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Biomass and Bioenergy
journal homepage: www.elsevier.com/locate/biombioe
Research paper
How and why does willow biochar increase a clay soil water retention
capacity?
Kimmo Rasaa,∗, Jaakko Heikkinena, Markus Hannulab, Kai Arstilac, Sampo Kuljua,
Jari Hyväluomaa
aNatural Resources Institute Finland, Luke, FI-31600, Jokioinen, Finland
b BioMediTech Institute and Faculty of Biomedical Sciences and Engineering, Tampere University of Technology, FI-33101, Tampere, Finland
cUniversity of Jyvaskyla, Department of Physics, P.O. Box 35, FI-40014, Finland
A R T I C L E I N F O
Keywords:
Biochar
X-ray tomography
3D image analysis
Helium ion microscopy
Soil water retention
Plant available water
A B S T R A C T
Addition of biochar into a soil changes its water retention properties by modifying soil textural and structural
properties. In addition, internal micrometer-scale porosity that is able to directly store readily plant available
water affects soil water retention properties. This study shows how precise knowledge of the internal micro-
meter-scale pore size distribution of biochar can deepen the understanding of the biochar-water interactions in
soils. The micrometer-scale porosity of willow biochar was quantitatively and qualitatively characterized using
X-ray tomography, 3D image analysis and Helium ion microscopy. The effect of biochar application on clay soil
water retention was studied by conventional water retention curve approach. The results indicate that the in-
ternal pores of biochar, with sizes of at 50 and 10 μm (equivalent pore diameter), increased soil porosity and the
amount of readily plant available water. After biochar addition, changes in soil porosity were detected at pore
size regimes 5–10 and 25 μm, i.e. biochar pore sizes multiplied by factor 0.5. The detected pore size distribution
of biochar does not predict directly (1:1 compatibility) the changes observed in the soil moisture characteristics.
It is likely that biochar chemistry and pore morphology affect biochar-water interactions via e.g. surface
roughness and contact angle. In addition, biochar induced changes in soil structure and texture affected soil
moisture characteristics. However, the approach presented is an attractive pathway to more generalized un-
derstanding on how and why biochar internal porosity affects soil moisture characteristics.
1. Introduction
While biochar is considered as a potential measure to sequester
carbon into soil [1], its secondary effects on soil properties are often
controversial [2]. Biochar has been shown to increase soil water
holding capacity (WHC), but no solid understanding exists on the ef-
fects of, e.g., soil type, biochar quality or climate. Addition of biochar
into a given soil changes soil textural and structural properties. These
changes modify soil moisture characteristics in a specific manner de-
pending on biochar type and soil properties (indirect mechanism).
However, biochar as a highly porous material also directly affects soil
water holding capacity via its internal porosity. The aim of this paper is
to study how this internal porosity and pore size distribution are related
to the amount of plant available water in a clay soil.
A large number of papers describe biochar effect on soil hydraulic
properties and water holding capacity [3–6] with heterogeneous de-
scriptions of biochar physical quality. Physical characteristics of
biochar is most commonly studied by gas adsorption techniques ac-
companied by the Brunauer-Emmett-Teller (BET) modelling to de-
termine the specific surface area [7] or Barrett-Joyner-Halenda (BJH)
modelling to determine pore size distribution [8]. However, pore space
analysis based on gas adsorption measurements is limited to pores
smaller than 300 nm, whereby it does not tell much about the porosity
in the size range that is important for plant water uptake (i.e. micro-
meter-scale pores).
The inadequacy of gas adsorption studies and drawbacks related to
porosity measurements using mercury porosimetry have been ad-
dressed in earlier studies [9,10]. NMR Cryoporometry have also been
used to study porous materials at nanometer length-scale, however,
resolution in the upper end is limited to few micrometers [11]. Kinney
et al. [4] addressed the need for quantitative techniques to characterize
micrometer-scale pores within the biochars due to their expected im-
portance on soil WHC. All these studies support theory that large in-
ternal micrometer-scale pores of biochar may have remarkable direct
https://doi.org/10.1016/j.biombioe.2018.10.004
Received 21 December 2017; Received in revised form 28 September 2018; Accepted 2 October 2018
∗ Corresponding author. Natural Resources Institute Finland, Luke, Production systems, Biorefinery and Bioproducts, Finland.
