, field layer roots and bottom/litter layer was 16.8 kg ha -1 , 42.8 kg ha 1 and 78.2 kg ha respectively (Tables 1, 2 and 3). Fertilization did not significantly affect the N contents of different layers (Fig. 2). Most of the understorey vegetation P was found in the bottom/litter layer, i.e. 6.4 kg ha -1 . The corre sponding figure for the above-ground field layer was 1.6 kg ha -1 , and that for the field layer roots 2.6 kg ha -1 . Fertilization increased the amounts of P only in the bottom/litter layer on the PK fertilized plots. The average amount of K prior to fertilization in the above-ground field layer, field layer root systems and bottom/litter layer was 5.0 kg ha-1 , 4.1 kg ha -1 and 16.0 kg ha-1, respectively. Fertilization did not increase the amounts of K in different layers. The average amount of Ca in the above-ground field layer was 9.8 kg ha -1 before fertilization. The corresponding figure for the field layer root sys tems was 9.2 kg ha -1 , and for the bottom/litter lay er, 35.7 kg ha-1 . Fertilization increased the amounts of Ca only in the bottom/litter layer on the PK Fig. 1. Changes from 1984 to 1987 in the amount of dry mass stored in the above-ground field layer vegetation and the bottom/litter layer. The chang es do not differ statistically significantly between treatments. Standard deviations are indicated by lines in the bars. fertilized plots. The average amount of Mg in the above-ground field layer, field layer root systems and bottom/litter layer was 2.2 kg ha-1 , 2.2 kg ha-1 and 5.7 kg ha~', respectively. There were no significant changes in the amounts of Mg after fer tilization. The average amount of B in the above ground field layer prior to fertilization was 0.021 kg ha -1 . The corresponding figure for the field lay er root systems was 0.032 kg ha" 1 , and for the bottom/litter layer 0.042 kg ha~' . The bottom/litter layer B contents increased significantly after fer tilization both on the PK- and NPK-fertilized plots. Table 1. Dry mass and the amounts of nutrients stored in the above ground field layer vegetation in the control (= 0) plots and the PK and NPK fertilized plots prior to fertilization in 1984. Standard deviations in parentheses. 0 PK NPK Dry mass, kg ha" 1 2291 (149) 2140 (222) 1874 (41) N, kg ha-1 18.2 (0.8) 16.8 (1.4) 15.3 (0.9) P, kg ha~' 1.7 (0.1) 1.5 (0.3) 1.5 (0.0) K, kg ha-1 5.1 (0.3) 5.0 (0.9) 4.8 (0.4) Ca, kg ha -1 10.3 (1.1) 9.4 (1.3) 9.8 (0.9) Mg, kg ha-1 2.3 (0.1) 2.1 (0.3) 2.2 (0.2) B, g ha -1 23.2 (1.1) 20.7 (4.4) 20.1 (4.1) Dry Mass and the Amounts of Nutrients in Understorey Vegetation ... Finer and Nieminen 395 Toble 2. Dry mass and the amounts of nutrients stored in the bottom/ litter layer in the control (= 0) plots and PK and NPK fertilized plots prior to fertilization in 1984. Standard deviations in parentheses. Table 3. Amounts of nutrients stored in the field layer roots in the control (= 0) plots and PK and NPK fertilized plots prior to fertilization in 1984. Standard deviations in parentheses. 3.2 Total Plant Biomass The field and the bottom/litter layers together accounted for < 20 % of the dry mass accumulat ed in the total plant biomass and litter (Fig. 3). The proportion of B (17 %) in the field and bottom/litter layers was also small, whereas the proportions of the other nutrients were greater. More than 40 % of N, 38 % of P, 30 % K, 38 % of Ca and 31 %of Mg was found in the field and bottom/litter layers. Neither the dry mass of the tree layer nor the combination of the field and bottom/litter layers was significantly affected by fertilization (Table 4). Nitrogen accumulation in total plant biomass and litter increased by 46.2 kg ha -1 on the PK fertilized plots and by 93.7 kg ha 1 on the NPK fertilized plots. This increase was almost evenly distributed between the tree biomass and under storey vegetation and litter. The total fertilizer induced increase in the amount of P was 17.3 kg ha 1 on the PK-fertilized plots and 13.3 kg ha -1 on the NPK-fertilized plots. Most of this in crease (65 % on the PK fertilized plots and 59 % on the NPK fertilized plots) had occurred in the field and bottom/litter layers. The amount of K in the total plant biomass and litter increased by 32 kg ha 1 on both the PK- and the NPK-ferti lized plots after fertilization. A high proportion of the K (87 % on the PK-fertilized plots and 79 % on the NPK fertilized plots) had accumulated in the tree layer. Fertilization decreased the amount of Ca in the tree layer. However, the amounts of Ca in the field and bottom/litter layers increased signifi cantly and the change in the total Ca accumula tion was thus positive on both the PK- (7.9 kg ha-1 ) and NPK-fertilized (8.7 kg ha ') plots. Fer 0 PK NPK Dry mass, kg ha -1 7934 (700) 7805 (813) 7213 (1252) N, kg ha -1 81.8 (9.5) 79.3 (9.1) 73.6 (13.2) P, kg ha-1 6.8 (0.3) 6.3 (0.4) 6.1 (0.9) K, kg ha -1 16.4 (0.9) 16.2 (1.9) 15.5 (3.0) Ca, kg ha -1 38.9 (2.2) 34.0 (2.7) 34.1 (2.0) Mg, kg ha-1 6.2 (0.6) 5.4 (0.5) 5.4 (0.5) B, g ha -1 47.8 (5.7) 36.9 (1.2) 41.5 (5.7) 0 PK NPK N, kg