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Author(s): Helena Soinne, Riikka Keskinen, Mari Räty, Sanna Kanerva, Eila Turtola, Janne 
Kaseva, Visa Nuutinen, Asko Simojoki and Tapio Salo 
Title: Soil organic carbon and clay content as deciding factors for net nitrogen 
mineralization and cereal yields in boreal mineral soils 
Year: 2020 
Version: Published version 
Copyright:   The Author(s) 2020 
Rights: CC BY 4.0 
Rights url: http://creativecommons.org/licenses/by/4.0/ 
 
Please cite the original version: 
Soinne H, Keskinen R, Räty M, et al. Soil organic carbon and clay content as deciding factors for net 
nitrogen mineralization and cereal yields in boreal mineral soils. Eur J Soil Sci. 2020;1–16. 
https://doi.org/10.1111/ejss.13003. 
S P E C I A L I S S U E AR T I C L E
Soil organic carbon and clay content as deciding factors for
net nitrogen mineralization and cereal yields in boreal
mineral soils
Helena Soinne1 | Riikka Keskinen2 | Mari Räty3 | Sanna Kanerva4 |
Eila Turtola2 | Janne Kaseva2 | Visa Nuutinen2 | Asko Simojoki4 | Tapio Salo2
1Natural Resources Institute Finland,
Helsinki, Finland
2Natural Resources Institute Finland,
Jokioinen, Finland
3Natural Resources Institute Finland,
Maaninka, Finland
4Department of Agricultural Sciences,
Division of Environmental Soil Science,
University of Helsinki, Helsinki, Finland
Correspondence
Helena Soinne, Natural Resources
Institute Finland, Latokartanonkaari 9,
FI-00790 Helsinki, Finland.
Email: helena.soinne@luke.fi
Funding information
Ministry of Agriculture and Forestry of
Finland; The Strategic Research Council
of the Academy of Finland, Grant/Award
Number: 327236
Abstract
To achieve appropriate yield levels, inherent nitrogen (N) supply and biological N
fixation are often complemented by fertilization. To avoid economic losses and
negative environmental impacts due to over-application of N fertilizer, estimation
of the inherent N supply is critical. We aimed to identify the roles of soil texture
and organic matter in N mineralization and yield levels attained in cereal cultiva-
tion with or without N fertilization in boreal mineral soils. First, the net N miner-
alization and soil respiration were measured by laboratory incubation with soil
samples varying in clay and organic carbon (C) contents. Secondly, to estimate
the inherent soil N supply under field conditions, both unfertilized and fertilized
cereal yields were measured in fields on clay soils (clay 30–78%) and coarse-tex-
tured soils (clay 0–28%). In clay soils (C 2.5–9.0%), both the net N mineralization
and the cereal yields (without and with fertilization) decreased with increasing
clay/C ratio. Moreover, in soils with high clay/C ratio, the agronomic N use effi-
ciency (additional yield per kg of fertilizer N) varied considerably, indicating the
presence of growth limitations other than N. In coarse-textured soils, the yield
increase attained by fertilization increased with increasing organic C. Our results
indicate that for clay soils in a cool and humid climate, the higher the clay con-
tent, the more organic C is needed to produce reasonable yields and to ensure
efficient use of added nutrients without high N losses to the environment. For
coarse soils having a rather high mean organic C of 2.3%, the organic C appeared
to improve agronomic N use efficiency. For farmers, simple indicators such as the
clay/C ratio or the use of non-N-fertilized control plots may be useful for site-spe-
cific adjustment of the rates of N fertilization.
Highlights
• We aimed to identify simple indicators of inherent soil N supply applicable
at the farm level.
Received: 15 January 2020 Revised: 17 April 2020 Accepted: 22 May 2020
DOI: 10.1111/ejss.13003
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2020 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science.
Eur J Soil Sci. 2020;1–16. wileyonlinelibrary.com/journal/ejss 1
• In clay soils, the net N mineralization was found to correlate negatively with
the clay/C ratio.
• In coarse-textured soils, agronomic N use efficiency improved with increas-
ing soil organic C.
• Clay soils with high clay/C ratio are at risk of low yield levels.
KEYWORD S
agricultural soil, clay, N mineralization, soil carbon mineralization, soil fertility, yield response
1 | INTRODUCTION
Synthetic nitrogen (N) fertilizers produced via the Haber-
Bosch process from atmospheric N2 have greatly contrib-
uted to the substantial increases in crop yields during the
past decades (Sinclair & Rufty, 2012; Smil, 1999; Tilman,
Cassman, Matson, Naylor, & Polasky, 2002). However,
currently only half or less of the global fertilizer N input
is converted into harvested crop products (Lassaletta,
Billen, Grizzetti, Anglade, & Garnier, 2014; Tilman
et al., 2002), indicating major waste of fossil energy and
losses of reactive N to the environment (Fields, 2004;
Gruber & Galloway, 2008; Mosier, 2002). This excess N
creates many negative impacts, including eutrophication
and acidification of terrestrial and aquatic systems, a
decrease in biodiversity and accelerated global warming
(e.g., Fields, 2004). Although synthetic N fertilization
increases biomass production, and thus the potential
input of crop residues into soil and accumulation of soil
organic carbon (OC), it may also increase microbial oxi-
dation of the residue and native soil OC, resulting in loss
of soil organic matter (OM) and total soil N (Khan,
Mulvaney, Ellsworth, & Boast, 2007; Mulvaney, Khan, &
Ellsworth, 2009). According to Singh (2018), however, N
fertilizer-induced acceleration of soil OC mineralization
takes place only when fertilizer N is applied at excessive
rates, whereas Ladha, Reddy, Padre, and van
Kessel (2011) concluded that in agricultural soils, inor-
ganic fertilizer N significantly reduces the rate at which
soil OM declines. Thus, considering all these aspects and
the cost of the fertilization, site-specific adjustment for an
optimal rate of synthetic N input is essential for sustain-
able N management.
