Geoderma 195–196 (2013) 184–191

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Gas transport and subsoil pore characteristics: Anisotropy and long-termeffects of compaction

F.E. Berisso a,⁎, P. Schjønning a, T. Keller b,c, M. Lamandé a, A. Simojoki d, B.V. Iversen a,L. Alakukku e,f, J. Forkman g

a Aarhus University, Department of Agroecology, Blichers Allé 20, P.O. Box 50, DK-8830 Tjele, Denmarkb Agroscope Research Station ART, Department of Natural Resources & Agriculture, Reckenholzstrasse 191, CH-8046 Zürich, Switzerlandc Swedish University of Agricultural Sciences, Department of Soil & Environment, P.O. Box 7014, SE-75007 Uppsala, Swedend University of Helsinki, Department of Food and Environmental Sciences, P.O. Box 11, FI-00014 University of Helsinki, Finlande University of Helsinki, Department of Agricultural Sciences, P.O. Box 28, FI-00014 University of Helsinki, Finlandf MTT Agrifood Research Finland, FI-31600 Jokioinen, Finlandg Swedish University of Agricultural Sciences, Department of Crop Production Ecology, P.O. Box 7043, SE-75007 Uppsala, Sweden

⁎ Corresponding author. Tel.: +45 8715 4756.E-mail address: Feto.Esimo@agrsci.dk (F.E. Berisso).

0016-7061/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.geoderma.2012.12.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 April 2012Received in revised form 29 November 2012Accepted 5 December 2012Available online 7 January 2013

Keywords:AnisotropyCompactionGas diffusivityAir permeabilityPore morphological indices

Anisotropy of soil pore functions significantly affects the transport of gas and water in soil. This paper quantifiesanisotropy of subsoil pores and investigates the long-term impact of soil compaction by agricultural machinery.Two long-term field experiments on soil compaction formed the basis for the investigation, one established in1981 on a clay soil in Finland (60°49′N, 23°23′E) and another in 1995 on a sandy clay loam in Sweden(55°49′N, 13°11′E). In 2009/2010, soil cores were sampled in vertical and horizontal directions from 0.3, 0.5,0.7 and 0.9 m depth (the two lower depths only in Sweden). In the laboratory, water content, air-filled porosity(εa), air permeability (ka) and gas diffusivity (Ds/D0)were determined at selectedmatric potentials. For the sandyclay loam, morphological characteristics of pores (effective pore diameter, dB; tortuosity, τ; the number of effec-tive pores per unit area, nB) were calculated using a tortuous tube model at−100 hPa matric potential. Blockedair-filled porosity (εb) and a pore continuity index (N) were estimated from the relationship between ka and εafor a range of matric potentials. A factor of anisotropy (FA) was determined as the ratio of a given property mea-sured in the horizontal direction to that in the vertical direction. ka showed anisotropic behaviour (FAb1) for theclay soil and for the 0.3 m depth of the non-compacted sandy clay loam soil, whileDs/D0 displayed anisotropy forthe clay soil (FAb1). In the sandy clay loam soil, dB and nB displayed significant anisotropy (FAb1) except at0.9 m. We interpreted this as effects of biological activities and physical processes in the B-horizon not beingactive in the C-horizon (0.9 m depth). Compaction generally reduced ka, Ds/D0, dB, nB and increased τ for bothsampling directions. Compaction had an effect on anisotropy for soil drained to −100 hPa, but only for ka anddB in the sandy clay loam at 0.3 m depth. Compaction reduced anisotropy for the N parameter, i.e. effects on soilpore continuity at themacropore scale, while it increased the anisotropy for εb. Our data thus indicate that compac-tion had persistent effect on soil physical properties and also affects anisotropy, especially that of macropores.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The term anisotropy is used to describe a property having differentvalues in different directions. Anisotropy in soil is commonly attributedto the presence of parallel horizontal soil layers (e.g. Jenny, 1941). How-ever, many researchers (e.g. Pozdnyakov et al., 2009) argued that the soilexhibits different physical properties in different directions evenwithin agiven soil layer. In this paper, we define anisotropy as the ratio of a givensoil property in the horizontal direction to that in the vertical direction(Pozdnyakov et al., 2009).

rights reserved.

Anisotropy of sediments has been addressed in several studies(e.g. Oda, 1972). In that context, anisotropy has been associated with apreferential alignment of the elongated axis of the soil grains parallel tothe bedding plane during deposition and soil formation processes. Thisinherent anisotropy may directly affect pore morphology and therebythe anisotropy of fluid dynamics at the macroscopic scale. However, bio-turbation (mixing of soil by animals or plants) andphysical drivers for soilstructure formation in pedogenesis may remove or change the preferen-tial orientation of elementary soil particles and even that of aggregatesand hence modify the degree of anisotropy.

Wiermann et al. (2000) measured 100 and 20 mm displacement inthe vertical and horizontal direction, respectively, at 10 cm depth dueto wheeling in a structured silty loam soil. An unequal displacement invertical and horizontal directions changes the position of soil particles/

185F.E. Berisso et al. / Geoderma 195–196 (2013) 184–191

aggregates relative to one another. This may affect the morphology ofthe soil pore system and the associated fluid transport properties asdemonstrated in a recent study (Berisso et al., under review). Impactsof human activity (soil compaction, soil tillage) do not only modify theporemorphology of agricultural soils, butmay also change their anisotro-py, which may have tremendous impacts on key processes and vital eco-system services such aswater transport and storage, soil–atmosphere gasexchange and soil aeration. This in turn affects biological activity andfluxes to plant roots.