ha -1 48.6 (6.4) 34.3 (4.6) 45.5 (3.2) P, kg ha-1 2.7 (0.2) 2.2 (0.4) 2.9 (0.1) K, kg ha - ' 4.6 (0.2) 3.4 (0.7) 4.3 (0.1) Ca, kg ha-1 9.7 (2.2) 7.2 (1.3) 10.8 (0.3) Mg, kg ha -1 2.3 (0.5) 1.7 (0.6) 2.5 (0.4) B, g ha -1 32.0 (7.1) 27.1 (3.3) 36.3 (8.2) research articles Silva Fennica 31(4) 396 Fig. 2. Changes from 1984 to 1987 in the amounts of nutrients stored in the above-ground field layer vegetation, field layer roots and the bottom/litter layer. Standard deviations are indicat ed by lines in the bars. Values marked by the same letter differ statistically significantly (p < 0.05) from each other. tilization did not significantly affect the amounts of Mg in either the tree layer or the field and bottom/litter layers. More B was accumulated in the tree layer than in the understorey vegetation and litter. More than 70 % of the total fertilized induced accumulation of B (0.30 kg ha-1 on the PK-fertilized plots, 0.32 kg ha -1 on the NPK fertilized plots) occurred in the tree layer. The total increase in the stores of P caused by fertilization (the change in the control plots is Dry Mass and the Amounts of Nutrients in Understorey Vegetation ... Finer and Nieminen 397 Fig. 3. Average percentual distribution of dry mass and nutrients in the tree, field and bottom/litter layers prior to fertilization in 1984. accounted for as in Table 4) was 33 % of the applied P on the PK-fertilized plots. For the oth er elements, the percentages on the PK-fertilized plots were K 31, Ca 6, and B 13. On the NPK fertilized plots, the corresponding figures were N 62 %, P 25 %, K 32 %, Ca 6 %, and B 13 %. 4 Discussion 4.1 Dry Mass The dry mass of the above-ground field layer (2100 kg ha" 1 ) was within the range reported for drained mires in Fennoscandia (Tamm 1954, Paavilainen 1980, Vasander 1987, Braskke 1988, Laiho 1996). The average root biomass of the field layer prior to fertilization was 6300 kg ha-1 on our study site (Finer 1991b). This was greater than that reported by Paavilainen (1980) for a drained low-shrub pine bog and by Laiho and Finer (1996) for tall-sedge pine fens in Finland, but almost similar to that presented by Häland and Brakke (1989) for a pristine bog in Norway. Only 25 % of the total field layer biomass was in the above-ground parts. Our results are thus in Table 4. Fertilizer-induced change in dry mass and nutrient content (kg ha~" of the combined field layer vegetation and bottom/litter layer and tree layer during the study period (from 1984 to 1987). Fertilizer-induced changes have been calculated by either adding the decrease on the control plots from 1984 to 1987 to the change on the fertilized plots or subtracting the increase on the control plots from the change on the fertilized plots. Ferti lizer-induced changes in the field and bottom/ litter layers are based on this study and those in the tree layer on Finer' s (1991 b) study at the same site. The tree layer includes the following com partments: cones, needles, living branches, dead branches, stembark, stemwood, stump and coarse roots, and small and fine (0 10 mm) roots. NS indicates that the change from 1984 to 1987 does not differ statistically significantly (p > 0.05) be tween treatments (0, PK, NPK) according to the analysis of variance. accordance with previous ones showing that most of the dry mass of the field layer vegetation is located in the root systems on drained bogs (Paavilainen 1980, Wallen 1986). The dry mass of the bottom/litter layer was Trees Field and bottom/litter layers Dry mass PK +711 +1923 NPK + 1377 +1859 NS NS N PK +22.7 +23.5 NPK +51.9 +41.8 P PK +6.0 +11.3 NPK +5.4 +7.9 K PK +27.3 +4.1 NPK +25.1 +6.7 NS Ca PK -18.9 +26.8 NPK -9.7 + 18.4 Mg PK +0.82 + 1.83 NPK +0.86 +1.32 NS NS B PK +0.22 +0.08 NPK +0.23 +0.09 Silva Fennica 31(4) research articles 398 only half of that on an old drained oligotrophic pine bog studied by Braekke (1988) in southern Norway. However, Jäppinen and Hotanen (1990) reported a lower biomass for the bottom layer vegetation on a drained herbrich pine mire and spruce mire, as well as Vasander (1982) for a drained ombrotrophic bog and Laiho (1996) for drained tall-sedge pine fens. These differences probably result from differences in sampling. On Braekke' s (1988) site the living mosses had al most disappeared due to shading by the trees, and mainly tree litter that had accumulated on the soil surface was sampled. Jäppinen and Ho tanen (1990), Vasander (1982) and Laiho (1996) sampled only living mosses, not litter. The dry mass of the above-ground field layer did not respond to fertilization. These results are contradictory to those from poorly stocked raised bogs, where NPK-fertilization increased the above-ground dry mass of the field layer (Vas ander 1982, Vasander etal. 1993). Tamm (1954) and Päivänen (1970) also reported a positive field layer response to fertilization on open mires. This positive response of field layer vegetation to fertilization is, however, probably a short term phenomenon, since the field layer species are eventually shaded out due to the