Persistent effects of subsoil compaction on pore size distribution and gas transport in a loamy soil

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on14esx Soil & Tillage Research 122 (2012) 4251Contents lists available at SciVerse ScienceDirectSoil & Tillag.e1. IntroductionIn recent decades, the weight of agricultural machines hasincreased in order to meet the demands of modern agriculture.This ever-increasing weight of agricultural machines causes stresspenetration to deeper soil layers (Carpenter et al., 1985; Kelleret al., 2007; Lamande et al., 2007; Lamande and Schjnning, 2011;Zink et al., 2010) which may result in compaction at greater depthsthan reported previously. Wet conditions in autumn, winter andspring in the Nordic countries aggravate the effect of heavymachinery and lead to serious subsoil quality degradation.Arvidsson et al. (2000) showed that the risk of subsoil compactionwith commonly used machinery in southern Sweden is 100% forspring slurry application and more than 60% after October in sugarbeet harvesting.Compaction is a reduction in total porosity in a given soil mass.However, not all pores reduce proportionally. Various authors havereported a reduction due to compaction of primarily larger pores(Bullock et al., 1985; Dorner et al., 2010; Matthews et al., 2010;Richard et al., 2001; Schaffer et al., 2007). Such pores maycompletely disappear after repeated wheeling (Pagliai et al., 2003;Servadio et al., 2005; Startsev and McNabb, 2001). This preferentialloss of larger pores can potentially change many of the mostimportant soil ecological functions, such as transmission andstorage of water, and support of plant growth and microbialactivities (Ball, 1986).Soil compaction reduces saturated hydraulic conductivity(Horn et al., 1995) and may thus trigger surface runoff and watererosion. It may also induce preferential ow in macropores (Kulliet al., 2003; Etana et al., In review), which has been shown tofacilitate colloid transport of otherwise immobile pollutants suchas phosphorus and pesticides to receiving water bodies (Jarvis,2007). Studies of the effect of compaction on unsaturated hydraulicconductivity have produced conicting results. Richard et al.(2001) measured higher unsaturated hydraulic conductivity incompacted soil than in uncompacted. Zhang et al. (2006), on theother hand, did not observe any signicant changes in unsaturatedhydraulic conductivity. Soil compaction reduces soil aeration(Czyz, 2004) and increases emissions of the greenhouse gas N2Othrough denitrication at anaerobic sites (Bakken et al., 1987;Article history:Received 13 October 2011Received in revised form 14 February 2012Accepted 16 February 2012Keywords:SubsoilCompactionSoil poreGas diffusivityAir permeabilityPersistencyThe ever-increasing weight of agricultural machines exacerbates the risk of subsoil compaction, acondition believed to be persistent and difcult to alleviate by soil tillage and natural looseningprocesses. However, experimental data on the persistency of subsoil compaction effects on soil porefunctioning are scarce. This study evaluated and quantied persistent effects of subsoil compaction onsoil pore structure and gas transport processes using intact cores taken at 0.3, 0.5, 0.7 and 0.9 m depthfrom a loamy soil in a compaction experiment in southern Sweden (Brahmehem Farm). The treatmentsincluded four repeated wheelings with 10 Mg wheel loads. Water retention characteristics (WRC), airpermeability (ka) and gas diffusivity (Ds/Do) were measured. A dual-porosity model tted the WRC well,and there was a reduction in the volume of macropores >30 mm in compacted compared with controlsoil for all soil depths. Averaged for all sampling depths and also for some individual depths, both ka andDs/Do were signicantly reduced by compaction. Gas transport measurements showed that theexperimental soil was poorly aerated, with local anoxic conditions at water regimes around eldcapacity in all plots and depths, but with signicantly higher percentage anoxia in compacted soil. Ourmain ndings were that: (1) commonly used agricultural machinery can compact the soil to 0.9 m depth,(2) the effect may persist for at least 14 years, and (3) important soil functions are affected. 2012 Elsevier B.V. All rights reserved.