Published September 22, 2016
Technical Notes
Preferential Transport in
Macropores is Reduced by
Soil Organic Carbon
Mats Larsbo,* John Koestel, Thomas Kätterer,
and Nick Jarvis
Core Ideas
•Soils of larger SOC content showed
weaker preferential transport.
•These soils also had larger volumes
of pores <600 μm in size.
•Results suggest a threshold SOC
content above which effects were
negligible.
It has been suggested that some management practices and farming systems that promote C sequestration may exacerbate the risk of groundwater
pollution due to fast preferential transport in soil macropores. However, soil
organic C (SOC) may also impact the soil pore structure at scales smaller
than the macropore scale, where complexes of SOC and clay form microaggregates that may increase pore volumes in the micrometer size range.
These effects of SOC per se on pore network architecture, water flow, and
solute transport have hardly been investigated. Therefore, to investigate this
question, we measured tracer transport through soil cores sampled along
a transect on a field under grass–clover ley with a natural gradient in SOC
content. The strength of preferential transport was characterized at two flow
rates (2 and 5 mm h−1) and related to the volume, size distribution, heterogeneity, and connectivity of pore networks quantified by X-ray tomography.
The results showed that soils with a larger SOC content had larger volumes
of pores in the smallest imaged size range (200–600 mm) that were also
more uniformly distributed. These effects of SOC on the imaged pore networks were only apparent up to a threshold value of the ratio between clay
and SOC of 10:1, which is assumed to correspond with the amount of SOC
needed for C saturation of the clay fraction. The increased flow capacity of these smaller macropores in soil columns with larger SOC contents
prevented flow from being activated in larger pores, which significantly
reduced the strength of preferential transport.
Abbreviations: SOC, soil organic carbon
Soil organic C is intimately associated with the structure of the soil, which may
M. Larsbo, J. Koestel, and N. Jarvis,
Dep. of Soil and Environment, Swedish
Univ. of Agricultural Sciences, Uppsala,
Sweden; T. Kätterer, Dep. of Ecology,
Swedish Univ. of Agricultural Sciences,
Uppsala, Sweden. *Corresponding
author ([email protected]).
Vadose Zone J.
doi:10.2136/vzj2016.03.0021
Received 24 Mar. 2016.
Accepted 20 June 2016.
Open access.
Vol. 15, Iss. 9, 2016
© Soil Science Society of America.
This is an open access article distributed
under the CC BY-NC-ND license
(http://creativecommons.org/licenses/
by-nc-nd/4.0/).
be defined as the three-dimensional arrangement of the soil solids and pore space (i.e.,
the volume, size distribution, and connectivity of pores). Soil structure is critical for
sustainable crop production and food security because it enhances infiltration and
drainage of water, reduces surface runoff and erosion, and improves soil aeration, plant
root growth, and nutrient uptake (Lal, 2004a, 2004b; Johnston et al., 2009; Powlson
et al., 2011). Indeed, it has been suggested that the global value of soil structure for
crop production could be on the order of US$300 billion annually (Clothier et al.,
2008). Historically, arable cropping has depleted soils of organic C, which has contributed significantly to CO2 emissions (Lal, 2004a, 2004b). Thus, changing land
use and soil and crop management practices to reverse this process by sequestering
organic C in degraded agricultural soils has the potential to both contribute to mitigating climate change and restore the productive capacity of degraded agricultural
land (Lal, 2004b). However, SOC sequestration may also involve adverse effects and
trade-offs that are not yet fully understood. One concern that has been expressed (e.g.,
Sojka and Upchurch, 1999; Bates et al., 2008) is that some management practices
(e.g., fertilization, organic amendments) and farming systems (e.g., rotational leys,
cover crops, conservation tillage) that promote SOC sequestration will enhance soil
macroporosity (Jarvis, 2007) and may therefore exacerbate the risk of groundwater
pollution due to preferential flow.
Vadose Zone Journal | Advancing Critical Zone Science
However, the effects of SOC per se on solute transport in arable
soil have hardly been investigated and are not well understood.