E-mail address: kimmo.rasa@luke.fi (K. Rasa).
Biomass and Bioenergy 119 (2018) 346–353
0961-9534/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license 
(http://creativecommons.org/licenses/BY/4.0/).
T
effect on soil moisture characteristics (indirect effect refers to changes
in soil texture and formation of soil aggregates through bindings be-
tween biochar and soil particles, see Ref. [12]). However, the effect of
biochar micrometer-scale porosity on soil moisture characteristics has
not been quantitatively analysed.
We anticipate that the way forward in increasing understanding of
biochar-water interactions goes through fundamental research on in-
ternal pore structure of biochars. Lately, X-ray tomography and 3D
image analysis methods have been adapted to biochar research. This
approach is able to visualize the internal pore structure of biochar, and
to directly determine pore characteristics such as pore size distribution
and pore continuity. In recent studies X-ray tomography has been used
to characterize pore structure of biochars derived from various raw
materials (e.g. Pinus sylvestris, Miscanthus, Populus spp. L., cottonseed
hull, hay) with resolution ranging from 0.74 to 21 μm [13–17]. Hyvä-
luoma et al. imaged various types of biochars and hydrochars with ca.
1 μm resolution and found that considerable part of the biochar volume
consist of pores in size range relevant to hydrological processes and
storage of plant available water [18]. In addition, they found high
variation in porosity, pore size distribution and structural anisotropy
between different biochars. Also aggregates of biochar-amended soil
have been imaged with X-ray tomography, although at this length-scale
X-ray tomography resolution cannot capture the internal porosity of
biochar [19].
The lack of precise data on micrometer-scale porosity might be one
reason that the importance of biochar internal porosity directly af-
fecting the soil water holding capacity has been overlooked. In this
study we used X-ray microtomography, 3D image analysis and Helium
ion microscopy to study structural properties of willow biochar with
emphasis on micrometer-scale pores contributing to storage of plant
available water. Further, we conducted an incubation experiment to
study the effect of this specific biochar on soil moisture characteristic
and to identify pore regimes altered by biochar amendment. The re-
lationship between biochar pore system and its specific effect on soil
moisture characteristics is an attractive pathway towards development
of precisely tailored biochars aimed to enhance water use efficiency.
2. Materials and methods
2.1. Experimental soil
Soil for incubation experiment was taken from Kotkanoja long-term
drainage experiment site [20] located in Jokioinen in southern Finland
(N 60.82°, E 23.51°). The study material was collected from the topmost
10 cm layer of an annually ploughed plot in spring 2016. The heavy
clay soil had 64.8%, 30.5% and 4.7% of clay, silt and sand, respectively
(mass fractions determined by pipette method [21]). Soil organic
matter content was 9.2% (loss of ignition at 550 °C), carbon content
2.9% (Leco analysator), pH 6.3 and electrical conductivity
63.1 μS cm−1 (soil to water ratio 1:5).
2.2. Biochar
The biochar used in the experiment was pyrolysed in an indirectly
heated pilot-scale batch-type pyrolysis facility using Willow stem wood
(Salix sp, with bark) as raw material. Temperature profile of pyrolysis
process consisted of two steps. First, the temperature was raised to
280 °C, and then further to 320 °C with rates of 2.2 and 0.2 Kmin−1,
respectively.
The elemental analysis (CHNSO) of biochar was carried out using
FLASH 2000 series analyser and the results were used to calculate O:C
and H:C atomic ratios. The biochar was analysed for pH (SF EN 13037,
1:5 char to water ratio), electrical conductivity (EC, SF EN 13038, 1:5
char to water ratio), nutrients and heavy metals (SFS-EN 13650 Aqua
Regia extraction and ICP-measurement), BET surface area (ISO
9277:2010(E) Brunauer–Emmett–Teller (BET) Surface Area) and Ash
content (SFS 3008, loss of ignition at 550 °C).
The BET surface area of biochar produced was 6.8 ± 0.43m2 g−1.
The elemental mass fractions of C, H, N, S and O were 74.0 ± 0.0%,
4.1 ± 0.1%, 0.4 ± 0.0%, 0.0 ± 0.0% and 15.8 ± 0.3%, respec-
tively. The ash mass fraction of the biochar was 3.13 ± 0.15%. The
corresponding mole ratios of H:C and O:C were 0.66 and 0.16, re-
spectively. Biochar had low content of main nutrients (1.5 g kg−1,
3.6 g kg−1, 9.1 g kg−1 of P, K, Ca respectively) and heavy metals Cd, Cu
and Pb (0.79 mg kg−1, 6.8 mg kg−1,< 3mg kg−1, corresponding limits
are 1.5, 600 and 100mg kg−1, respectively) contents were below limits
set for soil amendments in Finland.