The bulk of soil N reserves is organic, and therefore,
the cycles of N and C are tightly coupled (Castellano,
Kaye, Lin, & Schmidt, 2012; Gruber & Galloway, 2008;
Schulten & Schnitzer, 1998; Stevenson, 1982). Microbial
decomposition of soil OM continuously releases ammo-
nium ions, most of which are assimilated by microbial
biomass (Jarvis, Stockdale, Shepherd, & Powlson, 1996).
Net N mineralization, which is the difference between
gross N mineralization and N immobilization in the
microbial biomass, determines the amount of labile
inorganic N available for plant uptake. In soils with
inherently high N availability, the response to added fer-
tilizer N is low and the fertilizer N use efficiency
remains weak (Schindler & Knighton, 1999). Meta-ana-
lyses of Finnish N fertilization trials of spring cereals
and perennial grasses showed that optimum fertilizer
rates were clearly related to the yields achieved without
added N fertilizer in a given soil, and these yields, in
turn, depended on soil-OM content (Valkama et al., 2016;
Valkama, Salo, Esala, & Turtola, 2013). To avoid unprof-
itable and unsustainable over-application of fertilizer N,
the inherent N supply in soil should thus be considered
in fertilizer recommendations. In field conditions, the
difference between yield at optimum N application rate
and yield with no fertilizer N added can serve as an indi-
rect measure to determine fertilizer N need (Lory &
Scharf, 2003).
The capacity of soil to supply N depends largely on
the content and quality of its OM (Berendse, 1990; Matus
& Rodríguez, 1994). Ros, Hanegraaf, Hoffland, and van
Riemsdijk (2011a), for instance, found that total and
extractable OM explained 78% of the variation of miner-
alizable N in soil. As arable soils in boreal regions tend to
be rather rich in OC (Heikkinen, Ketoja, Nuutinen, &
Regina, 2013), their indigenous soil N availability may be
higher than in warmer climates. The rate of N minerali-
zation is related to the activity of the microbial decom-
poser community (Bengtsson, Bengtson, & Månsson,
2003), as influenced by factors such as temperature and
moisture (Guntiñas, Leirós, Trasar-Cepeda, & Gil-Sotres,
2012; Paul et al., 2003), cultivation practices (Jarvis et al.,
1996; Silgram & Shepherd, 1999), and pH and soil chemi-
cal fertility (Ros, Temminghoff, & Hoffland, 2011b). In
addition, mineralization rates tend to be lower in fine-
textured than in coarse-textured soils due to the ability of
clay to protect OM against decay (Castellano et al., 2012;
Hassink, 1997; van Veen, Ladd, & Amato, 1985). Instead
of the amount of soil OC per se, the ratio of OC to clay or
the fine textural fraction can be a more useful tool in the
2 SOINNE ET AL.
prediction of grain yields (Quiroga, Funaro, Noellemeyer,
& Peinemann, 2006; Schjønning et al., 2018).
By the law of diminishing returns, the agronomic N
use efficiency should be higher in soils that have low
inherent N supply than in soils with properties indicating
high inherent N supply. Because of the stabilizing effect
of clay on soil OC, we hypothesize that to reach similar N
supply originating from OM mineralization, soils with
high clay content should have higher OC content than
soils with lower clay content. With this background, the
aim of this study was to build knowledge for developing
science-based practical tools for farmers to adjust N fertil-
ization for more sustainable N management. The specific
objectives were (a) to identify the roles of soil OC and
clay content in the net N mineralization and inherent N
supply and (b) to assess the effects of varying soil texture
and OC contents on cereal yields in boreal mineral soils.
2 | MATERIAL AND METHODS
2.1 | Soils and climatic conditions
Eighteen fields from two research stations (Jokioinen,
south-western Finland, and Maaninka, east-central Fin-
land) of the Natural Resources Institute Finland (Luke)
and 16 fields of private farmers located similarly in south-
western Finland and in east-central Finland (Table 1, Fig-
ure 1) were selected for this study. All fields were in soil
regions with a predominantly boreal continental to temper-
ate continental climate (BGR 2005). The annual precipita-
tion in southern and central Finland varies typically
between 600 and 700 mm and mean yearly temperature is
c. 5
C in southern Finland and declines towards the north.
The formation of soils in Finland has been influenced by
the last Weichsel glacial period, ending about 11,500 years
ago, and by evolutionary stages of the Baltic Sea. Owing to
the young pedogenetic age, most of the soils are relatively
weakly developed. Cultivated clay soils of Finland are typi-
cally classified as (Vertic Luvic) Stagnosols or as (Luvic)
Gleysols in wet depressions. Silt soils are classified as
(Stagnic) Regosols, very fine sand or fine-sandy moraine
soils are commonly classified as (Endogleyic) Cambisols
and podzolized coarse mineral soils as (Gleyic) Podzols
(Lilja et al., 2017). The majority of the fields are artificially
drained due to the excessive moisture (Puustinen, Merilä,
Palko, & Seuna, 1994).
Only fields growing spring cereals in the study years
2016 and 2017 were chosen from the selected locations.
Farmers were advised to leave an unfertilized area of at
least one seed drill width and of 20 m in length at a rep-
resentative site at an appropriate distance from the field
edge. The observed part within each field (approximately
20 m × 20 m) thus included adjacent fertilized and
unfertilized areas that were otherwise similar with
regard to soil type and productivity according to the
farmer's judgement.