increased growth and consequent shading of the tree layer (Braekke 1988, Laine et al. 1995). The root biomass of the field layer decreased from 1984 to 1987 in all treatments (Finer 1991b). Finer (1991b) attributed this to the exceptionally cold winter in 1986-1987, which had probably damaged the living roots. Compared with the control plots, the decrease in root biomass was lower on the fertilized plots. This was interpret ed to indicate a positive effect of fertilization on the root biomass (Finer 1991b). The dry mass of the bottom/litter layer was not affected by fertilization. This may at first seem to be contradictory to the observations indicat ing that bryophytes suffer from direct contact with high doses of nutrients, especially those comprising readily soluble nitrogen and potassi um (Jäppinen and Hotanen 1990, Finer and Braekke 1991, Dirkse and Martakis 1992, Vas ander et al. 1993). However, Pleurozium schre beri was the most common bryophyte on our site and it is known that fertilization has a more destructive effect on peat mosses than on forest mosses (Heikurainen and Laine 1976, Jäppinen and Hotanen 1990, Finer and Brack ke 1991, Vas ander et al. 1993). Because the bottom layer vegetation and litter were included in the same sample, the dry mass of the bottom layer vegeta tion may have decreased as in the previously mentioned studies, if the dry mass of the litter had increased. However, this was probably not the case, since the dry mass of the tree litterfall was not affected by fertilization on our study site (Finer 1991 a). 4.2 Nutrient Content The amounts of nutrients in the above- and be low-ground parts of the field layer vegetation were inside the range reported for ombrotrophic pine bogs by Paavilainen (1980), Vasander (1981), Häland and Braekke (1989) and Braekke and Häland (1990). However, they were sub stantially higher than those presented by Braekke (1988) for a drained and fertilized oligotrophic pine bog, but of the same magnitude or lower than those reported by Braekke for only a drained bog. The distribution of nutrients between the root systems and the above-ground parts of the field layer differed from that of the biomass. While almost 70 % of the total field layer dry mass was in the root systems, only N was clearly more abundant in the roots than in the above ground parts. These results indicate that the be low-ground parts of field layer species accumu late relatively more carbon than nutrients. The amounts of nutrients in the bottom/litter layer were generally lower in our study than those reported by Braekke (1988). Päivänen (1970) studied the changes in the nutrient contents of the above-ground parts of the field layer during the first three years after fertilization of an open low-sedge bog. He found that 36 kg ha~' more N (36 % of applied N), 3.1 kg ha -1 more P (7 % of applied P) and 16 kg ha 1 more K (19 % of applied K) were fixed in the field layer on the fertilized plots. Haveraaen (1967) studied nutrient amounts in the above ground field layer biomass of an afforested so ligenous mire four years after fertilization. He found that fixation by the field layer accounted for 13 % of the applied 145 kg K ha 1 and 13 % Dry Mass and the Amounts of Nutrients in Understorey Vegetation ... Finer and Nieminen 399 of the applied 50 kg P ha -1 . The corresponding percentages for 72.5 kg K ha -1 and 25 kg P ha -1 were 11 and 14. These figures are high com pared to those found in this study. Because of the dense tree layer, light was probably more of a limiting growth factor for the field layer species than the deficiency of nutrients on our site. According to Finer (1991 a), the amounts of nutrients in needle or other tree litter were not significantly influenced by fertilization on our study site. Thus, the fertilizer-induced changes in the nutrient contents of the bottom/litter layer are probably mainly due to changes in the nutri ent contents of bryophytes and lichens, and not to changes in the litter. The high increase in the Ca contents of bot tom/litter layer was opposite to that found in the tree and field layers, where Ca accumulation decreased after fertilization. There were also no changes in the Ca contents of needle or other tree litter after fertilization (Finer 1991 a). The dissolution rate of Ca in Moroccan rock phos phate is probably slow. Thus, the bottom/litter layer samples probably still contained residual unreactive rock phosphate at the time of sam pling. The solubility of phosphorus in rock phos phate is also slow (Yli-Halla and Lumme 1987). The high increase in P amounts in the bottom/ litter layer after fertilization could thus partly result from the same reason. The nitrogen amounts in the understorey vege tation and litter did not increase only on the NPK-fertilized plots, but also on the PK-ferti lized plots after fertilization (Table 4). This indi cates that the PK treatment had a priming effect on peat N mineralization and vegetation N up take. Except for B and K, the accumulation of ferti lizer nutrients in understorey vegetation and lit ter was of the same magnitude or greater than the uptake by tree layer. The field and bottom/ litter layers are thus of great importance in the biogeochemical nutrient cycle of ombrotrophic pine bogs. References BMDP Statistical Software Manual 1990. Volume 1. BMDP Statistical Software, Inc. 629 p. ISBN 0- 520-07112-3. Braekke, F. H. 1979. Boron deficiency in forest plan tations on peatland in Norway. Meddelelser fra Norsk Institutt for Skogsforskning 35(3): 213 236. 