* Corresponding author. Tel.: +45 8715 4756.E-mail address: Feto.Esimo@agrsci.dk (F.E. Berisso).0167-1987/$ see front matter 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.still.2012.02.005Persistent effects of subsoil compaction transport in a loamy soilF.E. Berisso a,*, P. Schjnning a, T. Keller b,c, M. LamaB.V. Iversen a, J. Arvidsson b, J. Forkman daAarhus University, Department of Agroecology, P.O. Box 50, DK-8830 Tjele, Denmarkb Swedish University of Agricultural Sciences, Department of Soil & Environment, Box 70cAgroscope Research Station ART, Reckenholzstrasse 191, Department of Natural Resourcd Swedish University of Agricultural Sciences, Department of Crop Production Ecology, BoA R T I C L E I N F O A B S T R A C Tjou r nal h o mep age: w wwn pore size distribution and gasde a, A. Etana b, L.W. de Jonge a,, SE-75007 Uppsala, Sweden and Agriculture, CH-8046 Zurich, Switzerland7043, SE-75007 Uppsala, Swedene Researchl s evier . co m/lo c ate /s t i l lF.E. Berisso et al. / Soil & Tillage Research 122 (2012) 4251 43Hansen et al., 1993; Simojoki et al., 1991). Poor root growth due todense and poorly aerated soil can reduce crop yield (Alakukku,1999; Hakansson and Reeder, 1994) and nutrient use efciencyand hence induce leaching of soil nitrogen.There are only limited experimental data on the persistency ofcompaction effects on functioning of soil pores. Most existingstudies have focused on the short-term compaction effect in thetopsoil and plough-pan layers. However, detrimental structuralchanges and associated adverse effects on transport propertiesmay be especially serious in the deeper subsoil, where regenera-tion through biological activity, wettingdrying and freezethawcycles occurs at a slower rate. Compaction of deeper layers isespecially problematic since it is invisible, cumulative andpersistent (Alakukku, 2000; Hakansson and Reeder, 1994; Hornet al., 1995; Voorhees, 2000).The objective of this study was to examine whether subsoilcompaction induced by repeated trafc with 10 Mg wheel loadshad persisted 14 years after the compaction event. A further aimwas to quantify the compaction effect on the soil pore system andits gas transport properties.2. Materials and methods2.1. SoilIn 2009, we revisited a eld soil compaction experimentestablished in 1995 at Brahmehem Farm (558490N, 138110E) nearKavlinge village, southern Sweden. The soil has developed onglacial till deposits and is classied as a Mollic Endogleyic Luvisolaccording to the FAO soil classication system (IUSS WorkingGroup WRB, 2006). The soil has a sandy clay loam texture, with aclay content ranging from 0.19 to 0.27 g g1. The soil organicmatter content ranges from 0.003 to 0.024 g g1. The sand contentranges from 0.45 to 0.54 g g1. We observed high texturalvariability between experimental plots at 0.7 and 0.9 m depths(data not shown).We tted the RosinRammler (1933) distribution function toour textural data in order to characterise the mass-size distributionof the soil:PX > x 100ex=ab (1)where P(X > x) is the percentage of particles by weight greater thanparticle size x, e is Eulers number (base of natural logarithm), and aand b are adjustable parameters. The a parameter represents theparticle size corresponding to the 37.78th percentile of thecumulative probability distribution (Perfect et al., 1993). The greaterthe a value, the larger the soil fragment that dominates thedistribution and vice versa. The b value describes the spread ofthe particle size: the smaller the b value, the wider the spread of thefragment mass and vice versa.For most combinations of plots and depths, the a and b valueswere in the range 73 to 128 and 0.3 to 0.48, respectively.However, there were two striking outliers, the soil samples from0.7 m depth (a = 218; b = 0.62) and 0.9 m depth (a = 330; b = 0.56)in a compacted plot, where the sand content dominated othertextural classes (data not shown). The combination of a and b forthese sampling spots indicates a relatively well-sorted, sandymaterial, and probably reects local hydraulic conditions duringthe deposition of the glacial till material.2.2. Compaction experimentThe original eld experiment aimed to study the effect of trafcwith heavy sugar beet harvesters on soil physical properties andcrop yield (Arvidsson, 2001). The experiment had a randomisedblock design with four replicate plots. A detailed description of theexperimental set-up can be found in Arvidsson (2001). In thepresent study, plots that were not wheeled during the experimentwere used as controls, while plots subjected to four repeatedwheelings (track-by-track to cover 100% of the area in the plots)with a 35 Mg sugar beet harvester in 1995 comprised thecompacted plots.The plots were run as a eld experiment until 1999 and thenreintegrated into the larger eld, which had been managed accordingto local farming practices in a 7-year crop rotation (winter rapewinter wheatsugar beetspring wheatwinter wheatsugar beetspring barley). The tillage regime in the eld includes mouldboardploughing (to 0.25 m depth), with occasional reduced tillage (toabout 0.1 m depth).2.3. Vertical stress in the soil prole at the time of compactionThe sugar beet harvester used in 1995 was equipped with a0.8 m wide tyre inated to 240 kPa and the wheel load was 10.4 Mg(Arvidsson, 2001). We estimated the vertical stresses in the soilprole below such a tyre as follows. First we calculated the tyresoil contact area and the stress distribution within the contact areausing the FRIDA model (Keller, 2005; Schjnning et al., 2008). Inputparameters for FRIDA were predicted from the tyre type, tyredimensions, wheel load, and