2.3. X-ray tomography
The X-ray computed microtomography imaging was conducted with
Zeiss Xradia MicroXCT-400 (Zeiss, Pleasanton, CA, USA) device. Source
voltage was 40 kV and source current was 250 μA. The pixel size was
1.14 μm. A 20× objective was used with 2 binning. 1600 projections
were taken in full 360°. Each projection was exposed with X-rays for 3 s.
No filters were used in the imaging process. Zeiss XMReconstructor
software with the filtered back projection algorithm was used in the
reconstruction of the image stacks.
For quantitative structural analysis the original grey-scale image
was de-noised and segmented into solid and void phases. Details of
these image processing steps have been described by Hyväluoma et al.
[18] and reviewed in Appendix A. The subvolume used in the further
image analysis consisted of 1.9·108 voxels (sample size ca. 0.5mm).
The porosity of the sample was calculated by dividing the number of
void voxels by the total number of voxel in the analysed volume. The
pore-size distribution was determined using an approach based on
mathematical morphology [22] by successively applying morphological
opening operations (sphere with radius r was used as the structuring
element) on the pore space [23]. The pore size based on morphological
opening closely relates to the stationary distribution of wetting and
nonwetting fluids in pore space and the related capillary pressure via
the Young-Laplace equation
=p γcosθ
r
2
c (1)
where γ is the surface tension, θ the contact angle, and r the pore radius
defined as the radius of the structuring element used in the morpho-
logical opening [24]. This method is particularly suitable for our pur-
poses where the pore-size distribution determined from imaged pore
space is linked to measured soil moisture characteristic curve. The
structural anisotropy of the sample was quantified by calculating the
degree of anisotropy using the method based on grey-scale gradient
tensor [25].
2.4. Helium ion microscopy
Helium ion microscopy (HIM) is a novel development within the
family of scanning beam microscopes. Instead of using electrons for
beam particles, as in conventional scanning electron microscopes
(SEM), helium ions are used. Helium ions have a focal depth that is
5–10 times larger than electrons in SEM, a beam spot size below 0.5 nm,
and very small interaction volume producing secondary electrons, re-
sulting in images with very high resolution. Another important ad-
vantage of HIM is its capability to image insulating materials without
charging effects (no metal coating is needed) as the positive surface
charging from helium ions and secondary electrons can be neutralized
with an electron flood gun. A Zeiss Orion NanoFab Helium ion micro-
scope was used in this work.
2.5. Soil sample preparation
Experimental soil was air-dried in room temperature and sieved
K. Rasa et al. Biomass and Bioenergy 119 (2018) 346–353
347
through 6mm mesh. Four different kinds of samples were prepared for
incubation experiment. Half of the samples had natural (NAT) ag-
gregate structure (< 6mm). The soil for other half was homogenized
(HOM) using a roller-mill and then sieved (2mm) in order to destroy
soil macro aggregates. Samples with NAT and HOM structure received
0% or 5% mass fraction biochar amendment (on dry matter basis).
Biochar used in the experiment was crushed into fine powder using
blender. Altogether 14 samples were prepared.
The incubation experiment was conducted in 190 cm3 (7.1 cm in
diameter) metal cylinders. One end of each cylinder was covered with a
thin fabric to prevent sample to loss from the cylinder. The cylinders
were filled with soil or mixture of soil and biochar. The samples were
gently shaken and pressed using 10 kg weight. Finally, the excess ma-
terial was removed by spatula.
2.6. Incubation
Soil samples were saturated with water for six weeks. Saturated
samples were moved to sandbox (Eijkelkamp) and the matric potential
was gradually adjusted to −10 kPa. Thereafter, the samples were in-
cubated under constant matric potential (−10 kPa) and temperature
(5 °C) conditions for six weeks. Cool incubation temperature was used
to hinder the microbial activity in the samples.