2.2 | Incubation study for soil
respiration and potential net nitrogen
mineralization
To determine the potential net N mineralization, samples
from six fields (included also in the field study, Table 1)
were taken in autumn 2016 after the harvest of a spring
cereal crop. The fields were selected based on records of
their yields over the past 5 years and their clay contents
to include both fields of high and low productivity and
variation in texture. Each field had a sampling location
placed on both fertilized and unfertilized areas of the
field. On three of the six fields, an additional third sam-
pling location was included to account for unexpected
visually observed variability in the crop productivity. The
final set of soil samples used for respiration and N miner-
alization studies in the laboratory thus represented 15
locations (Table 2). At each sampling location, two to
four replicate soil samples were taken c. 5 m apart with
an auger from the 0–20-cm soil layer. In the laboratory,
the samples were passed through a 5-mm sieve and then
stored field moist at +5
C for 2 weeks prior to esta-
blishing the incubation experiment. The sieved samples
were analysed for organic carbon (C%) and total nitrogen
(N%), as well as for pH (1:2 H2O) and electrical conduc-
tivity (EC) (1:2.5 H2O) (Table 2). Soil texture was mea-
sured from one sample per location.
For the incubation, 10 g (dry matter, DM) of each soil
was weighed in a 120-ml infusion bottle, after which the
soil moisture was adjusted to 60% of the water-holding
capacity (WHC) of the particular soil. The WHC was
measured separately for each of the sampling locations
with a funnel method (20 g soil (DM) weighed into a fun-
nel lined with filter paper, saturated with water for 1 hr
and left to drain freely for 24 hr). Each replicate soil sam-
ple was used to fill four infusion bottles. Two of the bot-
tles were assigned for N measurements carried out on
days 0 and 14, the third one was frozen and reserved for
determination of water-extractable organic carbon
(WEOC) on day 0 and the last one used for measuring
respiration throughout the study period and N at the end
of the incubation (day 30). The infusion bottles were cov-
ered loosely with aluminium foil and incubated at 20
C
in the dark for a maximum of 30 days. At the beginning
of the incubation and on days 14 and 30, a set of sample
bottles was frozen for later extraction of inorganic and
total dissolved N.
SOINNE ET AL. 3
T
A
B
L
E
1
P
ro
pe
rt
ie
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of
th
e
fi
el
d
so
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g
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ay
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(s
ilt
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ti
cl
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si
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0.
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;C
E
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;S
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in
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F
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Sa
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C
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5
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ar
s
20
16
20
17
%
cm
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k
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1
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il
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20
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O
rg
an
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fe
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C
ro
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B
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B
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87
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–3
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8
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−
V
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%
8
B
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99
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20
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O
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13
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4
82
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–3
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%
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4 SOINNE ET AL.
The WEOC was extracted with deionized water (1:10)
at room temperature for 1 hr, filtered (0.2-μm syringe fil-
ters) and analysed with high-temperature catalytic oxida-
tion (HTCO) using a Shimadzu TOC-V CPH analyser
(Shimadzu Scientific Instruments, Kyoto, Japan). Soil
mineral N was extracted with 1 M KCl (1 hr agitation at
soil-to-solution ratio 1:5), and after filtration (0.7 μm glass
fibre filter) the NO3-N and NH4-N concentrations of the
extracts were analysed with a continuous flow analyser
(Skalar San++ System, Skalar Analytical BV, Breda, The
Netherlands). The total dissolved N (TDN) content of the
KCl extracts was analysed after oxidative digestion with
potassium persulphate (K2O8S2, Skalar San++ system).
Finally, net nitrogen mineralization was calculated as the
difference between soil mineral N contents (sum of NO3-
N and NH4-N) at the end and at the beginning of the
incubation period (0–14 days and 14–30 days).
For the soil respiration measurements conducted on
days 1, 3, 5, 10, 15, 21 and 30 from the start of incubation,
the incubation bottles were first aerated with pressurized
air and then closed with a chlorobutyl septum for 24 hr
before sampling of gas from the headspace. The sample
for background concentration of CO2 was taken from an
infusion bottle aerated and incubated without soil. Gas
samples of 8 ml were taken with an injection needle
pushed through the septum and introduced to He-flushed
and evacuated 3-ml Exetainer® vials. The CO2 concentra-
tions of gas samples were analysed with a gas chromato-
graph (Agilent 7890B GC system, customized, equipped
with thermal conductivity detector (TCD), flame ioniza-
tion detector (FID) and electron capture detector (ECD)
and an autosampler; Agilent Technologies, Santa Clara,
CA, USA). After subtracting the background concentra-
tion from the concentrations in the bottles incubated
with soil, the respiration rate at each sampling time was
calculated as mg of CO2-C per kg of soil dry matter (DM,
oven drying at 105
C) per day and as mg of CO2-C per kg
of soil OC per day. Respiration between sampling times
was estimated with linear interpolation.
2.3 | Nitrogen fertilization and yield
responses
For estimating the inherent soil N supply under field con-
ditions, unfertilized areas within fertilized cereal fields
were established at 13 fields in 2016 and at 33 fields in
2017 (Table 1). In 2016, fields of the research stations of
the Natural Resources Institute Finland (Luke) in
Jokioinen in south-western Finland and Maaninka in
east-central Finland were used. In 2017, in turn, 17 fields
of the experimental stations, including 12 sampled
already in 2016 (Table 1), were complemented with 16T
A
B
L
E
1
(C
on
ti
n
ue
d)
F
ie
ld
C
ro
p
Sa
n
d
Si
lt
C
la
y
C
N
C
/N
C
la
y/
C
C
E
C
p
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SOINNE ET AL. 5
fields of private farmers. In each unfertilized area and
adjacent fertilized area, a representative central sampling
point was chosen, and three surrounding sampling points
were placed on a circumference at an approximate dis-
tance of 5 m from the central point. Yield samples were
collected from unfertilized and fertilized areas, but soil
samples for agronomic soil testing and textural analysis
were only taken from one point per field and were
expected to represent the texture of the uniform sam-
pling area.