The results from two earlier studies that attempted to address
this question were inconclusive (Vendelboe et al., 2013; Soares
et al., 2015), probably because the range of variation in SOC content (17–22 g kg−1) was too small. Other studies performed at
the field and catchment scales for a limited number of soil columns have suggested that preferential transport may be weaker
in conventionally tilled arable topsoils with larger SOC content
(Jarvis et al., 2007; Ghafoor et al., 2013; Paradelo et al., 2016).
Furthermore, a recent global meta-analysis of the effects of soil
properties and site attributes on tracer transport also identified
SOC as a potentially important factor reducing preferential flow
in soil, although a correlation between SOC and clay content
made unequivocal interpretation of the data difficult (Koestel
and Jorda, 2014). One likely explanation of these results is that
SOC and pore space architecture are linked at multiple scales,
not only at the scale of millimeter-sized macropores but also
at smaller scales where SOC sequestration may increase the
volume of pores in the micrometer size range (Peth et al., 2008;
Schlüter et al., 2011). In this respect, complexes of SOC and clay
are thought to form the fundamental building blocks of soil
microaggregation that provides long-term physical protection
of organic matter against decomposition (Hassink, 1997; Six et
al., 2002, 2004) and improves structural stability in tilled soils
(Watts and Dexter, 1997; Dexter et al., 2008).
Extensive, well-connected pore networks in the micrometer size
range would tend to promote more uniform transport by increasing the near-saturated hydraulic conductivity, thereby reducing
the risk of “triggering” and sustaining fast preferential transport
in the larger macropores (Jarvis et al., 2012; Larsbo et al., 2014;
Katuwal et al., 2015). The objective of this study was to test this
hypothesis by quantifying the effects of SOC on soil structure
and non-reactive solute transport for a clay soil. To achieve this
objective, we conducted non-reactive tracer transport experiments
under steady water flow through soil cores sampled in a field with a
large natural gradient in SOC content. The strength of preferential
transport, characterized by the normalized 5% arrival time of the
applied solute, was measured at two flow rates (2 and 5 mm h−1)
and related to the volume, size distribution, heterogeneity, and
connectivity of macropore networks quantified by X-ray tomography on the same samples.
66Materials
of a slight slope (<1°) at the site, with the largest SOC contents
at the valley bottom, close to a drainage ditch. The SOC gradient is probably a legacy of decreasing water table depths toward
the valley bottom. The field was under grass and clover ley (50%
timothy [ Phleum pratense L.], 25% red clover [Trifolium pratense
L.], 20% meadow fescue [Festuca pratensis Huds.], and 5% white
clover [Trifolium repens L.]) at the time of sampling and had not
been plowed since autumn 2012.
On 20 May 2015, 20 soil columns were taken in polyvinyl chloride
(PVC) cylinders (height 20 cm, inner diameter 12.7 cm) along a
transect, starting close to the drainage ditch and finishing 100 m
farther upslope. The soil samples were taken from the topsoil
using a tractor-mounted hydraulic press. The soil surface was left
undisturbed. Measurements made on samples extracted from each
column (see below) showed that the organic C content decreased
from 151 to 42.4 g kg−1 along the transect in the upslope direction, while clay contents (expressed as a fraction of the mineral
soil mass) increased slightly, from 63.7 to 70.5% (Fig. 1). Because
of the considerably larger relative variation in organic C content
compared with clay content, we assume that any differences in pore
network characteristics and transport can be attributed mainly to
differences in SOC content.
X-ray Tomography
The GE Phoenix v|tome|x m X-ray scanner installed at the
Department of Soil and Environment at the Swedish University of
Agricultural Sciences was used to scan the columns. It has a 240-kV
X-ray tube, a tungsten target, and a GE 16² flat panel detector. We
collected 2000 radiographs per column with a discretization of
2024 by 2024 pixels. The X-ray scans were performed at a voltage
of 150 kV and a current of 300 mA. The exposure time for each
radiograph was 333 or 500 ms, varying among columns depending on the soil’s density. A 3-mm-thick Cu filter was attached to
the detector to reduce beam hardening artifacts. The radiographs
were inverted to a three-dimensional image using the GE image
and Methods
Field Site and Sampling
The site is located at Måtteby (59°43¢59² N, 16°55¢7² E), approximately 50 km west of Uppsala in eastern Sweden. The USDA
texture class of the soil is clay (63.7–70.5% clay, 28.9–34.9% silt,
0.6–1.6% sand). This site was chosen because preliminary measurements indicated a large gradient in SOC content in the direction
VZJ | Advancing Critical Zone Science
Fig. 1. Variations in clay and soil organic C (SOC) content along the
sampling transect.