2.7. Soil moisture characteristic curve
Volume fraction of soil water in matric potential range from−0.3 to
−10 kPa was determined using sandbox (Eijkelkamp), whereas pres-
sure plate extractor (Soilmoisture Equipment Corp.) equipped with
100 kPa, 300 kPa bar and 500 kPa ceramic plates was used to produce
lower matric potentials. In each step the soil samples were let to
equilibrate before determination of the water content. The lowest ma-
tric potentials−1500 kPa and−4000 kPa, were created osmotically by
placing 1 g soil sample into desiccators equipped with containers with
saturated solutions of ammonium oxalate ((NH4)2C2O4) or sodium
chloride (NaCl), respectively. Thereafter the samples were removed
from desiccators and the water content of the samples were determined
by weighing the samples before and after drying them at 105 °C.
Equilibration time varied between 1 day (at−0.3 kPa) and 6 weeks (at
−1500 and −4000 kPa) depending on the matric potential.
2.8. Effect of biochar amendment on soil porosity
Data used to determine the soil moisture characteristic curve (SMC)
was also used to study the effect of added biochar on the pore size
distribution (See Eq (2). below). However, instead of using volume
fraction of soil water, the water contents were expressed as ratio of
water volume to dry mass of sample. This ratio was used to ensure that
results contain only the pore volume related to biochar for each pore
size interval. First, the soil water potentials were converted to ap-
proximate pore sizes using the Young-Laplace equation (Eq. (1)). While
Young-Laplace equation is derived for straight capillaries with circular
cross section and does not directly apply to soils with more complex
pore geometry, it can be used to determine so-called equivalent pore
diameter (EPD) d= 2r. Then, the water contents of the samples were
interpolated on 2.75 μm pore size interval using linear interpolation.
This increment corresponds to the increment in size of structure ele-
ment used in determination of pore size distribution from tomography
images.
Finally, the effect of biochar on soil porosity was calculated as
follow:
= − − −
− − −
v W W M W W MΔ ( )/ ( )/ ,p p p p p p p p p[ , Δ ] Δ 5% Δ 0% (2)
where Δv is the effect of biochar amendment on specific pore volume of
soil in each pore size interval (cm3/g), W is the volume fraction of soil
water of the sample (=pore volume, cm3), M is the dry matter weight
of the sample (g), p is the upper limit and p-Δp lower limit of pore size
interval (p= 2.75 μm, 5.5 μm, …; Δp= 2.75 μm) and ˂˃ 5% and ˂˃ 0%
represents the average of the samples with and without biochar.
Calculation was performed separately for two different soil structures
(HOM, NAT).
2.9. Statistical analysis
The dependence of plant available water on the biochar addition
and soil structure was studied using two-way analysis of variance.
Dependent variables were proportions (values between 0 and 1) and
therefore logit transformation was used to make the model assumption
applicable. For the presentation of the results, the estimated means
were transformed back to the original scale. Bonferroni correction was
applied to multiple comparisons. All statistical analyses were performed
using Matlab with Statistics Toolbox.
3. Results and discussion
3.1. X-ray tomography of willow biochar
The visual observation of the X-ray tomography image revealed that
the pore structure of willow biochar pyrolysed at 320 °C retained the
initial structural characteristics of fresh stem wood (Fig. 1). Obviously,
the vascular tissues (xylem) in sapwood and/or heartwood appear as
biochar porosity. Analogously to living wood tissue, these pores are
Fig. 1. X-ray tomographic reconstruction of
willow biochar (left). The 3D visualization
of tomography data was produced with
Paraview [29] and Blender. The tomo-
graphy data consisting of the imaging slices
were first rotated with ImageJ [30] in a way
that pores are perpendicular to the cutting
face. After this a cuboid piece was cut from
a sample. A 57 μm thick slice cut from the
middle of a cuboid piece is shown on right.
K. Rasa et al. Biomass and Bioenergy 119 (2018) 346–353
348
mainly responsible for water storage and transport within the biochar.
The biochar pore system was quantified by image analysis of the X-
ray tomography image. The results show that the total porosity of
biochar is 0.6 and the pore size distribution is bimodal with local
maxima around diameters 10 and 50 μm (Fig. 2 top panel). The degree
of anisotropy was 32.0 (see Hyväluoma et al. [18] for reference values
of degree of anisotropy for several wood based biochars) which proves
that porosity of the biochar is highly anisotropic and consists of parallel
cylindrical pores with minimum number of lateral connections (verified
by HIM, see below). In general, these findings are well in line with
earlier published 2D scanning electron microscopy images indicating
bimodal characteristics of fresh willow wood structure e.g. Refs.