The anisotropy of agricultural soils exposed to vehicle traffic has notbeen extensively studied. Exceptions are studies by Dörner and Horn(2006, 2009),who reported anisotropy for saturatedhydraulic conductiv-ity and air permeability. The lack of adequate knowledge on anisotropy ofsoil functions and the limited understanding of impacts of agriculturalmanagement practices on the anisotropymay lead to erroneous assump-tions in most soil process and crop models, i.e. models do not generallyconsider anisotropy in soil functions (Kühne et al., 2012).

The objectives of this study were: (i) to investigate the anisotropy ofsubsoil pore characteristics and gas transport properties, and (ii) to exam-ine the effect of compaction on the anisotropy for these properties. Wesampled undisturbed soil cores from several depths in two long-termfield experiments in two principal axes, namely normal (vertical) andparallel (horizontal) to the soil pedological horizons. Compaction effectson soil pore functioning in vertically sampled soil cores from one of thesites are addressed in a separate study (Berisso et al., 2012). In the presentstudy we combined gas diffusion and air permeability measurements forthe vertically and horizontally sampled cores.

2. Materials and methods

2.1. Experimental sites

Soil samples were collected from long-term compaction experimentsin Finland (Jokioinen; 60°49′N, 23°23′E) and Sweden (BrahmehemFarm;55°49′N, 13°11′E) as part of a joint Scandinavian research effort on theeffects of subsoil compaction on soil ecological services and functions(www.poseidon-nordic.dk). The soil at Jokioinen was formed on thebed of the Yoldia sea about 10,000 YBP (Yli-Halla et al., 2009). The subsoilhas a clay texture and is classified as Vertic Endostagnic Cambisol (WRB,2006). The soil at Brahmehem was developed on glacial till and has asandy clay loam texture. According to theWRB system the Brahmehemsoil is classified as a Mollic Endogleyic Luvisol (Berisso et al., 2012).Some physical properties of the two soils are shown in Table 1.

Both experiments had a randomised block design with four repli-cate plots. In the present study, soil that was not compacted duringthe experiments is referred to as the ‘control’, while ‘compacted’ refersto soil that was subjected to four repeated wheelings with machineryat the year when the experiment was established (at Jokioinen in1981, at Brahmehem in 1995). At Brahmehem, a sugarbeet harvesterwith 10 Mg wheel load/200 kPa inflation pressure was used (Arvidsson,2001), while a tractor–trailer combination with wheel loads of 3.2 Mgon the tractor rear wheel and 9.3 Mg on the trailer (inflation pressures250 and 700 kPa, respectively) was applied at Jokioinen (Alakukku,1996).

Table 1Basic soil properties at the experimental sites.

Soil location Depth(m)

Clayb2 μm g g−1

Silt 2–63μm g g−1

Brahmehem 0.3 0.272 0.2710.5 0.259 0.2780.7 0.225 0.2670.9 0.212 0.290

Jokioinen 0.3 0.642 0.2970.5 0.635 0.334

After the initial, one-event experimental compaction, no heavytraffic has been applied to the soil at the two experimental sites.The Brahmehem plots have been managed in a 7-year arable crop rota-tion (see Berisso et al., 2012 for additional details). The tillage regime inthe field includes mouldboard ploughing to about 0.25 m depth, withoccasional reduced tillage to about 0.1 m depth. The Jokioinen plotshave been managed in an arable crop rotation (spring cereals) since1981, with ploughing to about 0.2 m depth until 2001 andwith conser-vation tillage since then.

2.2. Soil sampling

Soil samples were collected at Brahmehem in May 2009 and atJokioinen in June 2010. Before sampling, access pits (2 m by 2 mwide and 1.5 m deep) were dug in each plot and horizontal planeswere sequentially exposed for sampling at 0.3, 0.5, 0.7 and 0.9 mdepth (Brahmehem) or 0.3 and 0.5 m depth (Jokioinen).

Soil cores of 100 cm3 were collected in vertical and horizontal di-rections into steel cylinders. The total number of cores extracted ateach site and depth is listed in Table 2. Approximately 1 kg ofremoulded soil was also collected for each combination of plot anddepth.

2.3. Measurements

Remoulded soil was air-dried at room temperature (25 °C) andused to determine soil particle size distribution, total carbon contentand particle density by standard techniques. The soil coreswere storedat 2 °C until analyses could take place. Before the analyses, the coreswere carefully trimmed with a sharp-edged knife and saturated in asandbox. Then the cores were sequentially drained to −6, −30 and−100 hPa matric potential (Brahmehem soil) or−100 hPa (Jokioinensoil).

Air permeability (ka) wasmeasured at−6,−30 and−100 hPa (onlyat −100 hPa for Jokioinen) using the steady-state method described byIversen et al. (2001). Prior to measurements, the soil at the very edge ofthe cores was gently pressed to the inner wall of the cylinder tominimiseflow of air between cylinder and soil. Air pressurised to 5 hPa was thenapplied to the sample from one end. When steady state was established,volumetric air flow rate through the soil core was recorded and ka wascalculated using Darcy's law.