p. 2 of 7
reconstruction software datos|x. The resulting spatial resolution
of the reconstructed three-dimensional images was 100 mm in all
directions. X-ray tomography imaging was performed in June 2015,
prior to the solute transport experiments.
Image Processing
Image processing and analysis were performed using the Fiji
distribution (Schindelin et al., 2012) of the software ImageJ
(Abramoff et al., 2004) including the plugin BoneJ (Doube et
al., 2010) and R (R Core Team, 2016). First, the resolution of
each image was reduced by a factor of two in all directions to
limit the computation time during the following image processing steps. As a result, each image voxel had an edge length of
200 mm. A three-dimensional median filter with a radius of one
voxel was then applied to reduce noise. Illumination differences
between and within scans in the vertical direction were corrected
using the gray values in the PVC cylinder and the air inside the
column as references. There were no differences in illumination
in the radial direction. A single gray value threshold was then
applied to all images. This approach resulted in a satisfactory
separation between the pore space and the soil solids. All visible
pores are referred to as macropores based on the definitions given
by Jarvis (2007).
Pore Space Characteristics
The total imaged porosity and volumes of pores in three size
(“thickness”) classes (0.2–0.6 , 0.6–1.2, and >1.2 mm) were
calculated using BoneJ (Doube et al., 2010). The pore thickness is defined for each pore voxel as the diameter of the largest
sphere that fits into the pore and contains the voxel. The Particle
Analyzer in BoneJ was used to label all individual pore clusters
in an image. The files containing the labeled pore clusters were
imported into R to calculate the connected porosity, which is
defined as the porosity connected to both the top and the bottom
of the sample. The mass fractal dimension, a measure of the heterogeneity of the spatial distribution of imaged porosity (i.e.,
how well the macropore network “fills up” the sample volume),
was calculated with the box-counting algorithm in ImageJ. Pore
networks were analyzed both for the whole sample volume and
for cylindrical subvolumes (8 cm high, 10-cm diameter) located
centrally within the soil columns 3 cm below the average depth
of the soil surface. The subvolumes, which were used for analysis
of the effects of SOC on pore network measures, were assumed
to be free from artifacts created at sampling close to the column
walls. We used pore network measures calculated for the whole
sample volume in the analysis of effects of differences in structure on preferential transport.
Solute Transport Experiments
The columns were set up in a laboratory irrigation chamber
allowing free drainage at the base. Tracer breakthrough experiments were performed at two different irrigation rates (2 and
5 mm h−1). In Sweden, a rainfall intensity of 5 mm h−1 has a
VZJ | Advancing Critical Zone Science
return period of 0.4, 4, and 30 yr for durations of 2, 6, and 12
h, respectively (Hernebring, 2006). In each experiment, the columns were first irrigated for 5 d with artificial rainwater prior
to the first solute application to achieve stable flow rates and
background electrical conductivities in the effluent. A 2-mL
pulse of KBr solution (250 mg Br mL−1 water) was applied
during approximately 30 s at the surface of each column using
a pipette. Application within 10 mm of the column walls was
avoided to limit transport through any artificial pores created
at sampling.
Bromide breakthrough curves were derived from electrical conductivity measured in the effluent at 5-min intervals, following the
method described by Larsbo et al. (2014). The soil columns were
weighed directly after the irrigation nozzles had been turned off to
enable calculation of the degree of saturation for the whole column
and in the imaged pore space during the transport experiments
at both flow rates from measured water contents and porosities
(see below). Ponding occurred on one column during irrigation at
2 mm h−1 and seven columns at 5 mm h−1. These were excluded
from the subsequent analysis to ensure that flow rates were identical through all columns.