[26,27]. In addition, a recent paper [28] discussing the effect of pyr-
olysis temperature on micrometer-scale porosity of willow (Salix
schwerinii ‘Amgunskaja') biochar shows similar bimodal pore char-
acteristics as in the present paper.
The used imaging resolution prevents observation of pores in sub-
micrometre size range. However, the low BET surface area
(6.8 ± 0.43m2 g−1) suggests poorly developed nanoporosity of the
studied biochar. This suggestion is in line with previous studies re-
porting development of pyrogenetic nanopores only in higher pyrolysis
temperatures [10,31,32]. To verify this hypothesis, we studied nano-
porosity of biochar pore walls using HIM, which indicated that cell wall
structures remaining in the biochar do not contain visible nano-scale
pores (Fig. 3). HIM observations and degree of anisotropy together
suggest that water storage and flow within willow biochar takes place
in “bundle of cylindrical capillaries” as is assumed in Eq. (1) (see also
discussion in Sec. 3.3).
3.2. Effects of biochar amendment on soil-moisture characteristics
Addition of biochar reduced the bulk density of NAT (from 0.91 to
0.87 g cm−3, P= 0.001) and HOM (from 1.08 to 0.93 g cm−3,
P < 0.001) samples compared to samples without biochar. The effect
was more pronounced in HOM samples, because in HOM samples bio-
char replaces more mineral soil than in the case of NAT. In NAT samples
biochar falls partially in pores between aggregates and thus affects the
soil bulk density less. These indirect effects of biochar amendment ex-
plain changes observed in the wet-end of SMC (related to pores with
larger size than the size of internal pores of biochar, see below).
Biochar application influenced soil moisture characteristics over the
entire range of measured matric potentials. The major changes occurred
at matric potentials above −316 kPa, while differences levelled out at
the dry-end of the curve (Fig. 4). According to Young-Laplace equation,
in matric potential of −300 kPa water remaining in soil is stored in
pores with EPD<1 μm. Biochar addition thus modifies soil porosity
especially at pore size regime EPD>1 μm. In general, this finding in-
dicates that biochar with high micrometer-scale porosity, observed with
X-ray tomography and 3D image analysis, reflects directly to soil WHC
in regime important for plant water uptake (Fig. 4, Table 1).
Closer inspection of SMC for NAT and HOM samples (Fig. 2) reveals
that biochar application increased porosity in two specific regimes. In
both soils (irrespective of soil structure), a clear increase in porosity
occurred at pore sizes around 25 μm (EPD). The other maximum is
approximately at EPD 5 μm and 10 μm for NAT and HOM samples, re-
spectively. These peaks correspond the pore sizes observed in the bio-
char (10 and 50 μm, EPD) multiplied by factor 0.5 for larger pores. This
factor differs slightly between NAT and HOM samples for smaller pore
class. The fact that a bimodal shape is observed both in the pore-size
distribution determined with image analysis and derived from SMC
provide strong support for the inference that biochar affects the soil
moisture characteristics greatly via direct mechanism. In next section
some biochar-related properties affecting how biochar internal porosity
translates to changes observed in SMC are discussed in more details.
3.3. Biochar pore sizes vs. changes in soil moisture characteristics
There are several reasons why it is unlikely that a one-to-one
compatibility between 3D image analysis and SMC results will be ob-
tained. Conversion of the SMC to porosity response curve was by using
the Young-Laplace equation with generalized assumptions which are
typically not fully valid for any practical porous material including the
pore system considered here. One assumption is that pore space consists
of a bundle of cylindrical capillaries with uniform circular cross section
of different sizes and perfectly smooth surfaces. In addition it is as-
sumed that pore walls are chemically homogeneous and perfectly
wetting, i.e., in the Young-Laplace equation cos θ=1 at all pore sur-
faces.
Fig. 2. Pore size distribution determined by 3D image analysis of X-ray tomo-
graphy image (top panel) and the change in the pore size distribution due to
biochar addition for NAT (middle panel) and HOM (bottom panel) samples.
Effect of biochar on soil porosity (Δv, cm3 g−1 soil) was determined by sub-
tracting the soil water content (expressed as a ratio of water volume to dry mass
of sample) of pure soil from that of soil-biochar mixture as described in Sec. 2.8.