Gas diffusivity was measured at −100 hPa by a non-steady statemethod proposed by Taylor (1949) using a one-chamber apparatusdescribed by Schjønning (1985). Atmospheric O2 was used as theexperimental gas. The diffusion coefficient (Ds)was calculated accordingto Fick's second law and converted to gas-independent diffusivity (Ds/D0)by relating it to the diffusion of oxygen in air (D0=0.205 cm2 s−1 at at-mospheric pressure and 20 °C; Smithsonian Physical Tables).

2.4. Modelling of soil pore characteristics

The effect of compaction on soil physical properties is usuallyquantified from total porosity, air-filled porosity (εa) and pore sizedistribution. Further quantification in terms of pore ‘morphological’

Sand 63–2000μm g g−1

Organic matterg g−1

Particle densityg cm−3

0.457 0.007 2.690.463 0.005 2.700.508 0.004 2.690.498 0.003 2.710.061 0.013 2.780.031 0.007 2.79

Table 2Summary of soil cores sampled at Brahmehem and Jokioinen.

Soil Depth(m)

Directiona Treatment numberof samples

Total numberof cores

Control Compacted

Brahmehem 0.3, 0.5, 0.7and 0.9

V 6 6 192 cores=4 depths×2treatments×4 plots×6cores

H 3 3 96 cores=4 depths×2treatments×4 plots×3cores

Jokioinen 0.3 and 0.5 V 6 6 96 cores=4 depths×2treatments×4 plots×6cores

H 6 6 96 cores=4 depths×2treatments×4 plots×6cores

a Vertically sampled cores at Brahmehem were also used in the study by Berisso et al.(2012).

186 F.E. Berisso et al. / Geoderma 195–196 (2013) 184–191

characteristics such as length, diameter and orientation gives moreinsight about the changes. For this purpose, we used the tortuoustube model proposed by Ball (1981) to estimate three morphologicalindices of pores: effective pore diameter (dB), tortuosity (τ) and thenumber of pores per unit cross-sectional area (nB):

dB ¼ 32kaDs=Do

� �1=2ð1Þ

τ ¼ εaDs=D0

� �1=2ð2Þ

and

nB¼ε1=2a Ds=D0ð Þ3=2

8πka: ð3Þ

This model does not give detailed information on how pore con-nectivity (pore network) affects transport processes in the soil. How-ever, Ball (1981) showed that lateral interconnections betweenmainmacropores did not disqualify model predictions. In addition, Ball(1981) reported that the model produced realistic dimensions forthe average continuous air-filled pore paths in undisturbed soil sam-ples equilibrated to several matric potentials. Also, Blackwell et al.(1990) showed that subsoil macropore dimensions could be quiteaccurately predicted from combinations of basic equations for gasflow in tubes. Our model estimates for the clay soil at Jokioinenwere inaccurate because of very low values of ka and Ds/D0, and lowand uncertain estimates of εa. Thus, we calculated pore morphologi-cal indices for the Brahmehem soil only.

Based on themeasurements of ka at−6,−30 and−100 hPamatricpotential, we modelled the relationship between ka and εa for theBrahmehem soil with a simple exponential equation (Ball et al., 1988):

ka ¼ MεaN ð4Þ

which can be written as:

log kað Þ ¼ log Mð Þ þ N log εað Þ ð5Þ

whereM and N are empirical fitting parameters.N is regarded as a porecontinuity index reflecting the rate of opening of continuous pore pathswith decreasing matric potential (Ball et al., 1988).

Ball et al. (1988) considered a soil with ka=1 μm2 as effectivelyimpermeable. Thus we regarded the intercept of Eq. (5) on the abscissa

(log[ka=1]=0) as an estimate of blocked air-filled porosity (εb), whichwas calculated as:

εb ¼ 10− logM=N: ð6Þ

2.5. Statistics

Following Pozdnyakov et al. (2009), we defined ‘factor of anisotro-py’ (FA) as the ratio of a given soil property in the horizontal directionto that in the vertical direction. A given property is said to be aniso-tropic if FA is significantly different from 1.

Before the statistical analysis, all data were log-transformed inorder to facilitate statistical inference about the FA. This practicehelps to transform ratios into differences, which can be tested usinglinear models. Each depth was analysed separately, using the MIXEDprocedure in SAS version 9.2 (Littell et al., 2006). The linear statisticalmodel comprised the random factor block and the fixed factors treat-ment, direction, and treatment-by-direction interaction. This modelcan be written as:

yijk ¼ μ þ αj þ δk þ αδð Þjk þ bi þ pij þ eijk ð7Þ

where yijk is the observation of the jth treatment and the kth directionin the ith block, μ is an intercept, αj is the effect of the jth treatment,δk is the effect of the kth direction, (αδ)jk is the interaction betweenthe jth treatment and the kth direction, bi is the effect of the ithblock, pij is the effect of the plot with the jth treatment in the ithblock, and eijk is a residual error. The hypothesis that the expected dif-ference between vertical and horizontal measurements was 0 on thelog scale was tested, which corresponds to the hypothesis FA=1. TheKenward and Roger (1997) method was used for calculating the de-grees of freedom. The percentage of clay was included in the modelas a covariate for the Brahmehem soil from 0.7 to 0.9 m depth to ac-count for textural variability (see Berisso et al., 2012 for details). Wefurther evaluated the overall effect of compaction on Ds/D0 in thesoil profile (for both Brahmehem and Jokioinen) by applying a statis-tical model with a repeated measures analysis and an autoregressiveco-variance structure.