Plant samples for determining cereal yields were col-
lected from the same four sampling points before harvest
in late summer 2016 and/or 2017. The yield samples were
harvested after ripening by cutting down the entire plant
stand within a 75 cm × 75 cm frame. The plant material
was separated into straw and spikes, dried at 60
C and
weighed. The spikes harvested from the central sampling
point were further threshed to separate grains from the
chaff, after which the share of grains in the spikes was
calculated. This share was used in converting the spike
yields to grain yields also in the three surrounding
sampling points. Finally, the grain yield in each site was
calculated as a mean of the four sampling points. For the
crop N uptake, the content of total N (TN) in the grains
and straw from the central sampling point were analysed
by the Kjeldahl method (Foss Kjeltec™ 8,400; Hilleroed,
Denmark). Crop N uptake per hectare was calculated as
a sum of N in the grains and straw. This crop N uptake
was compared with grain yield and with the clay soil N
mineralization potential estimated on the basis of the lin-
ear regression between the clay/C ratio and N minerali-
zation in the laboratory incubation. Furthermore,
apparent N recoveries were calculated as percentages of
added N measured in the grains and straw at harvest. For
this aim, the difference in crop N uptake between fertil-
ized and unfertilized areas was divided by the amount of
applied N. The agronomic N use efficiency was calculated
by dividing the difference in crop yield between fertilized
and unfertilized areas by the amount of applied fertil-
izer N.
Soil samples for the agronomic soil test (STP) (0.5 M
ammonium acetate-acetic acid extraction (AAAc,
FIGURE 1 Locations of fields
where soil and yield samples were
collected. Number (≥2) indicates the
number of fields in the location
shown by the circle
6 SOINNE ET AL.
pH 4.65), Vuorinen & Mäkitie, 1955), textural analysis,
pH and potential cation exchange capacity (CEC) were
collected from the 0–20-cm surface layer at each central
sampling point. For analyses of OC and total N concen-
trations, a 4.5-cm-diameter soil core (0–20 cm) was
augered, dried at 40
C, weighed and milled through a 2-
mm sieve. The C and N were analysed by dry combustion
(Leco TruMac CN, LECO Corporation, St. Joseph, Michi-
gan, USA), assuming that in these acidic soils very low in
carbonates, the amount of total C is practically equal to
the amount of OC. The C and N concentrations were cal-
culated per mass of oven dry (105
C) soil.
2.4 | Statistical analyses
The relationship between soil C and mineralized N (mg
N kg−1 day−1), and the clay/C ratio and mineralized N in
clay soils during the incubation study was assessed with
linear regressions. A broken-stick regression model was
fitted to C% or clay/C and CO2 production (mg C kg
−1
day−1) in clay soils and the change points were modelled
with a nonlinear model using least squares through an
NLIN procedure in SAS.
The linear relationship between clay/C ratios and N
mineralization in the incubation study was applied in
estimating the net N mineralization in clay soils of the
field experiment. To characterize the relationship
between the calculated estimates of the N mineralization
and actual crop N uptake in the field, a linear regression
line was fitted to the results.
The relationships between selected soil properties (C
%, N%, Clay%, Clay/C and CEC) and grain yields were
investigated with Spearman correlations. Based on the
preliminary screenings the fields were divided into two
soil types; coarse (clay <30%, in 2016 n = 4 and in 2017
n = 16) and clay (clay ≥30%, in 2016 n = 9 and in 2017
n = 17) soils. A linear regression line was fitted to the
grain yield (unfertilized or fertilized) and soil C% or
Clay/C of clay soil fields. Similarly, a linear regression
line was fitted to the grain yield (unfertilized or fertilized)
and CEC of coarse-textured fields. To analyse whether
the relationship between the soil properties (C%, Clay/C
in clay soils and CEC in coarse-textured soils) and grain
yields differed between fertilized and unfertilized areas,
linear mixed models with C% (or Clay/C or CEC in other
models) and the soil type and their interaction as fixed
effects, were used. Correlated years from the same sites
were taken into account with a G-side random effect, and
unfertilized and comparable fertilized areas with an R-
side random effect having a compound symmetry covari-
ance structure. Comparisons of the slopes were
TABLE 2 Properties of the soils used in the laboratory incubation study in ascending order of clay %
Field Location n pH
% mg kg−1
Clay C N Clay/C WEOCa NH4-N NO3-N TDN
b
4 A 4 6.4 5 2.1 0.21 2 83 0.29 14.0 21.4
4 b 2 6.2 13 2.3 0.22 6 76 0.33 17.7 26.5
8 C 4 6.8 14 2.0 0.20 7 80 0.37 10.5 15.4
8 d 4 6.6 15 2.1 0.19 7 76 0.33 9.7 14.7
4 e 2 5.8 30 9.7 0.63 3 170 0.77 19.9 30.8
20 f 4 6.0 44 3.3 0.33 13 160 0.81 15.5 23.3
20 G 4 5.9 47 3.5 0.34 13 157 0.86 21.4 28.2
27 h 4 5.0 68 8.5 0.69 8 157 0.73 23.3 40.1
27 I 4 5.0 69 7.2 0.60 10 148 0.63 21.6 35.8
28 J 4 6.3 71 3.2 0.33 22 213 0.75 12.9 19.9
29 K 4 6.0 71 3.6 0.35 20 196 0.94 14.0 22.3
29 l 4 6.0 73 3.5 0.35 21 240 0.70 10.1 17.6
29 m 4 6.0 73 3.8 0.35 19 252 0.67 17.2 25.6
28 n 4 6.3 74 3.2 0.31 23 230 0.70 11.1 17.3
28 o 4 6.5 75 3.0 0.30 25 165 0.80 11.3 17.8
The field numbering corresponds to that used in Table 1. Locations marked in capital and small letters refer to samples taken from fertilized
and unfertilized areas, respectively. Values are means of 2–4 replicate samples (n) taken from each sampling location. Soil texture (clay%)
was measured from one sample per location.
aWater extractable organic carbon.
bTotal dissolved nitrogen.