Matric potential was converted to equivalent pore size using Eq. (1).
K. Rasa et al. Biomass and Bioenergy 119 (2018) 346–353
349
Practically all pore surfaces are rough. Wenzel's model is widely
used to describe the effect of surface roughness on the contact angle
[33]. Another effect of surface heterogeneities is so-called contact angle
hysteresis which means that contact angle is not unique but varies
between two extreme values known as receding and advancing contact
angles. Also, assumption of perfectly wetting pore walls is not realistic.
To account for both non-perfect wetting and roughness, one can write
=
∗cos θ r cos θ (3)
where r is the roughness factor defined as the ratio of the true surface
area to the apparent (projected) area and θ* is so-called intrinsic con-
tact angle. In reality biochars are subcritically hydrophobic
(0° < θ* < 90°) and pore surfaces rough, whereby cos θ*< 1 and
r>1. Therefore, depending on the physical and chemical properties of
pore walls, pore sizes can be either larger or smaller than those deduced
from SMC. The importance of wetting properties on the hydrologic
effects of biochar on soil has been demonstrated by Suliman et al. [34].
It must also be emphasized that simple approaches to wettability
(including the discussion above) do not consider complicated time-de-
pendent behavior observed in many natural systems. Wettability of
many natural materials, including soil and biochar, is a highly complex
phenomenon which is not yet properly understood.
In practice, it is difficult to determine the contact angle needed to
properly use the Young-Laplace equation. The commonly used method
that is based on optical determination of the contact angle for a sessile
drop does not measure intrinsic contact angle as the measurement is
performed on a rough surface of material. Sessile drop method has been
used to determine contact angle for biochars [15,35]. It is important to
note that this large-scale roughness does not correspond to the small-
scale roughness used in Eq. (2). The intrinsic contact angle should be
determined at the three-phase contact line within the pore which is not
possible in practice. Furthermore, the derived roughness factor r in Eq.
(2) is the roughness at pore walls which also is practically impossible to
determine.
The discussion above explains the factors why there is not one-to-
Fig. 3. Helium ion microscopy images of willow
biochar. Size of the images (a) 1 mm, (b) 50 μm, (c)
4 μm, and (d) 2 μm. In image (a) overall porosity of
biochar is shown while image (b) is focused on pores
with EPD of approx. 5–10 μm. Image (c) shows pore
wall between the biochar pores and (d) is the same
pore wall with higher magnification indicating that
there is no visible connections between individual
pores.
Fig. 4. Soil moisture characteristics curve for each studied soil structure and
biochar combinations. Presented results are average of each treatments (n= 4
and n=3 for samples with and without biochar addition, respectively). Error
bars denote standard deviations.
K. Rasa et al. Biomass and Bioenergy 119 (2018) 346–353
350
one correspondence between the pore sizes determined by image ana-
lysis and those derived from SMC. We also observed that there is dif-
ferent transform coefficient between the large pores and small pores.
One explanation could be the existence of constrictions or throats in the
biochar pores that could be formed, e.g., by tar compounds which could
partially clog the pores.
Image analysis and porosity response derived from SMC can be di-
rectly compared only if all pores are connected to the sample surface
directly or through larger pores. The Image analysis determines pore
size distribution while SMC method is related to the size distribution of
pore throats. Even though the pores in the used biochar were tubular
and did not have obvious throats (see Secs. 3.1 and 3.2), we never-
theless estimated the effect of pore throats on the comparison by de-
termining the throat size to pore size ratio using the method described
in Appendix B.
In the analysis, we separated the two pore size classes present in the
studied biochar by using pore diameter 25 μm as threshold between
larger and smaller pores. The average throat size to pore size ratios
were 0.97 and 0.78 for the larger and smaller pores, respectively. Thus
for larger pores the throat effect was practically negligible while for
smaller pores a clear effect was observed. This explains at least partly
why the positions of modes related to smaller and larger pores are not
shifted by same factor when comparing SMC results to pore size dis-
tribution determined by image analysis (Fig. 2).
Although the hypothetical consideration of biochar direct effect on
the SMC is well established, the methodological limitations have hin-
dered studies on this issue. However, the SMC results for sandy soils and
SEM images of biochar presented by Abel et al. [36] could possibly be
interpreted in similar way than in our study. Liu et al. found that bio-
char amendment shifted soil pore size distribution toward smaller
pores, but biochar effect was not unambiguously established [5].