3. Results and discussion

In this study, we decided to analyse soil anisotropy for the mea-sured gas flow expressions per se and then in addition to addressthree model-derived fingerprints of soil structure. Kühne et al.(2012) discussed anisotropy of gas diffusivity normalized by the vol-ume of air-filled pores (labelled ‘diffusion efficiency’ in their study).This approach may seemmost logical as the specific gas flow (diffusionor convection)would be easier compared across soil depthswith differ-ent volumes of air-filled pore volumes.We choose, however, to evaluateanisotropy based on themeasured gas transport properties, for two rea-sons. First, because the normalization by air-filled pore space would in-troduce an additional uncertainty in data, as air-filled pore space is notmeasured directly but derived fromestimates of total porosity andmea-sured water content. And second, we note that the anisotropy of thenormalized and the ‘raw’ gas flow is identical because the air-filled porespace is a direction-independent value.

The results are discussed below for each parameter – measured andmodel-estimated – by first addressing the compaction effect, then the an-isotropy for the parameter in question and finally the compaction effecton the anisotropy (i.e. the interaction between compaction treatmentand direction of sampling).

187F.E. Berisso et al. / Geoderma 195–196 (2013) 184–191

3.1. Air permeability

In the sandy clay loam soil at Brahmehem, ka at−100 hPa was gen-erally higher for the control than for the compacted soil, although theresults were significant (Pb0.05) only for the vertical direction at 0.3and 0.9 m depth and the horizontal direction at 0.5 and 0.9 m depth(Table 3). For the clay soil at Jokioinen, the compaction inflicted nearlythree decades prior to our sampling was still detectable as a significantreduction in vertical ka at 0.3 and 0.5 m depth and in horizontal ka at0.5 m depth (Table 3).

In the Brahmehemcontrol soil, horizontal ka tended to be smaller thanvertical ka (i.e. FAb1) but the difference was significant only at 0.3 mdepth (Table 3). This was probably due to vertical biopores facilitatingconvective flow in the vertical as opposed to the horizontal direction;while the εa was similar in both directions (data are not shown). In thecompacted Brahmehem soil, no significant anisotropy was observed.We found that FA exceeded a value of 1 at 0.3 m depth, which implieshigher horizontal than vertical ka. This may be interpreted as the effectof roots being forced to grow horizontally while attempting to findtheir way through the severely compacted upper part of the subsoil(Jakobsen and Dexter, 1987).

For the Jokioinen clay soil at 0.3 mdepth, horizontal kawas an order ofmagnitude smaller than vertical ka and FAwas significantly lower than 1(Table 3). The difference between vertical and horizontal ka at 0.5 mdepth was rather small and no significant anisotropy was observed. Theprogressive decrease in ka anisotropy with increasing depth supportsfindings by Jing et al. (2008) and also implies isotropy of deeper subsoilpore structure. This could be due to decreasing root penetration (Ehlerset al., 1983) and earthworm burrows (Blanco-Canqui and Lal, 2008), orreduced frequency of drying and crack formation (Shipitalo et al., 2004)(or any combination of these processes) with depth.

A compaction effect on anisotropywas quantified as a significant in-teraction between compaction and direction of sampling (Table 3). Thevery high P-values for ka, in all our tests for this interaction (except forBrahmehem soil at 0.3 m depth, as discussed below) indicate clearlythat compaction generally did not affect the anisotropy of ka at a matricpotential of−100 hPa. In the Brahmehem soil at 0.3 m depth; however,wemeasured a considerable difference in ka between the vertical and thehorizontal direction for the control soil, while compaction reduced verti-cal ka to the level of that in the horizontal direction (Table 3). Such areduction in vertical ka at 0.3 m depth may worsen the aeration of this

Table 3Air permeability (ka) and relative gas diffusivity (Ds/D0) at –100 hPa for the Brahmehem anvertically sampled cores at Brahmehem were also used in the study by Berisso et al. (2012

Soil Depth ka,100 (μm2)

m Va Ha

Brahmehem 0.3 Control 9.01 2.95Compacted 2.18 2.30P-value 0.02 0.59

0.5 Control 7.18 5.14Compacted 4.27 2.56P-value 0.13 0.05

0.7 Control 12.68 6.14Compacted 6.50 2.96P-value 0.20 0.17

0.9 Control 6.08 5.11Compacted 2.25 1.67P-value b0.01 0.01

Jokioinen 0.3 Control 8.16 0.16Compacted 0.58 0.03P-value 0.02 0.22

0.5 Control 1.08 0. 90Compacted 0.03 0.02P-value 0.04 0.03

a P-values in column are for compaction effect.b P-values in column are for interaction between compaction and direction of sampling (c P-values in column are for significance of the Factor of Anisotropy (FA).

dense soil and increase the risk of anaerobic microsites in the soil profile,whichmay play a critical role in the production as well as mobilization ofgreenhouse gases. The air permeability is known to be correlated to thesaturated hydraulic conductivity (Iversen et al., 2003; Loll et al., 1999),and our results therefore indicate that compaction at 0.3 m depth de-creases infiltration rate and consequently increases the risk of surfacerunoff and preferential flow (Etana et al., 2013).