1983. Micronutrients - prophylastic use and cure of forest growth disturbances. Communicationes Instituti Forestalis Fenniae 116: 159-169. 1988. Nutrient relationships in forest stands: field vegetation and bottom-litter layer on peatland. Meddelelser fra Norsk Institutt for Skogsforsk ning 40(7): 1-20. & Finer, L. 1991. Fertilization effects on surface peat of pine bogs. Scandinavian Journal of Forest Research 6: 433-449. & Haland, B. 1990. Mineral elements in the root biomass of a low- shrub pine bog community. Scandinavian Journal of Forest Research 5: 29- 39. Dirkse, G.M. & Martakis, G.F.P. 1992. Effects of fertilizer on bryophytes in Swedish experiments on forest fertilization. Biological Conservation 59: 155-161. Finer, L. 1991 a. Effect of fertilization on dry mass accumulation and nutrient cycling in Scots pine on an ombrotrophic bog. Acta Forestalia Fennica 223. 43 p. 1991b. Root biomass on an ombrotrophic pine bog and the effects of PK and NPK fertilization. Silva Fennica 25(1): 1-12. & Braekke, F. H. 1991. Understorey vegetation on three ombrotrophic pine bogs and the effects of NPK and PK fertilization. Scandinavian Journal of Forest Research 6: 113-128. Haland, B. & Braekke, F. H. 1989. Distribution of root biomass in a low-shrub pine bog. Scandinavian Journal of Forest Research 4: 307-316. Haveraaen, O. 1967. Vekst- og naeringsstudier i et gjödslings forsok med svartgran, Picea mariana (Mill.), pä myr. Summary: Growth and nutrient studies in a fertilized experiment with Black spruce, Picea mariana (Mill.), on peatland. Meddelelser fra det Norske Skogforsoksvesen 23: 137-175. Halonen, 0., Tulkki, H. & Derome, J. 1983. Nutrient analysis methods. Metsäntutkimuslaitoksen tiedon antoja 121. 28 p. research articles Silva Fennica 31(4) 400 Heikurainen, L. & Laine, J. 1976. Lannoituksen, kui vatuksen ja lämpöolojen vaikutus istutus-ja luon nontaimistojen kehitykseen rämeillä. Summary: Effect of fertilization, drainage and temperature conditions on the development of planted and nat ural seedlings on pine swamps. Acta Forestalia Fennica 150. 38 p. & Pakarinen, P. 1982. Mire vegetation and site types. In: Laine, J. (ed. ). Peatlands and their utili zation in Finland. Finnish Peatland Society, Hel sinki. p. 14-23. Jäppinen, J.-P. & Hotanen, J.-P. 1990. Effect of ferti lization on the abundance of bryophytes in two drained peatland forests in Eastern Finland. An nales Botanici Fennici 27: 93-108. Laiho, R. 1996. Changes in understorey biomass and species composition after water level drawdown on pine mires in southern Finland. Tiivistelmä: Vedenpinnan alenemisen vaikutus sararämeen pintakasvillisuuden biomassaan ja lajistoon. Suo 47(2): 59-69. & Finer, L. 1996. Changes in root biomass after water-level drawdown on pine mires in southern Finland. Scandinavian Journal of Forest Research 11:251-260. Laine, J., Vasander, H. & Laiho, R. 1995. Long-term effects of water level draw-down on the vegeta tion of drained pine mires in southern Finland. Journal of Applied Ecology 32: 785-802. Meshechok, B. 1968. Om startgjodsling ved skogkul tur pä myr. Summary: Initial fertilization when afforesting open swamps. Meddelelser fra det Nor ske Skogforsoksvesen 25(1). 140 p. Paavilainen, E. 1979. Turvemaiden metsänlannoitus tutkimuksista. Summary: Research on fertiliza tion of forested peatlands. Folia Forestalia 400: 427^142. 1980. Effect of fertilization on plant biomass and nutrient cycle on a drained dwarf shrub pine swamp. Communicationes Instituti Forestalls Fen niae 98(5). 77 p. Päivänen, J. 1970. Hajalannoituksen vaikutus lyhyt kortisen nevan pintakasvillisuuden kenttäker rokseen. Summary: On the influence of broadcast fertilization on the field layer of the vegetation of open low-sedge bog. Suo 1: 18-24. Reinikainen, A., Vasander, H. & Lindholm. T. 1984. Plant biomass and primary production of southern boreal mire-ecosystems in Finland. Proc. 7th Inter national Peat Congress, Dublin. Vol. 4. p. 1-20. Tamm. C. O. 1954. Some observations on the nutrient turn-over in a bog community dominated by Erio phorum vaginatum L. Oikos 5 II: 189-194. Vasander, H. 1981 Luonnontilaisen keidasrämeen sekä lannoitetun ojikon ja muuttuman ravinnevarat. Summary: Nutrients in an ombrotrophic bog eco system in the virgin state and after forest-improve ments. Suo 32(4-5): 137-141. 1982. Plant biomass and production in virgin, drained and fertilized sites in a raised bog in south ern Finland. Annales Botanici Fennici 19: 103- 125. 