In addition to the direct influence of biochar internal pore system,
biochar affects soil moisture characteristics via indirect mechanisms,
i.e. soil-biochar interactions affecting soil structure (aggregate forma-
tion) and/or changes in soil particle size distribution [6,37]. The effect
of soil structural changes probably explains why the modes of HOM
treatment are wider than those of NAT treatment (Fig. 2). Interactions
between soil-soil and soil-biochar are more common in HOM treatment
with destroyed soil aggregates than in NAT treatment where biochar
acts more independently in cavities between existing soil aggregates.
In any case, observed bimodal pore size distribution of willow
biochar and consequent increase in soil water retention properties at
corresponding pore size regimes (multiplied by factor 0.5) provides
strong evidence that biochar internal micrometer-scale porosity con-
tributes directly to soil water storage. This outcome encourages further
studies and methodological development in order to improve under-
standing on relationship between biochar micron scale porosity and its
direct effect on soil moisture characteristics.
3.4. Effects of biochar amendment on soil plant available water
Biochar application increased the plant available water (PAW) by
17% and 32% for HOM and NAT samples in comparison to samples
with no biochar addition (Table 1). It is notable that in both studied soil
structures the biochar induced increase in PAW occurred mostly matric
potential regime from −10 to −316 kPa (Fig. 2), considered here as
readily plant available water (RPAW), which corresponds to pores with
EPD > 1 μm.
With respect to crop production at dry conditions and/or during
vigorous growth, increased storage of RPAW helps plants to maintain
potential transpiration rate for longer time. It is well documented that
dry matter production of plants virtually ceases when they lose turgor
pressure and this may occur far before permanent wilting point (PWP)
[38]. Meyer and Green reported that wheat could use 52–57% of PAW
before reduction in growth became evident [39]. In wider perspective,
measures like biochar amendment to increase water productivity in
agriculture (i.e. output yield per unit water used) are of importance in
the struggle against water scarcity threatening global food security
[40].
The present study highlights the importance of a direct link between
the biochar micrometer-scale porosity and RPAW. Irrespectively to soil
structure, biochar amendment increased soil porosity at the same pore
size regimes (EPD 5–10 and 25 μm, Fig. 2), which refer to biochar in-
duced changes via direct mechanism (biochar internal porosity). It
could be hypothesized that this specific biochar would result in similar
changes in other soil types as well. If the hypothesis holds, the approach
presented above could provide a fascinating tool-box enabling devel-
opment of tailored biochars that could modify soil moisture char-
acteristics at well-defined moisture regimes over the range of soil types.
Such tailored biochar products could improve biochar performance in
crop production at different climatic areas, soil types and production
systems, which is essential when sustainability of biochar is considered
in global scale [1]. In any case, this hypothesis calls for further careful
studies. However, it should also kept in mind that while biochar effects
on SMC via direct mechanism may be independent of soil type, the
biochar induced changes in soil bulk density, texture and structure are
strongly depend on the properties of receiving soil and the type of
biochar used.
4. Conclusions
The present study addresses the importance of biochar micrometer-
scale porosity when used as soil amendment. It was shown that biochar
internal porosity detected by X-ray tomography modifies SMC via direct
mechanism. Bimodal porosity of willow biochar acted as traceable
marker when added to a clay soil. However, a correction factor to
predict relationship between biochar pore size distribution and its ob-
served effects on SMC was needed (in this case close to 0.5) due to e.g.
roughness and hydrophobicity of biochar pores. Because biochar was
shown to increase soil porosity due to its internal porosity (direct me-
chanism) it could be hypothesized that this effect is independent from
soil type. On the other hand, biochar induced changes in soil texture
and structure, and modified SMC simultaneously (indirect mechanism).
The latter mechanism is known to be strongly dependent on soil and
biochar properties, which may explain why results on biochar effect on
SMC are often controversial.
From the practical point of view, the willow biochar used in this
experiment can greatly increase amount of readily plant available water
(between matric potentials−10 and−316 kPa) in clay soil irrespective
of soil initial structure. The result suggests that biochar amendment
Table 1
Water volume fraction at matric potential of −10 kPa (Field capacity, FC) and −1500 kPa (Permanent wilting point, PWP), plant available water (PAW) between
these matric potentials and readily plant available water (RPAW) between matric potentials of −10Kpa and −316 kPa.