3.2. Gas diffusivity

The compaction inflicted 14 years prior to sampling at Brahmehemand 29 years prior to sampling at Jokioinen resulted in consistentlylower gas diffusivity in compacted than in control soil (generally Pb0.2if not Pb0.05; Table 3). For the Jokioinen soil, some zero values of Ds/D0

(values below the detection limit) were measured at −100 hPa, whileεa was greater than zero (data not shown). This implies that for thisclay soil, the pores that were drained at −100 hPa matric potentialwere either blocked to the diffusion pathway or not continuous throughthe experimental core.

When including Ds/D0 data from all depths, we found a significantimpact of compaction on vertical and horizontal Ds/D0 at both sites(analysis not shown). Thus, a single traffic event comprising four re-peated passes of heavy traffic machinery can persistently decrease thediffusion of gases in the soil profile. The reduction ofDs/Do in turn affectsthe dynamics of soil aeration andmay enhance formation of active sitesfor N2O production. Studies of N2O emission from the Brahmehem andJokioinen sites were not able to identify significant compaction effects(Simojoki et al., 2012). However, other studies (e.g. Simojoki et al.,1991) have found compaction to increase emissions of the greenhousegas N2O through denitrification at anaerobic sites.

The FA forDs/D0 in the Brahmehem sandy clay loam soilwas close to,and not significantly different from, 1 (Table 3). Thus, even if FA at thetwo upper soil depths was mostly larger than 1, and those at 0.7 and0.9 m depth lower than 1, we did not observe clear indication on anyanisotropy inversion (as defined by McKenzie and Dexter, 1985) withFA shifting fromhigh to lowvalueswith depth. In contrast to the patternobserved at Brahmehem, considerable (and generally highly signifi-cant) anisotropy for Ds/D0 was observed in the clay soil at Jokioinenwith higher vertical diffusivity than horizontal one (Table 3). The resultsfrom the clay-rich Jokioinen soil are in accordance with Kühne et al.

d Jokioinen soils: V=vertical, H=horizontal, FA=factor of anisotropy (H/V). Data for).

Ds/D0,100 (−)×1000

FAb P-valuec Va Ha FAb P-valuec

0.33 0.01 4.49 3.59 0.80 0.681.08 0.86 1.66 2.65 1.60 0.240.03 0.03 0.47 0.260.72 0.31 5.08 5.68 1.12 0.610.56 0.13 3.30 3.44 1.04 0.850.69 0.13 0.08 0.820.48 0.13 7.39 6.66 0.90 0.670.45 0.11 4.77 3.59 0.75 0.130.92 0.26 0.13 0.610.84 0.43 5.16 4.78 0.93 0.90.74 0.23 2.27 1.76 0.77 0.150.51 0.02 b0.01 0.260.02 0.01 2.92 0.34 0.11 0.010.04 0.04 0.85 0.12 0.14 0.020.67 0.12 0.19 0.840. 83 0.07 0.80 0.28 0.34 0.070.61 0.67 0.26 0.12 0.47 0.170.25 0.03 0.13 0.66

compaction effect on anisotropy).

188 F.E. Berisso et al. / Geoderma 195–196 (2013) 184–191

(2012), who found the same trend for a fine-textured forest soil (siltloam to silty clay loam).

In table 3we note a general trend of lower values in FA for ka than FAfor Ds/Do except for Jokioinen soil at 0.5 m depth. This indicates that gastransport anisotropy is not only dependent on soil pore structure butalso on the mode of gas transport. If anisotropy was not dependent onthe mode of gas transport, then the ratio between FA for ka and FA forDs/D0 would be equal to 1. The deviation from 1 can be explained bythe fact that gas diffusion is determined by the cross-sectional areaavailable for diffusion, while gas convection is governed mainly by theavailability of large, interconnected air-filled pores (Poiseuille's law).For Jokioinen soil at 0.5 m depth, we attribute the deviating resultfrom the observed-trend for other combinations of sites and depths touncertain estimates of ka and Ds/Do for this nearly-saturated soil.

3.3. Model-derived pore characteristics

3.3.1. Morphological characteristics of pores at−100 hPamatric potentialThe indices characterising poremorphologywere calculated using the

tortuous tube model of Ball (1981) (Eqs. (1)–(3)). We note that the Balleffective pore diameter, dB, may also be interpreted as a soil structureindex because it relates directly to the so-called gas transport characteris-tic (ka/(DS/D0)), with high values of this ratio reflecting a structured soil(Kawamoto et al., 2006; Moldrup et al., 2010). Further, the tortuosity pa-rameter, τ (Eq. (2)), is inversely proportional to the specific diffusivity((DS/D0)/εa) that has been used for describing soil structural characteris-tics in other studies (e.g. as continuity index by Schjønning (1989), as dif-fusion efficiencybyKühne et al. (2012)). Our results as presented throughthe (Ball, 1981)model parametersmay thus aswell be evaluated by theseinterpretations of the fingerprints.