SOINNE ET AL. 7
conducted with two-tailed t-tests. All models were cre-
ated using the GLIMMIX procedure of the SAS Enter-
prise Guide 7.1 (SAS Institute Inc., Cary, NC, USA).
3 | RESULTS
3.1 | Potential net N mineralization and
soil respiration
In laboratory incubation, net N mineralization generally
increased with increasing soil C%, but in clay soils with
similar C% the amount of N mineralized varied largely
(Figure 2a). The mean daily CO2-C production over the
30-day incubation increased with soil C% up to the
change point of C% 3.4, after which the broken stick
regression model showed a change in the relation and
the increase in respiration rate with soil C% slowed down
(Figure 2b). In coarse-textured soils (clay <30%), the C%
was lower than in clay soils, and consequently, the respi-
ration remained lower. When expressed as CO2-C pro-
duced per kg of soil C, in contrast, respiration rates were
lowest in soils with C > 7% (Supporting Information 1).
In clay soils, there was a clear negative linear rela-
tionship between net N mineralization and the soil clay/
C ratio (Figure 3a). The net mineralization was low in
coarse-textured soils, and similar rates of net N minerali-
zation were measured in coarse-textured soils with C% of
2.3 and in clay soils with C% of 3.2. Clay soils also
exhibited a clear negative relationship between the clay/
C ratio and respiration rate but only in soils with clay/C
ratios higher than 18 (Figure 3b). In clay soils with a
clay/C ratio below 18, the respiration was not related to
the clay/C ratio. In coarse-textured soils, respiration was
low despite the low clay/C ratio (Figure 3b).
3.2 | Soil net N mineralization and crop
N uptake
The estimated potential net N mineralization
(mg N kg−1 day−1) in the clay soils, acquired with the
regression equation accounting for the soil clay/C ratio
based on the incubation experiment, correlated fairly
well with the crop N uptake at the unfertilized sites in
the field (Figure 4). As expected, there was a significant
positive correlation between the measured crop N uptake
and grain yield despite the different crops, fertilization
levels and variation in the weather conditions between
sites and years (Figure 5). In the current grain yield range
of up to 7,000 kg ha−1, the correlation between crop N
uptake and grain yield appeared linear. Thus, the rela-
tionships between soil properties and grain yield are simi-
lar, as the relationships between soil properties and crop
N uptake would be.
3.3 | Yields in unfertilized and fertilized
areas in the field
In the unfertilized areas of the coarse-textured soils, no
significant correlation was found between soil C% or N%
(a) (b)
FIGURE 2 Net nitrogen mineralization (a) and respiration rate (b) over the 30-day laboratory incubation in coarse-textured (clay%
<30) and clay (clay% >30) soils and in clay soils with C% higher than 7 plotted against soil C%. Red colour indicates that the soil samples
were taken from the fertilized area. In (a) the linear regression line was fitted to the clay soil data points (p < .0001). In (b), the broken-stick
regression model showed a significant positive relationship below the change point C 3.4% (p < .0001) and after the change point (p < .0001)
8 SOINNE ET AL.
and crop yield (Table 3), but, however, the fertilized yield
and the agronomic N use efficiency increased with
increasing C% and N%. Because there was a significant
positive relationship between soil C% and N%
(Supporting Information 2), further results are presented
only in relation to soil C%. In clay soils, a positive
correlation was found between soil C% and both the fer-
tilized and unfertilized yields, but there was a large varia-
tion in yields especially among the soils with C%, ranging
between 2 and 4 (Figure 6). Clay and the clay/C ratio cor-
related negatively with yields in both unfertilized and fer-
tilized areas (Figure 7). The slope of the regression
(a) (b)
FIGURE 3 Net nitrogen mineralization (a) and respiration rate (b) over the 30-day laboratory incubation in coarse-textured (clay%
<30) and clay (clay% >30) soils and in clay soils with C% higher than 7 plotted against the soil clay/C ratio. Red colour indicates that the soil
samples were taken from the fertilized area. In (a), the linear regression line was fitted to the clay soil data points (p < .0001). In (b), the
broken-stick regression model showed a negative relationship between respiration rate and clay/C ratio in clay soils having a clay/C ratio
higher than 18 (p = .0135), whereas in clay soils with clay/C ratio smaller than 18, the respiration rate was not related to clay/C
ratio (p = .831)
0
20
40
60
80
100
120
140
0.2 0.4 0.6 0.8 1.0 1.2
N uptake  
kg ha-1
Estimated net N mineralization, mg kg-1 d-1
FIGURE 4 Net N mineralization rate in the clay soils as
estimated from the relationship between mineralization and soil
clay/C ratio in the incubation study plotted against crop N uptake
in unfertilized areas of the fields. Linear regression line was fitted
to the results (r2 = 0.5667, p < .0001)
0
20
40
60
80
100
120
140
160
0 2,000 4,000 6,000 8,000
N uptake  
kg ha-1
Yield, kg ha-1
FIGURE 5 Grain yields (kg ha−1) in fertilized (open black
circles) and unfertilized (grey) areas plotted against N uptake
(kg ha−1) in the central sampling points of the fields. Linear
regression line fitted to the results including both fertilized and
unfertilized areas (r2 = 0.8787, p < .0001)
SOINNE ET AL. 9
equation of yield over clay/C ratio did not differ between
unfertilized and fertilized soils (p = .722). In coarse soils,
the clay/C ratio was not related to the yields, but instead,
the yields in unfertilized and fertilized areas were posi-
tively related to CEC (Figure 8). However, fertilization
did not induce larger yield increases at lower CEC than
at higher CEC, as indicated by broadly parallel slopes of
the regression equations in unfertilized and fertilized
treatments (p = .731). The six unfertilized areas with no
fertilization for two sequential years did not produce
lower yields in the second year than in the first year
(Supporting Information 3).