FC (cm3 cm−3) PWP (cm3 cm−3) PAW (cm3 cm−3) RPAW (cm3 cm−3)
NAT 0% 0.36 p < 0.001 0.13 p=0.01 0.22 p < 0.001 0.08 p= 0.02
5% 0.41 0.12 0.29 0.14
HOM 0% 0.45 p=0.04 0.15 p < 0.001 0.30 p < 0.001 0.07 p < 0.001
5% 0.48 0.12 0.35 0.18
K. Rasa et al. Biomass and Bioenergy 119 (2018) 346–353
351
would help plants to withstand moisture stress during dry periods and/
or vigorous growth state. With respect to biochar use in growing media
or agronomic applications, we strongly recommend utilization of re-
search methods capable to reveal micrometer-scale pore structure of
biochars.
Acknowledgements
Riikka Keskinen, Johanna Nikama and Ilkka Sarikka are warmly
thanked for carrying out the laboratory work related to the soil
moisture characteristic curve. We also wish to thank Mirja Muhola and
Atte Mikkelson from VTT Technical Research Centre of Finland Ltd for
carrying out BET surface area and CHNSO analysis. This project has
received funding from the European Union's Horizon 2020 research and
innovation programme under grant agreement No 637020 −MOBILE
FLIP.
Appendix A
Image processing
Grey-scale X-ray tomography image was filtered with a three-dimensional median filter (radius 2). Filtered image was segmented into pore and
solid voxels by global thresholding. Threshold value was selected with the modified Otsu's method [41] described Hyväluoma et al. [18]. This
modified version utilizes ideas presented earlier by Hapca et al. [42]. Segmented image was then filtered with a majority filter (radius 2) and finally
isolated (“floating”) objects with volume less than 1000 voxels were removed from the image. A cross section of the grey-scale image and corre-
sponding cross-section of the segmented and filtered image is shown in Fig. A1.
Figure A1. Cross sections of the x-ray tomography images of the willow biochar. On left, grey-scale image with grey-scale histogram in the inset. Lighter color in the
histogram shows the grey-scale values interpreted as pore space and darker those interpreted as solid material. On right, final segmented and filtered image used in
analyses.
Appendix B
Pore throat analysis
The ratio of pore size and the smallest throat leading to that pore was estimated in the following way. As a starting point, a tomographic image
that was segmented into pores and solids was used. Method utilizes two scalar fields and one vector field derived from the image. First is distance
transform which determines the shortest distance to the pore walls for each pore voxels. Distance transform was computed using so-called d [3–5,7]
Chamfer metrics with a rescaling procedure as explained in detail by Svensson and Borgefors [43]. Secondly, for each voxel a pore size value was
determined by morphological opening, e.g. Ref. [24]. Finally, we computed pore-scale flow field for saturated pressure-driven flow with driving
pressure parallel with the pore direction. Here the lattice Boltzmann method was utilized for flow computation [44]. D3Q19 lattice Boltzmann model
with two-relaxation time collision operator was used and the no-slip boundary condition at fluid-solid boundaries was enforced with bounce-back
boundary condition. Sample size in the simulation was 454×400×638 μm3. Further details of the used method can be found from, e.g., [45].
Throat size was then determined by utilizing distance transform, pore size and flow fields.
Inspection points were selected as local maxima of distance transform which were determined using a 5×5×5 moving window. For these
point, the streamlines were integrated both upstream and downstream, and the smallest pore size found following the streamline was then de-
termined in both directions. As points along streamline do not coincide with the grid nodes, trilinear interpolation was utilized to determine the
pore-size values. This method produces two throat sizes, one upstream and another downstream from the inspection point. Larger of these throats
was selected as it is the easier route for the pore to be drained. The throat size thus determined is compared to the pore size at the inspection point to
determine the throat to pore size ratio.
It should be noted that, as the tomography image used in this analysis is smaller than the grain size used in the experiment, the results obtained
are only approximate. In real biochar grains routes to surface are longer and thus smaller throats could be present. Therefore, the results obtained
can slightly underestimate the effect of pore throats.
K. Rasa et al. Biomass and Bioenergy 119 (2018) 346–353
352
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