The compaction event 14 years prior to sampling at Brahmehem re-duced dB (Eq. (1)) in the vertical direction (significant at 0.3 and 0.5 mdepth; Fig. 1a). We observed a similar trend for horizontally sampledcores, but no significant differences among treatments were detectedat any single depth (Fig. 1b). The values for tortuosity (τ; Eq. (2)) hadthe reverse trend compared with dB, i.e. lower values of τ in the controlsoil than in the compacted soil, irrespective of direction of sampling.Statistically nearly significant differences (Pb0.1) were observed at0.3 and 0.9 m depth in the vertical direction (Fig. 1c), and at 0.5 and

(a)

Dep

th (

m)

0.0

0.2

0.4

0.6

0.8

1.0

(b)

dB(µm)0 100 200 300

0.0

0.2

0.4

0.6

0.8

1.0

(c)

(d)

τ(m

0 2

P = 0.01

P = 0.20

P = 0.02

P = 0.14

P = 0.03

P = 0.17

P = 0.17

P = 0.06

P = 0.78

P = 0.23

P = 0.31

P = 0.18

P = 0.75

P = 0.10

P = 0.30

P = 0.05

Fig. 1. (a, b) Effective pore diameter (dB); (c, d) tortuosity (τ); and (e, f) number of soil por(1981) for compacted (shaded) and control (open) soil at Brahmehem. Circles and rectanglemeans of medians observed in four replicate blocks.

0.9 m depth in the horizontal direction (Fig. 1d). As for dB, the valuesof nB (Eq. (3)) were smaller in the compacted soil than in the controlsoil and nearly significant differences (Pb0.1)were observed at all depthsin the vertical direction (except 0.7 m; Fig. 1e) and at the two upperdepths in the horizontal direction (0.3 and 0.5 m; Fig. 1f). Generally,ourmeasurements at−100 hPamatric potential indicate that the controlsoil at Brahmehem had more, wider and less tortuous pores than thecompacted soil.

Upon compression, macropores may progressively decrease in size orcollapse and disappear. Dexter and Richard (2009) suggested two hypo-thetical mechanisms through which changes occur to macropores (or towhat they called ‘structural porosity’) during compaction: 1) change atthe expense of macropore size alone (quantified by the effectivepore diameter, dB, in this study), and 2) change at the expense ofnumber of pores (here expressed by the number of pores per unitarea, nB). To evaluate these two mechanisms, we tested the hypoth-esis that: 1) dB changes with bulk density (BD), and 2) nB changeswith BD. The results showed a statistically significant correlation be-tween nB and BD (R2=0.31; P=0.02), but not between dB and BD(R2=0.07; P=0.15). Our results therefore support the hypothesisthat the macropores collapse rather than become progressively smallerin size during compaction. This is in agreementwithDexter and Richard(2009), who analysed data from two Polish soils under variousmanage-ment conditions. In contrast, Schäffer et al. (2008) analysed changes inartificialmacropores in soil cores during compression bymeans of com-puter tomography and observed a progressive reduction in pore radiuswith increasing compression. Schäffer et al. (2008) studied artificial,straight and perfectly spherical pores, while our study dealt with natu-ral pores.

The FA of dB for the Brahmehem soil was significantly lower than 1 at0.3, 0.5 and 0.7 m depth in the control and at 0.5 and 0.7 m depth incompacted soil (Fig. 2a), which indicates that the effective diameter ofthe vertical pores was wider than that of the horizontal pores. One canargue that this is due to the presence of biopores (burrows of anecicearthworms and channels of plant roots), which predominantly, butnot exclusively, run vertically down through the soil profile (Ellis andBarnes, 1980). However, the tortuosity of these pores did not displayanisotropy (Fig. 2b) in either control or compacted soil. The parame-ter nB displayed anisotropy in both control and compacted soil at all

CompactedControl

m -1)

4 6 8

CompactedControl

(e)

Ver

tical

(f)

nB cm-2 of soil

0 40 80 120

Hor

izon

tal

P = 0.03

P = 0.07

P = 0.45

P = 0.06

P = 0.07

P = 0.09

P = 0.18

P = 0.15

es in a given soil cross-section (nBf) at −100 hPa, derived using the tube model of Balls denote vertically and horizontally sampled soil, respectively. Values are least squares

(a)

-4

-2

0

2

VerticalHorizontal

(c)

log(

k a, µ

m2 )

-4

-2

0

2

(e)

-4

-2

0

2

(g)

0

2

(b)

0.3 m

VerticalHorizontal

(d)

0.5 m

(f)

0.7 m

(h)

0.9 m

Control Compacted Depth

189F.E. Berisso et al. / Geoderma 195–196 (2013) 184–191

depths (except 0.9 m; Fig. 2c) with FA>1. Thus, εa of the Brahmehemsoil at −100 hPa reflected a smaller number of pores with a largereffective diameter in the vertical direction in comparison to the hor-izontal direction, irrespective of compaction treatment.