Preferential transport is associated with an early arrival and pronounced “tailing” in tracer breakthrough curves. In this study, we
used the normalized 5% arrival time, t 0.05, as a measure of preferential transport (Knudby and Carrera, 2005; Koestel et al., 2011).
This measure, which reflects the tendency for early arrival of the
solutes, is defined as the time between solute application and the
arrival of the first 5% of the applied solute at the bottom of the
column normalized to the first temporal moment of the breakthrough curve. Small values of t 0.05 thus indicate a high degree of
preferential transport.
Soil Properties
After completion of the breakthrough experiments, the soil samples were split into three parts: the uppermost 3 cm, 3–11 cm
(corresponding to the location of the cylindrical subvolume
chosen for structure characterization), and the remaining soil
at the bottom of the columns. The top and bottom parts were
dried at 105°C and weighed. The central part was split into two
approximately equal-sized parts. One half was dried at 20°C
and then analyzed for particle densities, SOC content by dry
combustion on a TruMac CN (LECO Corp.), and particle size
distribution by the pipette method. The other half was dried
at 105°C and weighed. The average degree of saturation in the
imaged pores and total pore space during the solute transport
experiments was calculated from the weights of these samples,
the volumes of the samples, and the imaged pore volumes determined by X-ray, particle densities (assuming that the value
measured at 3–11 cm was representative for the whole column),
and the weights of the soil columns at the end of the solute transport experiments.
p. 3 of 7
Statistics
Spearman rank correlation coefficients, r, were calculated
between imaged pore space characteristics, soil properties,
and t 0.05. The statistical significance of the correlations
was calculated from exact permutation distributions in R
(R Core Team, 2016). Results were considered significant
for p values <0.05.
66Results
and Discussion
The column breakthrough experiments showed that soils
with larger SOC contents were associated with weaker
preferential transport. The 5% arrival time of the tracer,
t 0.05, was positively correlated with the SOC content at
both flow rates (Fig. 2; r = 0.90, p < 0.001 and r = 0.54,
p = 0.047, respectively). A threshold behavior can be discerned at the smaller of the two flow rates (2 mm h−1),
with t 0.05 values increasing at low SOC contents, reaching an average value of about 0.4 at SOC contents larger
than approximately 60 to 70 g kg−1. A t 0.05 value of
Fig. 2. Relationships between soil organic C (SOC) content and 5% arrival times
approximately 0.4 is within the range of values found for
(t0.05) at two flow rates. The degree of saturation (S) for the total pore system is
artificial homogeneous porous media as well as sieved and
given using the color scales.
repacked soils without any macrostructure (Koestel et al.,
2012), which suggests convective–dispersive transport in
the high-SOC columns at 2 mm h−1. Preferential transport was
porosity in the sample with a low SOC content was domistronger at the 5 mm h−1 flow rate. One soil column in particunated by a few large macropores, while the pore networks in
lar showed much stronger preferential transport (t 0.05 » 0.1) at
the samples representing soils of intermediate and high SOC
this flow rate despite a high SOC content (»140 g kg−1, Fig. 2),
contents were characterized by a dense, homogeneously diswhich suggests that flow in large biopores was triggered in this
tributed network of smaller pores in addition to the larger
column at the higher irrigation rate. The t 0.05 was negatively
macropores (Fig. 3). Similar effects of SOC on the soil pore
correlated with the degree of saturation of the total pore space at
space architecture have been documented in two other recent
the 2 mm h−1 flow rate (r = −0.52, p = 0.021), which confirms
X-ray tomography studies (Peth et al., 2008; Schlüter et al.,
that preferential solute transport was increasingly triggered as
2011). These differences in the soil pore space architecture
larger pores became active. Finally, Fig. 2 also shows that many
are illustrated quantitatively in Fig. 4b and 4c. Columns with
of the columns were far from fully saturated during the irrigahigh SOC contents were associated with large porosities in the
tion experiments.
smallest imaged size class (0.2–0.6 mm in diameter) (r = 0.59,
p = 0.0067), whereas no significant correlation was found
Figure 3 shows examples of the imaged pore networks for
between SOC and either the total imaged porosity (Fig. 4a)
columns of low, intermediate, and high SOC contents. The
or the porosity in the two largest pore classes (Fig. 4b). The
Fig. 3. Examples of imaged pore networks for cylindrical subvolumes (8 cm high, 10-cm diameter) of samples with low (left), moderate (center), and
high (right) soil organic C (SOC) contents.