1987. Diversity of understorey biomass in virgin and drained southern boreal mires in eastern Fen noscandia. Annales Botanici Fennici 24: 137-153. , Kuusipalo, J. & Lindholm, T. 1993. Vegetation changes after drainage and fertilization in pine mires. Summary: Kasvillisuuden muutokset rä meillä ojituksen ja lannoituksen jälkeen. Suo 44( 1): 1-9. Wallen, B. 1986. Above and below ground dry mass of the three main vascular plants on hummocks on a subarctic peat bog. Oikos 46: 51-56. Yli-Halla, M. & Lumme, I. 1987. Behaviour of cer tain phosphorus and potassium compounds in a sedge peat soil. Silva Fennica 21(3): 251-257. Total of 31 references IV Scand. J. For. Res. 11:321-326, 1996 Phosphorus Adsorption by Peat from Drained Mires in Southern Finland MIKA NIEMINEN and MAIJA JARVA The Finnish Forest Research Institute, Vantaa Research Centre, P.O. Box 18, FIN-01301 Vantaa, Finland INTRODUCTION The area of peatlands in Finland totals about 10 million ha, which corresponds to one-third of the total land area. About 50% of the peatland area has been drained for forestry purposes. For satisfactory tree production, one or more fertilizer applications with P during the rotation is often needed. Several studies on the leaching of applied P from drained peatlands have been done (Karsisto 1970, Särkkä 1970, Karsisto & Ravela 1971, Kenttämies 1981, Nie minen & Ahti 1993). The risk for high leaching rates of applied P is substantial, at least in the most nutrient-poor, acid Sphagnum peats. This is related directly to their low Fe and A 1 content and conse quent low P adsorption (Kaila 1959, Rannikko & Hartikainen 1980, Cuttle 1983). Owing to a lack of information about P adsorption capacities, as well as A 1 and Fe contents, of different kinds of peaty soils in Finland, the actual risk areas cannot be differentiated from those exhibiting high adsorption capacities. An understanding of the retention of P by peat is important both for assessing the risk for eutrophica tion of water bodies resulting from the leaching of phosphate from fertilized areas and for achieving efficient and economical use of different P fertilizers on drained peatland forests. Information about the P adsorption capacities of peat soils can also be used for assessing the effectiveness of peatlands to act as P-saturated waste-water filtration systems (Richard son 1985). Nieminen, M. and Jarva, M. (Finnish Forest Research Institute, Vantaa Research Centre, P.O. Box 18, FIN-01301 Vantaa, Finland). Phosphorus adsorption by peat from drained mires in southern Finland. Received Oct. 2, 1995; Accepted Febr. 1, 1996. Scand. J. For. Res. 11: 321-326, 1996. The phosphorus adsorption capacity of 20 peat soils and those soil properties likely to influence it (AI, Fe, Ca, Mg, pH, ash content) were studied. Phosphorus adsorption was correlated only weakly to the site type classification of peatlands. However, the P adsorption was strongly correlated -with the concentration of Fe in the peat. Thus, soil chemical analysis should be used to support site type classification in order to assess the effectiveness of peatlands to adsorb P. When expressed on a volume basis, the P adsorption capacity varied from 7 kg ha" 1 to 673 kg ha"' in the 0-30 cm layer. Compared with the amounts of P used in practical peatland forest fertilization in Finland (40-45 kg ha"'), the P adsorption capacity of several peat soils studied was substantially lower. It was thus concluded that slowly soluble P fertilizers or applications <4O-45 kg ha"' should be used where the P adsorption capacity is low. Key words: Adsorption capacity, adsorption isotherm , fertilization, iron, phosphorus. In this paper, the adsorption of P by peat from a range of drained peatlands is reported. The peatlands differ from each other with respect to peat chemical and physical properties, as well as with respect to peatland site type. The mechanisms controlling P adsorption were also studied. MATERIAL AND METHODS Study sites Samples from 20 sites were collected from different parts of southern Finland (between latitudes 60°N and 62°N) in the summer of 1994. The basic principle in the selection of individual peatlands was to obtain material representing the most common types of drained peatland sites in Finland (Table 1). Most samples were collected from permanent sample plots established on drained peatlands by the Finnish Forest Research Institute or by the University of Helsinki. Collection of peat samples At each site vertical volumetric peat samples (0-15 and 15-30 cm) were taken using a square-section corer (section area 25 cm 2 ) at 5, 10 and 15 m distance from the nearest ditch and combined by layers. Two adjacent samples were taken from each position: the first set for the chemical analyses to be made from moist samples, the second for the bulk density deter minations and for chemical analyses from dried sam- © 1996 Scandinavian University Press ISSN 0282-7581 322 M. Nieminen and M. Jarva Scand. J. For. Res. 11 (1996) Table 1. Peatland site type, drainage status and site quality index of the sampling areas "According to Heikurainen & Pakarinen (1982). ''According to