The yield differences between adjacent fertilized and
unfertilized areas within the same field corresponded to
on average 16 kg additional grain yield per 1 kg of
applied fertilizer N. However, the variation was large,
and in some areas, yield increase was not achieved at all
with N fertilization. Consequently, the apparent N recov-
ery ranged between 0 and 79%, being on average 35%. As
the yield gained without fertilization increased, the maxi-
mum additional yield per kg of fertilizer N decreased
(Figure 9). At sites with low inherent productivity (low
yields without fertilization), the effectiveness of N fertili-
zation varied considerably.
TABLE 3 Spearman correlations between soil parameters (organic carbon%, C%; total nitrogen, N%; clay content, clay%; clay/C ratio;
cation exchange capacity, CEC) and cereal yields (unfertilized yield, yield 0; fertilized yield, yield N; yield increase obtained with 1 kg of
fertilizer N, agronomic NUE [N use efficiency]) of (a) coarse-textured soils and (b) clay soils
(a)
Coarse-textured
soils
C% N% Clay% Clay%/C% CEC
N = 19 R p R p R p R p R p
Unfertilized yield 0.075 .7612 0.031 .9005 −0.064 .7931 −0.110 .6544 0.493 .0320
Fertilized yield 0.470 .0422 0.486 .0349 0.236 .3310 0.096 .6965 0.453 .0517
Agronomic NUE 0.729 .0004 0.805 <.0001 0.283 .2409 0.025 .9204 0.097 .6938
(b)
Clay soils
C% N% Clay% Clay%/C% CEC
N = 27 R p R p R p R p R p
Unfertilized yield 0.607 .0008 0.571 .0019 −0.395 .0417 −0.6815 <.0001 −0.315 .1090
Fertilized yield 0.498 .0082 0.500 .0079 −0.457 .0165 −0.6809 <.0001 −0.367 .0599
Agronomic NUE −0.074 .7139 −0.056 .7829 0.032 .8739 0.0092 .9638 −0.006 .9783
Statistically significant (P < .05) correlations indicated in bold.
 
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0 2 4 6 8 10
kg ha-1
C%
0 2 4 6 8 10
kg ha-1
C%
Clay 2016 Clay 2017
Coarse 2016 Coarse 2017
(a) (b)
FIGURE 6 Grain yields (kg ha−1) in 2016 and 2017 from (a) unfertilized and (b) fertilized soils plotted against soil C%. Regression lines
were fitted to the clay soil data points with a significant linear relationship between C% and (a) unfertilized yield (r2 = 0.3552, p = .001) and
(b) fertilized yield (r2 = 0.2925, p = .0037)
10 SOINNE ET AL.
4 | DISCUSSION
4.1 | Organic matter mineralization and
soil N supply affected by varying clay
contents
In favourable temperature and moisture conditions, the
soil respiration rate is generally limited by the supply of
substrate (Wang, Dalal, Moody, & Smith, 2003). As
expected, because soil CO2 production is largely depen-
dent on soil OC (Wang et al., 2003), we measured the
lowest respiration rates in our laboratory incubation in
soils with the lowest C%. The respiration rate increased
linearly up to C% 3.4, but the soils with C% higher than 7
produced less CO2-C than expected based on their C%. In
these high-C% soils, the WEOC did not clearly differ from
the other clay soils with much lower C% (coarse soils
were clearly lower in WEOC), indicating solubility-
related differences in OM quality. The dissolved OC is
probably the most bioavailable fraction of soil OM (Mar-
schner & Kalbitzb, 2003), and thus, in addition to the
total amount of OC and soil water content (Schjønning,
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0 5 10 15 20 25
kg ha-1
Clay/C
0 5 10 15 20 25
kg ha-1
Clay/C
Clay 2016 Clay 2017
Coarse 2016 Coarse 2017
(a) (b)
FIGURE 7 Grain yields (kg ha−1) in 2016 and 2017 from (a) unfertilized and (b) fertilized soils plotted against the soil clay/C ratio.
Regression lines were fitted to the clay soil data points with a significant negative linear relationship between the clay/C ratio and (a)
unfertilized yield (r2 = 0.5866, p < .0001) and (b) fertilized yield (r2 = 0.4873, p < .0001)
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0 10 20 30 40
kg ha-1
CEC, cmol kg-1
0 10 20 30 40
kg ha-1
CEC, cmol kg-1
Clay 2016 Clay 2017
Coarse 2016 Coarse 2017
(a) (b)
FIGURE 8 Grain yields (kg ha−1) in 2016 and 2017 from (a) unfertilized and (b) fertilized soils plotted against soil cation exchange
capacity (CEC). Regression lines were fitted to the coarse soil data points with a significant linear relationship between CEC and (a)
unfertilized yield (r2 = 0.2800, p = .0198) and (b) fertilized yield (coarse soils: r2 = 0.2117, p = .0475)
SOINNE ET AL. 11
Thomsen, Moldrup, & Christensen, 2003), the properties
of OM that determine the solubility of soil OC regulate
the soil OC mineralization.