VZJ | Advancing Critical Zone Science
p. 4 of 7
calculated fractal dimensions showed that the pore space in the
samples with smaller SOC content was less uniformly distributed (Fig. 4c; r = 0.50, p = 0.026). The fraction of the imaged
porosity that was connected to both the top and the bottom of
the analyzed subvolume ranged from zero for the two columns
with the smallest SOC contents to an average value of 0.56 for
the five columns with the largest SOC contents (Fig. 4a). The
SOC/clay ratio has been suggested to play an important role
in soil structure formation. This is because a fraction of the
clay is assumed to be uncomplexed below a threshold value of
this ratio when the soil becomes “saturated” with C (Hassink,
1997; Dexter et al., 2008). Based on an analysis of Polish and
French agricultural soils, Dexter et al. (2008) proposed that
this threshold should be around 0.1. Noting that clay contents
were between 63.7 and 70.5% (Fig. 1), Fig. 4b and 4c suggest a
similar threshold value above which the effects of SOC on the
volumes of pores <1.2 mm in diameter and the fractal dimension become negligible.
All pore network measures for the whole volume were significantly correlated with the same measures for the subvolume (not
shown). However, imaged porosities were, on average, about
twice as large for the whole sample. The reason for these differences is that the whole volume contained artificial pores close to
the column walls created at sampling. For the subvolumes, three
columns (all with SOC < 60 g kg−1) did not have a continuous
path connecting the top of the subvolume to the bottom. For the
whole volume, such a path existed for all columns. Figures 5a and
5b show that t 0.05 was positively correlated with the volume of
imaged pores <0.6 mm in size (r = 0.75, p < 0.001 and r = 0.85,
p < 0.001 for the 2 and 5 mm h−1 irrigation rates, respectively)
and with pores in the size class of 0.6 to 1.2 mm, although less
strongly in this case (r = 0.66, p = 0.0029 and r = 0.78, p =
0.0026). In contrast, the volume of macropores >1.2 mm in size
was not correlated with t 0.05 (Fig. 5c). The positive correlation
found between SOC and t 0.05 (Fig. 2) would probably be weaker
for flow rates >5 mm h−1 because more of these larger macropores would be active in the transport.
66Conclusions
Fig. 4. Relationships between soil organic C (SOC) content and pore
space characteristics for the cylindrical subvolumes (a) total imaged
porosity and connected (percolating) porosity, (b) imaged porosity in
three pore size classes, and (c) fractal dimension of the macropore volume.
VZJ | Advancing Critical Zone Science
The results from our study show that high-SOC soils were
characterized by pore networks in the micrometer size range
(200–1200 mm) that had a sufficiently large hydraulic conductivity to prevent the occurrence of fast preferential transport in
the larger macropores at the applied flow rates. These effects of
SOC on pore network characteristics and preferential flow were
most noticeable at the smallest flow rate of 2 mm h−1 and in
columns with smaller SOC contents. No apparent effects were
found above a threshold SOC content equivalent to C “saturation,” assuming that saturation occurs at a ratio of organic C to
clay content of 0.1. With this definition, about 30% of Swedish
arable topsoils have a deficit of SOC, which suggests that there
p. 5 of 7
may be considerable scope in Sweden for improving soil and
water quality through policies that promote C sequestration.
However, we reported data from only one field site, and further
work would be needed to establish the general validity of our
results for other soil types.
Acknowledgments
We thank the land owner, Dr. Martin Larsson, for letting us take samples from the field at
Måtteby. The work was funded by the Swedish Research Council Formas.
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