Heikurainen & Pakarinen (1982): "Recently drained peatlands (RDP) are characterized by a vegetation which has changed only slighfly from the original virgin phase...No clear response to drainage can be observed in the growth of trees and, as is the case with treeless sites, afforestation is often incomplete." "Transitional drained peatlands (TDP) exhibit more advenced changes in their ground vegetation. In this instance the tree stand shows a clear response to drainage...Cover of Sphagnum mosses usually decreases significantly at this phase, while certain shrubs (e.g.) Betula nana increase in abundance..." "Old peatland forests (OPF) are characterized by a rather stable ground vegetation which clearly differs from that of virgin peatlands, resembling more the vegetation associated with mineral soil forests. The tree stand can be likened to that of ordinary upland forests, showing clearly the effects of silvicultural treatments where performed." c Site quality index has been developed to depict the potential post-drainage tree growth afforded by sites of varying soil fertility, pre-drainage tree stand and location with respect to south-north direction in Finland (e.g. Heikurainen 1982). An arbitrary scale of 1-10 has been devised so that the most fertile sites, having a well-developed tree stand prior to drainage, is equal to 10 in southernmost Finland. The values presented here are for southernmost Finland. pies. Peat samples were taken from a level surface, not from hollows or hummocks. If there was a raw humus layer on the peat surface (Kaunisto & Paavi lainen 1988), it was removed prior to sampling. All the samples were stored in a freezer ( — lB°C). Laboratory analysis The analyses were carried out in the Central Labora tory of the Finnish Forest Research Institute. The moist samples were used for determining P adsorp tion isotherms (Bache & Williams 1971, Richardson 1985; see also Fig. 1). When increasing amounts of P are added to a series of soil suspensions, the P adsorbed (mg g" 1 soil) can be plotted against the equilibrium P concentration. The resulting curve is called a P adsorption isotherm. In this study the adsorption isotherms were determined as described below. After thawing at room temperature, the peat sam ples were coarsely shredded and thoroughly mixed. Larger roots (> 2 mm in diameter) were removed at the same time. A subsample was then weighed and dried at 105° C to determine the water content. A further subsample of moist peat (equivalent to 1 g dry weight) was added to bottles containing solutions (60 ml) with 0, 0.1, 0.3, 0.5, 0.7, 1, 2, 3, 5, 7, 9, 12, 15, Sample Peatland site typel)" Drainage status'' Site quality index" 1 Eutrophic pine fen OPF 8.0 2 Herb-rich hardwood-spruce swamp OPF 10.0 3 Herb-rich sedge birch-pine swamp OPF 7.0 4 Herb-rich sedge hardwood-spruce swamp TDP 9.0 5 Herb-rich tall-sedge fen TDP 6.5 6 Herb-rich sedge hardwood-spruce swamp RDP 9.0 7 Vaccinium myrtillus spruce swamp OPF 7.5 8 Vaccinium myrtillus spruce swamp TDP 7.5 9 Tall-sedge pine swamp OPF 5.0 0 Tall-sedge pine swamp TDP 5.0 1 Tall-sedge fen TDP 5.0 2 Care.x globularis pine swamp OPF 4.5 3 Spruce-pine swamp TDP 3.5 4 Low-shrub pine bog OPF 3.0 5 Low-shrub pine bog TDP 3.0 6 Cottongrass pine bog TDP 2.5 7 Cottongrass pine bog TDP 2.5 8 Small-sedge Sphagnum papillosum pine bog RDP 3.0 9 Ombrotrophic small-sedge bog TDP 3.0 2 0 Sphagnum fuscum pine bog-Cottongrass pine bog TDP 2.5 Phosphorous adsorption in drained peat soils Scand. J. For. Res. 1 1 (1996) 323 Fig. 1. Phosphorus adsorption iso therms for the 0-15 cm (solid line) and 15-30 cm (broken line) sampling depths. The sample number (see Table 1) is given in the upper left corner of each subfigure. Values for PSI (see text) are also given for each curve. 20, 25, 30, 35 or 40 mg l" 1 of P. The bottles were shaken vigorously on a reciprocating shaker for 1 h. The suspensions were then left to stand for 23 h, after which they were shaken again for 10 min. The suspensions were then centrifuged at 4000 r.p.m. for 10 min, passed first through a glass fibre paper and then through a membrane filter. The concentration of P remaining in the filtrate was determined imme diately after equilibration by inductively coupled plasma emission spectrometry (ICP/AES, ARL 3580). The amount of P adsorbed was calculated from the difference between the initial amount of P in the solution and that of the filtrate after treat ment. A preliminary study had established that there were no significant differences in adsorption curves made when using the ascorbic acid, the molybdate hydrazine, the stanno-chloride and the ICP/AES methods to determine phosphorus. For example, the interdependence between the ascorbic acid method and the ICP/AES method in two different data sets was as follows: The remainder of the moist samples was used for the determination of oxalate soluble A 1 (Al ox ) and Fe (Fe ox ). 