The sorption of organic molecules onto mineral parti-
cles affects soil OM turnover through reduced microbial
access to the substrate (Dungait, Hopkins, Gregory, &
Whitmore, 2012; Schmidt et al., 2011). Lower soil OM
mineralization with increasing soil clay content has been
reported (Cote, Brown, Pare, Fyles, & Bauhus, 2000;
Hassink, 1997; McLauchlan, 2006; Wang et al., 2003),
and in accordance with this, our results showed that
among the soils with similar C%, the lowest respiration
was observed in soil with the highest clay content
(Supporting Information 1). In soils with a clay/C ratio
above 18, the respiration rate clearly decreased with
increasing clay/C ratio, suggesting that in soils with a
clay/C ratio below 18 the accessibility of substrate does
not significantly reduce respiration.
Because of the tightly coupled cycles of C and N,
higher OC mineralization rates may indicate also higher
potential of soils to supply N. We measured clearly higher
respiration rates for clay soil with C% of 3.2 than for
coarse soil with C% of 2.1; however, the net N mineraliza-
tion rate did not differ between these two soils. This sug-
gests that the factors regulating mineralization of organic
N partly differ from those of organic C. The incubation
experiment showed a linear decrease in the net N miner-
alization with an increase in the clay/C ratio and may
indicate a more important role of clay in controlling the
availability and mineralization of organic N than in regu-
lating the respiration. Thus, our results support the find-
ing that N-rich organic molecules that sorb directly onto
mineral surfaces (Dümig, Häusler, Steffens, & Kögel-
Knabner, 2012; Kopittke et al., 2020) are better protected
by clay particles than molecules with a higher C/N ratio.
This is in agreement with the previous research reporting
higher C/N ratios in unprotected OM than in organic
molecules adsorbed onto clay particles (Guggenberger,
Christensen, & Zech, 1994; Kleber, Sollins, & Sut-
ton, 2007).
Soil aggregation and pore structure are linked inti-
mately to the processes that stabilize OM and protect it
from microbial degradation. The incubation experiment
was performed at room temperature with sieved
(<5 mm) soil samples, and the soil pore structure was
changed by loosening in pretreatments. Thus, the deter-
mined respiration and net N mineralization reflect the
potential for mineralization, and the results are not
directly applicable to field conditions (e.g., Moinet et al.,
2016). Hassink (1992) observed that sieving caused a tem-
porary increase in the mineralization of C and N, which
interacted with soil texture such that the relative increase
in C and N mineralization due to sieving was larger for
loamy and clayey soils than for sandy soils. On average,
the daily net N mineralization in our incubation experi-
ment was 0.57 mg kg−1. By using typical bulk densities
and accounting for the textural differences, this minerali-
zation rate can be calculated to equal
1.23 kg N ha−1 day−1 (0.7–2.0 kg N ha−1 day−1). This
value is somewhat higher than available earlier estimates
from field experiments in comparable climatic condi-
tions. Lindén, Lyngstad, Sippola, Søegaard, and
Kjellerup (1992) reported an average N mineralization
rate of 0.40 kg ha−1 day−1 in field experiments conducted
in Denmark, Finland, Norway and Sweden in different
soils. Delin and Lindén (2002) reported that daily net N
mineralization within one field in south-western Sweden
varied between 0.10 and 0.93 kg N ha−1 day−1. Compared
with the reported field observations, the results of the
laboratory incubation agree quite well with the observa-
tion by Stenger, Priesack, and Beese (1995) that minerali-
zation rates of disturbed samples were approximately
twice as high as those of undisturbed samples.
4.2 | Yield levels as affected by soil
organic carbon and texture
According to Ros (2012), irrespective of geographical
location and land use, soils with higher OC content have
higher N mineralization rates, and thus, higher inherent
N supply. Likewise, Hijbeek et al. (2017) and Schjønning
et al. (2018) indicated a reduction in the need for mineral
N with an increase in soil OM content. In previous stud-
ies, both positive or conditionally positive effects (e.g. on
a limited range of C%) (Majumder et al., 2008; Oldfield,
FIGURE 9 Relationship between unfertilized grain yields
(kg ha−1) and agronomic N use efficiency (agronomic NUE,
additional grain yield kg ha−1 per kg of fertilizer N) in clay and
coarse-textured soils in 2016 and 2017
12 SOINNE ET AL.
Bradford, & Wood, 2019; Oldfield, Wood, & Bradford,
2018) and a lack of a relationship or even negative influ-
ences in experiments where N supply had been secured
with fertilization (Hijbeek et al., 2017; Schjønning et al.,
2018) of soil OM on yields have been reported. In the
results of Oldfield et al. (2018) showing on average higher
maize and wheat yields with higher soil OC, the yield
increases levelled off at c. 2% OC. In our field measure-
ments, a positive relation was found between soil C% and
grain yields in fertilized and unfertilized clay fields,
although in all studied clay soils C% was higher than 2.
In the coarser unfertilized soils with C% varying between
1.3 and 3.7, the relation between the yields and soil C%
was insignificant but the yields increased with increasing
CEC, suggesting that the availability of nutrient cations
was more important in regulating the yields in these
fields. Further in coarse soils, the agronomic N use effi-
ciency was higher in soils with higher C%, also indicating
that the OC enables efficient use of added fertilizer N.
This further highlights the importance of soil OC for
other soil productivity factors (water retention, structure,
macro- and micronutrients) besides N supply.
In agreement with the results of laboratory incuba-
tion of clay soils, unfertilized areas in fields with a
higher clay/C ratio resulted in lower grain yields,
suggesting lower mineralization rates due to the N-rich
molecules being protected by clay mineral particles.