As with the directly measured soil properties (ka and Ds/D0), ourresults on pore morphology indicated no effect of compaction on soilpore anisotropy. Thus for all soil depths below 0.3 m, there was no sig-nificant interaction between sampling direction and compaction treat-ment (P>0.05, Fig. 2). However, an important exception was 0.3 mdepth at Brahmehem,where compaction reduced anisotropy for the ef-fective pore diameter, dB (FA closer to 1 for the compacted treatment inFig. 2a). This fits well with our observations for Ds/D0 and ka (Table 3)and gives a physical/morphological basis for the observations.

3.3.2. Blocked soil porosity and pore continuity across matric potentialsThe regression lines of ka versus εa of the Brahmehem soil at three dif-

ferent matric potentials corresponding to the exponential model of Ballet al. (1988) in Fig. 3 showed a strong positive linear log–log relationshipbetween ka and εa. Similar relationships have been reported previously(e.g. Ball et al., 1988; Dörner and Horn, 2006; Schjønning et al., 2002).

Values ofN in the control soil revealed a significant difference betweenthe vertical and horizontal directions at 0.3 and 0.5 m depth, with consis-tently higher values in the horizontal direction (FA>1; Table 4). A similar,although non-significant, difference was observed for the 0.7 m depth.This higher value ofN for horizontal pores indicates a higher rate of open-ing of continuous air paths as the soil drained from−6 to−100 hPa. Theka values at −6 hPa were consistently greater in the vertical than in thehorizontal direction, despite similar εa (Fig. 3). This is most probablydue to vertical orientation of biopores/cracks, which resulted in betterpore connectivity in the vertical than in the horizontal direction in wetsoil conditions. As the soil drained further to−100 hPa, the difference be-tween vertical and the horizontal ka decreased (Fig. 3). This means thatthe increase in horizontal ka was greater than that in vertical ka for agiven increase in εa.

(a)

dBh/dBv

0.0 0.5 1.0 1.5

Dep

th (

m)

0.0

0.2

0.4

0.6

0.8

1.0

(b)

0.0 0.5 1.0 1.5

(c)

nBh/nBv

0 1 2 3

Dep

th (

m)

0.0

0.2

0.4

0.6

0.8

1.0

P = 0.03

P = 0.90

P = 0.90

P = 0.85

P = 0.25

P = 0.80

P = 0.79

P = 0.80

P = 0.63

P = 0.90

P = 0.52

P = 0.34

τh/τv

Fig. 2. (a) Factor of anisotropy for effective pore diameter (dBh/dBv); (b) tortuosity (τh/τv);and (c) number of soil pores in cm2 of soil (nBh/nBv) as a function of depth for control(open) and compacted (shaded) Brahmehem soil. Significant anisotropy denoted by aster-isks (open for control and shaded for compacted soils). P-values indicated in the diagramwere obtained from tests of the interaction between main effects (treatment and samplingdirection). Subscripts v and h denote vertical and horizontal, respectively.

log(εa, m3m-3)-2.0 -1.6 -1.2 -0.8

-4

-2

-2.0 -1.6 -1.2 -0.8

Fig. 3. Relationshipbetweenair permeability andair-filledporosity for Brahmehemsoil sam-pled in the vertical (circles) and horizontal (square) direction at depths (a, b) 0.3 m; (c, d)0.5 m; (e, f) 0.7 m; and (g, h) 0.9 m. Graphs on the left (a, c, e, g) show control soil,those on the right (b, d, f, h) compacted soil. Measurements were taken at −6, −30 and−100 hPa matric potentials from left to right. The error bars represent±1 standard error(n=4; for each block and treatment combination).

The regression model parameters, N (Eqs. (4) and (5)) given inTable 4 suggest a slight possibility for compaction effects on bothvertical and horizontal pores at the two upper soil depths, while noeffects were observed for the two lower depths (Table 4). More im-portantly, compaction significantly affected anisotropy for the N pa-rameter at 0.3 and 0.5 m depth (Table 4). Generally, compactionreduced the N parameter in the vertical direction, while it had theopposite effect in the horizontal direction. This means that soil com-paction reduced the anisotropy of this pore characteristic.

Compaction had only a slight effect on the blocked porosity, εb, thatmay be regarded as the εa of the soil sample that does not take part inconvective gas flow. However, some tendencies (Pb0.1) of compaction-affected increase in εb of vertical pores and decrease of horizontal poreswere detected at 0.3 and 0.5 m depth (Table 4).

The εb displayed anisotropy for both control and compacted soil (ex-cluding control soil at 0.9 m and compacted soil at 0.3 and 0.9 m), withconsistently higher values in the horizontal than in the vertical direction(FA>1; Table 4). This is in agreement with Dörner and Horn (2009),who reported higher εb in the horizontal than in the vertical direction in

Table 4Regression parameters for air permeability (ka) as a function of air-filled porosity (εa) for the Brahmehem soil. The value εb=10−log(m)/n gives model prediction of blocked air-filledporosity at ka=1 μm2: V=vertical, H=horizontal, FA=factor of anisotropy. Data for vertically sampled cores at Brahmehem were also used in the study by Berisso et al. (2012).