28.5 ml of 0.05 M oxalic acid was added to 31.5 ml of 0.05 M ammonium oxalate. Moist peat (8 g) was added to a bottle containing 60 ml of this solution. The suspension was then shaken for 2 h, centrifuged at 4000 r.p.m. for 10 min and passed first through a Whatman GF/G filter paper and then through a Schleicher and Schuell Rundfilter (589 3 ). Al ox and Fe ox were determined by ICP/AES, ARL 3580. The second set of the samples were dried at 70° C and used for total analyses and bulk density determi nations. For total analyses, the samples were dry combusted and the ash taken up in HCI acid accord ing to the standard methods used in the Finnish acid 0.9398 • Picp + 0.015 r 2 = 0.999 n = 126 Pascorbic acid 0.9998 ■ PICP + 0.167 r 2 = 0.998 «= 36 324 M. Nieminen and M. Jarva Scand. J. For. Res. 11 (1996) Forest Research Institute (Halonen et ai. 1983). The samples were analysed for Fe (Fetot ), A 1 (Al lot), Ca (Ca tot) and Mg (Mgtot) by ICP/AES, ARL 3580. Soil pH was measured in deionized water (1 : 25). Ash content was determined by thermogravimetry at 500° C. Calculation of P adsorption index (PSI) and P adsorption capacity (PAC) There is no suitable method for determining maxi mum adsorption directly (Bache & Williams 1971). For example, when determining P adsorption isotherms, adsorption usually continues to increase with increasing P additions, and a well-defined maxi mum is not obtained. However, the adsorption data can be treated with a number of adsorption equations and adsorption indices from which adsorption can be calculated. The P adsorption index (PSI), which has frequently been used as an indicator of P adsorption (e.g. Bache & Williams 1971, Cuttle 1983, Richard son 1985, Mozaffari & Sims 1994), was used in this study. The PSI was calculated as: X/log C, where X is the quantity of P adsorbed by the sample (mg P (100 g) "' dry weight of peat) and Cis the concentra tion of P in equilibrium solution. The total P adsorption capacity (PAC) was calcu lated by multiplying the PSI value by the bulk density of the sample, and by expressing the adsorption as kg P sorbed ha"'. Table 2. Correlation coefficients between phosphorus adsorption index (PSI) values and site quality index (see Table I) and some peat properties for the 0-15 cm and 15-30 cm peat layers (n = 20) Simple Pearson correlation analysis was used to examine the dependence of P adsorption on the site quality index and peat chemical properties. In analy sis of regression, logarithmic transformations were used to normalize the distribution of variables. The statistical tests were made using the BMDP (1990) software package. RESULTS Phosphorus adsorption isotherms and adsorption indices (PSI) The only eutrophic peatland site included in the study (sample 1) exhibited the highest P adsorption (Fig. 1). The highest PSI value in the upper peat layer was 32.5 (sample 10) and 42.2 in the lower layer (sample 1). The lowest values for the upper and lower layers were —2.3 (sample 13) and 0.9 (sample 14), respec tively. High-fertility or intermediate-fertility sites fre quently exhibited high adsorption, whereas many low-fertility sites had low adsorption. However, there was no clear relationship between the site quality index and P adsorption (Table 2). It should be noted that several of the samples had very low adsorption values (samples 3, 9, 12, 13 and 14). Even with the highest additions of P, the amounts adsorbed by the peat samples remained negligible. Peat properties controlling adsorption In the upper peat layer, Alox contents varied from 194 to 4017 mg kg"', Al tot contents from 315 to 5782 mg kg"', Fe ox contents from 424 to 24 696 mg kg"', Fe tot contents from 533 to 33 780 mg kg"', Ca tot contents from 908 to 11 208 mg kg"', Mgtot contents from 143 to 1311 mg kg"', ash contents from 0.9 to 32.7%, and pH values from 3.3 to 4.5. In the lower peat layer, the ranges were: Al ox 171-5577 mg kg"'; Altot 263- 13 573 mg kg"'; Fe ox 297-28 135 mg kg"'; Fe tol 315-33 751 mg kg"'; Ca lot 689-18 870 mg kg"'; Mg tot 60-1 510 mg kg"'; ash content 0.8-77.8% and pH 3.6-5.0. Correlations between PSI and selected chemical properties of the peat likely to influence P adsorp tion showed that PSI was correlated most strongly to Fe contents (Table 2). Except for Fe and pH, no other statistically significant correlations could be observed. Total phosphorus adsorption capacity (PAC) The highest PAC value in the upper peat layer was 200 kg ha"' and 569 kg ha"' in the lower layer Significance level: *P < 0.05, ** P <0.01, ***P< 0.001 PS I 0-15 cm 15-30 cm Site quality index 0.216 0.363 pH 0.486* 0.561** Ash content 0.413 0.292 Fe.o, 0.859*** 0.935*** Fe 0 , 0.901*** 0.901*** Al,„, 0.197 0.297 AL 0.233 0.193 Ca tol -0.117 0.009 Mg tot -0.211 -0.049 325 Phosphorous adsorption in drained peat soils Scand. J. For. Res. II (1996) Table 3. Phosphorus adsorption capacity (PAC) values (Table 3). The lowest values for the same layers were okg ha -1 and 4kg ha -1 , respectively. The total adsorption capacity of the 0-30 cm peat layer was highest for sample 6 (673 kg ha -1 ) and lowest for sample 14 (7 kg ha"'). The large range in PAC values and the dependence of PAC upon the amount