However, a similar trend of decreasing yields with
increasing clay/C ratio was detected in the areas receiv-
ing mineral N fertilization. This conflicts with the gen-
eral assumption that in soils with properties indicating
low inherent N supply the yields would be increased
more with proper N fertilization. Especially in soils
with a clay/C ratio higher than 15, the variation in
yields in fertilized areas was large; this implies that
some of the areas suffered from yield restrictions that
prevented the utilization of fertilizer N. Yield restric-
tions non-compensable by N fertilization may derive
from, for instance, shortages of other nutrients or
defects in soil structure affecting root growth, water
infiltration and storage, and air–soil gas exchange (Olfs
et al., 2005). In poor structured soils with restricted aer-
ation, N limitation due to gaseous losses via denitrifica-
tion is also possible. In fact, high clay/C ratios have
been found to indicate poor structure in clay soils
(Johannes et al., 2017; Soinne, Hyväluoma, Ketoja, &
Turtola, 2016). Thus, for clay soils in a cool and humid
climate, the higher the clay content, the more OC is
needed to compensate for it and to maintain both soil
structural stability and efficient use of applied N.
In Denmark, Schjønning et al. (2018) found the yields
of non-N-fertilized winter wheat to range from less than
1,000 kg ha−1 to more than 9,000 kg ha−1. In the data set
of Valkama et al. (2013), which included 61 relevant N
fertilizer trials conducted with spring cereals in Finland
since the 1940s, the yields of non-N-fertilized control
areas ranged similarly, although with a lower upper
range, from 1,000 to 4,000 kg ha−1. In the present study,
the wide range of yields achieved without N fertilization
(from 400 to 6,000 kg ha−1) is in agreement with previous
findings and reflects substantial variation in the inherent
productivity of the studied soils. However, in fertilized
areas, the variation in yields was roughly similar to that
in the unfertilized areas. The very low apparent N recov-
eries occasionally recorded in the current study (<10% or
even zero) indicate a serious lack of synchrony between
the supply and demand of N. This will result in a nega-
tive economic impact and unnecessary environmental
hazards (Cassman, Dobermann, & Walters, 2002).
Together, the large differences in the yield response to
added N between fields (additional yield per kg of fertil-
izer N varying from −2 to 37 kg) and the high variation
in yields in the fertilized areas indicate a coinciding influ-
ence of growth factors other than N supply.
The observation that the incremental agronomic N
use efficiency decreased with increasing non-N-fertilized
yield level is in agreement with the previous findings of
Valkama et al. (2013). It is also compatible with
Mitscherlich's law of diminishing returns, which assumes
that the fraction of fertilizer addition used by the crop
becomes smaller and smaller as the production
approaches its maximum level (Ferreira, Zocchi, &
Baron, 2017). Overall, the results of the present and ear-
lier studies (Kachanoski, O'Halloran, Aspinall, & Von
Bertoldi, 1996; Lory & Scharf, 2003; Valkama et al., 2013)
demonstrate that the yields achieved with (near-) opti-
mum N fertilization are poor predictors of responsiveness
of a crop to added N at a given site. Therefore, instead of
estimating the fertilizer N requirement based on yield
expectancy, determination of the ability of added N to
increase the yield site-specifically by comparison of fertil-
ized and non-N-fertilized areas appears recommendable.
The efficiency of fertilizer N use could be greatly
enhanced by the allocation of higher rates of N to respon-
sive sites and reducing inputs to non-responsive sites,
which may, according to the current data and
Kachanoski et al. (1996), be the highest and lowest-yield-
ing sites, respectively.
5 | CONCLUSIONS
In boreal clay soils with varying cultivation and fertili-
zation history, both the net N mineralization and soil
productivity decreased with increasing clay/C ratio. Our
results suggest that for soils with high clay content, the
SOINNE ET AL. 13
protective effect of clay on soil organic N should be
considered as one factor when estimating the inherent
N supply of the soil. In soils with a high clay/C ratio,
the agronomic N use efficiency varied considerably,
suggesting that in these soils the growth is often limited
by poor soil structure. In a cool and humid climate, the
higher the clay content, the more OC is needed to
ensure production of high yields in an environmentally
sustainable way. In studied coarse-textured soils with a
narrower range of OC, the yields increased with
increasing CEC while OC enabled efficient use of added
fertilizer N.
ACKNOWLEDGEMENTS
This study was funded by the Ministry of Agriculture and
Forestry (Finland) as part of the project “The role of
organic matter in soil productivity” (ORANKI) and by
The Strategic Research Council of the Academy of Fin-
land as part of the project “Multi-benefit solutions to cli-
mate-smart agriculture” (MULTA, grant number
327236). We thank the technical staff of Luke Jokioinen
and Maaninka for smooth cooperation in field selection
and management, and for laboratory analyses, and Eeva
Lehtonen for the map of the sampling sites. The partici-
pating farmers are also gratefully acknowledged.
AUTHOR CONTRIBUTIONS
Study concept and design:T. Salo, E. Turtola, H. Soinne,
R. Keskinen, S. Kanerva, A. Simojoki and V. Nuutinen.
Acquisition, analysis and interpretation of data: H.
Soinne, R. Keskinen, M. Räty, S. Kanerva, A. Simojoki, J.
Kaseva and T. Salo. Drafting the manuscript: H. Soinne,
R. Keskinen, M. Räty, S. Kanerva, A. Simojoki and T.
Salo. Critical revision of the manuscript for important
intellectual content: E. Turtola and V. Nuutinen. Statisti-
cal analysis: J. Kaseva and H. Soinne. Obtained funding:
T. Salo.
CONFLICTS OF INTEREST
The authors have no conflicts of interest to declare.
ORCID
Helena Soinne https://orcid.org/0000-0002-7965-6496
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Soinne H, Keskinen R,
Räty M, et al. Soil organic carbon and clay content
as deciding factors for net nitrogen mineralization
and cereal yields in boreal mineral soils. Eur J Soil
Sci. 2020;1–16. https://doi.org/10.1111/ejss.13003
16 SOINNE ET AL.