Depth m Model predictions: log (ka)=log(M)+N log(εa)

Slope (N) Blocked porosity εb=(10−log(m)/n)

Va Ha FAb P-valuec Va Ha FAb P-valuec

0.3 Control 3.87 9.88 2.55 0.02 0.03 0.08 1.42 0.01Compacted 7.44 6.22 0.83 0.95 0.05 0.06 2.43 0.53P-value 0.09 0.15 0.01 0.06 0. 07 0.02

0.5 Control 3.49 20.23 5.79 0.01 0.04 0.08 1.46 0.01Compacted 7.89 13.04 1.65 0.01 0.05 0.07 2.11 0.01p-value 0.16 0.12 0.05 0.08 0.23 0.05

0.7 Control 3.68 10.07 2.73 0.11 0.04 0.08 1.22 0.01Compacted 6.96 10.5 1.49 0.3 0.06 0.07 2.08 0. 01P-value 0.44 0.87 0.64 0.39 0.59 0.81

0.9 Control 8.75 13.47 1.53 0.72 0.09 0.09 0.82 0.42Compacted 9.18 12.67 1.38 0.74 0.08 0.07 0.98 0.58P-value 0.98 0.72 0.9 0.93 0.93 0.88c

a P-values for compaction effect.b P-values in column are for interaction between compaction and direction of sampling (compaction effect on anisotropy).c P-values in column are for significance of the Factor of Anisotropy (FA).

190 F.E. Berisso et al. / Geoderma 195–196 (2013) 184–191

subsoils (from 0.4 to 0.9 m depth). They attributed this anisotropic be-haviour to the dimensions of soil aggregates. According to their study,aggregates with a dominant axis appeared to induce a smaller εb alongthis axis, which resulted in anisotropy. In the Brahmehem soil, duringsampling we noted that the soil had a very coarse prismatic structure(from ~0.45 to ~0.9 m depth), which might have induced a relativelylower εb in the vertical direction. This explanation is not applicable to con-trol soil at 0.3 m depth, where we observed aggregates with sub-angularstructure. For εb we observed a significant interaction between compac-tion and anisotropy at the upper two soil depth, where compaction in-creased εb in the vertical direction without inducing significant changesin the horizontal direction (Table 4).

In general, Fig. 3 indicates that the difference in ka values betweenver-tically and horizontally sampled cores decreases drastically as the soildrains; implying a decrease in FA with decreasing water content. Thistrend of dependency of anisotropy on degree of saturation is in agree-ment with results from theoretical analysis (Assouline and Or, 2006;Mualem, 1984), computer simulations (Bear et al., 1987) aswell as exper-imental investigations (Dörner and Horn, 2009; Reszkowska et al., 2011).

4. Conclusions

Compaction 14 (Brahmehem) and 29 years (Jokioinen) prior to sam-pling decreased soil air permeability, ka, and gas diffusivity, Ds/D0, andhence considerably reduced the soils' ability to conduct gases. For the ver-tically sampled Brahmehem soil, compaction decreased the effective porediameter, dB and the number of pores per unit cross-sectional area, nB,while soil pore tortuosity, τ, increased. Thus, our results indicate the per-sistency of subsoil compaction for at least more than a decade.

The Ds/D0 displayed anisotropy in the Jokioinen clay soil only,whereas the ka showed trend of anisotropy for both soils with decreas-ing tendency with depth. We noted anisotropy on dB, nB and other poreindices (blocked air-filled porosity, εb; index that reflects rate of open-ing of continuous pore path with decreasing matric potential, N). Com-paction had generally no influence on the anisotropy when soil wasdrained to −100 hPa, except for ka of the Brahmehem soil at 0.3 mdepth, where ka in the vertical direction was reduced to the level evenlower than that of the horizontal ka. For the upper two soil layers atBrahmehem, there was a significant impact of compaction on anisotro-py of N and εb (pore indices derived by includingmatric potentials from−6 to−100 hPa).

Knowledge on the effect of anisotropy on gas transport is importantfor evaluating and monitoring the aeration status of soil for crop pro-duction. Present models for soil profile aeration do not take anisotropy

of the soil pore system into account. Finally, efforts should be geared to-wards designing new sampling/measurement methods which allow forevaluation of gas transport anisotropy on the same soil volume.

Acknowledgements

The technical assistance of B.B. Christensen, M. Koppelgaard,J.M. Nielsen, S.T. Rasmussen, C. Öhman, M-L. Westerlund, andI. Sarikka is highly acknowledged. We thank A. Westlin, L. Evasdotter,E. Petersson, L. Börjesson andM. Ylösmäkithe for their assistance duringfield experimentation and sample collection. We would also like tothank G. Persson for allowing us to use his field for the experiment.The pedological description of the soil profile at Brahmehemwas carriedout by Søren B. Torp. This work is part of a Scandinavian cooperation onthe effects of subsoil compaction on soil functions (www.poseidon-nordic.dk). The study reported here was funded by the Danish Ministryof Food, Agriculture and Fisheries, the Swedish Research Council for Envi-ronment, Agricultural Sciences and Spatial Planning (Formas) and theFinnishMinistry of Agriculture and Forestry via the Nordic Joint Commit-tee for Agricultural Research (NKJ). The Graduate School of Science andTechnology (GSST) at Aarhus University supported the PhD study of thefirst author.

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