of iron in the peat, but not site quality index, are depicted in Figure 2. DISCUSSION "Adsorption isotherms or adsorption indices are in dicative of phosphorus retention but most probably overestimate the actual maximum adsorption in the field. This is because water movement occurs mainly in the large pores and channels, thus reducing contact with a large portion of the soil matrix" (Richardson 1985). Input-output data from fertilized basin areas is the only method to determine the actual retention of P. However, adsorption isotherms and especially adsorption indices offer a simple, yet adequate, method to study the dependence of P adsorption on the chemical and physical properties of the soil. Only the P adsorption of the uppermost 0-30 cm peat layer was determined. If the deeper peat layers Fig. 2. The relationship between PAC (see text) and the amount of Fe ox in the 0-30 cm peat layer. The samples are identified by their values for the site quality index (Table 2). Logarithmic scales. exhibit significant P adsorption, the total P adsorp tion capacity for the site would be underestimated accordingly. However, deeper peat layers are unlikely to exhibit P adsorption because conditions are anaer obic for most of the year. Anaerobiosis inhibits P adsorption due to the reduction and redistribution of Fe (Armstrong 1975). It was not possible to assess the relationship be tween PSI or PAC values and actual P adsorption in the field. However, the results of a fertilization exper iment (Nieminen & Ahti 1993) performed in one of the sampling areas used in this study (sample 20), which has both low PSI and PAC values (Fig. 1, Table 3), indicated high leaching rates of applied P. The PSI values in this study ranged from —2.3 to 42.2. Similar values have been reported by both Cut tle (1983) (values ranging from —0.4 to 43.7) and by Lopez-Hernandez & Burnham (1974) (values ranging from 0 to 58.7) for organic soils in Britain. Substan tially higher values were reported for freshwater wet lands (including both organic soils and mineral soil wetlands) in North America, where the PSI values ranged from 8 to 163 (Richardson 1985). There was no clear relationship between the site quality index and P adsorption. Thus, the P retention capacity cannot be evaluated by simply classifying soils on the basis of their vegetation cover. Soil analysis, particularly for Fe, should also be used. Compared with the amounts of P fertilizer applied to Finnish peatland forests in practice (40-45 kg ha"'), the PAC values (Table 3) for several of the peatlands studied must be regarded as substantially low. As previously mentioned, P adsorption indices Sample" PAC, (kg ha" ■') 0-15 cm 15-30 cm 0-30 cm 1 184 263 447 2 15 42 57 3 17 27 44 4 29 81 110 5 43 39 82 6 104 569 673 7 21 99 120 8 16 38 54 9 0 25 25 10 200 173 373 11 161 76 237 12 2 14 16 13 0 25 25 14 3 4 7 15 27 48 75 16 1 31 32 17 8 25 33 18 24 23 47 19 32 23 55 20 3 18 21 " see Table 1. 326 M. Nieminen and M. Jarva Scand. J. For. Res. 11 (1996) probably overestimate the actual P retention. Thus, those peats exhibiting a PAC value of <4O-45 kg ha"' probably have very little ability to retain dis solved phosphate against leaching. According to Finer (1989), the annual uptake of P from peat varies from 2.5 to 3.4 kg ha -1 in well-developed tree stands on drained peatlands, indicating that uptake by the tree stand is of minor importance in restricting leach ing losses. Consequently, where the P adsorption capacity is low, very slowly soluble P fertilizers or applications of less than 40-45 kg ha -1 ought to be used. ACKNOWLEDGEMENTS We are grateful to Tuija Hytönen and Maarit Niemi for their conscientious assistance in conducting the laboratory work in this trial, and to Inkeri Suopanki, Raija Linnainmaa and Johanna Ylinen for helping us in preparing the manuscript. We also wish to thank Professor Seppo Kaunisto and Dr Erkki Ahti for reading the manuscript and giving valuable advice. The English was revised by Dr Michael Starr. This study was supported financially by Kemira Ltd. REFERENCES Armstrong, W. 1975. Waterlogged soils. In Etherington, J. R. (ed.). Environment and Plant Ecology, pp. 184-216. 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Phosphorus availability and sorption in an Atlantic coastal plain watershed dominated by animal-based agriculture. Soil Sci. 157: 97-107. Nieminen, M. & Ahti, E. 1993. Leaching of nutrients from an ombrotrophic peatland area after fertilizer applica tion on snow. Folia For. 814: 1-22. (In Finnish with English summary.) Rannikko, M. & Hartikainen, H. 1980. Retention of ap plied phosphorus in Sphagnum peat. Proceedings of the 6th International Peat Congress, Duluth, Minnesota, August 17-23, 1980, pp. 666-669. Richardson, C. J. 1985. Mechanisms controlling phospho rus retention capacity in freshwater wetlands. Science 22: 1424-1427. Särkkä, M. 1970. On the influence of forest fertilization on water courses. Suo 21(3-4): 67-74. (In Finnish with English summary.) ISBN 951-40-1711-0 ISSN 0358-4283 Hakapaino Oy 1999