actual and rated tyre inationpressure (Table 13 in Schjnning et al., 2006). Finally, we useddata for stress distribution in the tyresoil contact area as input,and calculated vertical stresses in the soil prole using the Sohnesummation procedure (Sohne, 1953) by setting a concentrationfactor (Frohlich, 1934) to 6 (for wet soil conditions; Sohne, 1953).The calculated vertical stresses beneath the centre line of thesugar beet harvester wheel were 207, 160, 116 and 84 kPa at 0.3,0.5, 0.7 and 0.9 m depth, respectively. A range of studies inScandinavian soils at water contents close to eld capacity haveshown that plastic strain (persistent compaction) of subsoil layersoften occurs if vertical stress exceeds approximately 50 kPa(Arvidsson et al., 2002; Keller et al., 2002; Keller and Arvidsson,2004). We noted from our calculations that the trafc event in1995 was likely to have induced vertical stresses higher than thisthreshold for all depths at which we calculated the verticalstresses. Our starting hypothesis in soil analysis was thus that wewould nd compaction effects at these depths.2.4. Sample collectionSampling took place in May 2009, when the experimental eldwas cropped with winter wheat. Prior to sampling, access pits ofapproximately 2 m by 2 m and 1.5 m deep were dug in each plotand horizontal planes were exposed in sequence for sampling at0.3, 0.5, 0.7 and 0.9 m depth. At each plot and sampling depth, weidentied three sub-areas of 0.5 m 0.5 m each, locatedapproximately 1 m apart. From each sub-area we collected fourundisturbed soil cores and approximately 1 kg of bulk soil. Coredimensions (height [H] and diameter [D], m) were H = 0.05,D = 0.072 for two of the cores (200 cm3; hereafter labelled coretype A), and H = 0.034, D = 0.061 for the two other (100 cm3;hereafter labelled core type B).A total of 384 core samples were taken (two treatments fourblocks four depths three sub-areas two replicate cores = 192of each core type). Both core types were collected by hammeringsharp-edged cylinders into the soil. The cylinders were then gentlyremoved from the bulk soil and roughly trimmed before lids weretted at each end.Before analyses, the cores were stored at 2 8C, while the bulksoil was air-dried at room temperature (25 8C). Type A soil coreswere used for determination of soil water retention, while airF.E. Berisso et al. / Soil & Tillage Research 122 (2012) 425144permeability and gas diffusivity were measured at selected matricpotentials on type B soil cores as described below. Bulk soil wasused for analysis of soil texture, total carbon content and particledensity.2.5. Laboratory measurements and calculationsSoil texture was determined by a combination of wet sievingand hydrometer methods. Total carbon was determined using aFLASH 2000 organic elemental analyser coupled to a thermalconductivity detector (Thermo Fisher Scientic, MA, USA). Sub-samples from each combination of block (each block contained oneplot of each treatment) and depth were pooled for soil particledensity determination using a pycnometer.In the laboratory, before the actual measurements, type A and Bsoil cores were treated identically. The cores were carefullytrimmed with a sharp-edged knife and the bottom ends tted withnylon cloth and saturated step-wise with water from beneath.Type A soil cores were drained sequentially to 6, 10, 30, 50,100, 500 and 1600 hPa matric potential on sand boxes andceramic plates to determine the water retention characteristics athigher (>100 hPa) and lower (500 hPa and 1600 hPa) matricpotentials, respectively. Finally, the samples were oven-dried at105 8C for 24 h. Samples were weighed at each matric potential andafter oven-drying to determine the water retention characteristics ofthe soil. The water retention at 1.5 MPa was determined ondisturbed (remoulded) soil samples in a pressure plate system.The soil dry bulk density (BD) was calculated from the weight ofthe oven-dry soil and total soil volume. Total porosity wasdetermined from dry bulk density and particle density. Volumetricwater content at each matric potential was obtained fromgravimetric water content and bulk density. Air-lled porosity(ea) at a given matric potential was calculated as the differencebetween total porosity and volumetric water content.Type B soil cores were sequentially drained to 6, 30 and100 hPa matric potential on sand boxes. Air permeability (ka) wasmeasured at each matric potential by the steady state method asdescribed by Iversen et al. (2001), using a pressure head of 5 hPa.Prior to measurements, the soil at the extreme edge was gentlypressed to the edge of the metal ring to minimise leaking of airbetween the inner wall of the cylinder and the soil (Ball andSchjnning, 2002). A volumetric ow rate through the soil coreswas recorded at each matric potential and ka was calculated fromDarcys law.Gas diffusion was measured at 100 hPa matric potential bythe non-steady state method suggested by Taylor (1949), usingequipment described by Schjnning (1985) and oxygen as thediffusing gas. In short, the soil cores were attached by an O-ring to adiffusion chamber that was ushed with oxygen-free nitrogen. Thediffusion through the soil core was followed by recording theoxygen concentration in the diffusion chamber every 2 min forapproximately 2 h. The diffusion coefcient, Ds, was calculatedaccording to Ficks second law and converted to gas-independentdiffusivity by relating it to the diffusion of oxygen in air, Do(0.205 cm2 s1 at atmospheric pressure and 20 8C; SmithsonianPhysical Tables).3. Model and statistics3.1. Water retention characteristics and pore size distributionWe tted the double-exponential equation proposed by Dexteret al. (2008) to our water retention data. The double-exponential(DE) model has ve adjustable parameters, and can be written as:u C A1eh=h1 A2eh=h2 (2)where C is the asymptotic value of volumetric water content aspore water suction approaches innity (i.e. the water content ash ! 1), A1 and h1 describe the rst peak of pores, and A2 and h2describe the second peak of pores. Dexter et al. (2008) related theA1 and h1 parameters to what they called textural porosity, whilethe pores described by A2 and h2 were interpreted as structuralpores. This model is convenient for soils displaying a dual porosityand the model parameters also have a physical meaning. In thisstudy, we regarded the sum of C and A1 as the pore volumedetermined primarily by soil texture, and we did not address thephysical meaning of the C parameter.The parameters in the DE model were determined by ttingEq. (2) to measured water retention data by nonlinear regressionanalysis using the R software. The root mean square error (RMSE)and bias of prediction were calculated and used for evaluating thet of predicted to measured data:RMSE 1nXni1d2ivuut (3a)bias 1nXn11di (3b)where di is the difference between predicted and measured valuesof water content, and n is the number of measurements. The poresize distribution predicted by the DE model was obtained bydifferentiating Eq. (2) with respect to matric potential as suggestedby Dexter et al. (2008).3.2. Statistical analysisAt 6 and 30 hPa, some of the air permeability measurementswere below the detection limit of our equipment (0.3 ml s1,which is equivalent to 0.15 mm2) and we obtained zero values.Zero values not only affect the means, but also bias the respectivestatistical analysis based on the means. Median values, observedper plot and depth, were used to handle this problem. Theapproach with median estimates per plot and depth was then usedfor all parameters.Air-lled porosity, gas diffusivity and air permeability weretested for normality prior to statistical analysis using the ShapiroWilk test. Water retention and gas diffusivity values followed anormal distribution, while most air permeability values werefound to be positively skewed and not normally distributed. Forsubsequent analysis and to obtain a normal distribution, the airpermeability values were log-transformed. We also checked thenormality of residuals after tting a statistical model (described inthe next paragraph) to our data in order to ensure that thenormality assumption of the model was satised.The statistical analysis followed a completely randomised blockdesign. A linear mixed model with xed treatment effects andnormally distributed block effects was tted for each depth, usingthe MIXED procedure in SAS version 9.2 (Littell et al., 2006). TheKenward and Roger method was used for calculation of degrees offreedom in the statistical tests (Kenward and Roger, 1997). Toaccount for high textural variability at 0.7 and 0.9 m depth (seeSection 2), we made an analysis of covariance by including claycontent in the linear mixed model. The statistical analysis of the DEparameters followed the same procedure as for all other variablesin this study.The overall effect of compaction on the whole subsoil layer(0.30.9 m depth) was analysed by a repeated measurementanalysis with plots as subjects. An autoregressive, AR(1), covari-ance structure was used to account for correlation betweensamples from the same plot at different depths.4. Results and discussion4.1. Effect of compaction on soil water retention and pore sizedistributionThe compaction event 14 years prior to sampling signicantlyreduced the volumetric water content at 6, 10, 30 and50 hPa matric potential at all four depths studied except 0.3 m,where the differences were signicant only at 6, 10 and30 hPa (Fig. 1ad). These results are in line with previousndings, e.g. those of Startsev and McNabb (2001), who studiedthe effect of compaction by harvesting equipment on medium-textured soils. They collected soil from 14 sites across west-centralAlberta (USA) and reported a reduction in soil water retentionbetween saturation and 100 hPa matric potential in highlycompacted soil.During curve-tting to measured water retention data, wenoted that all the DE model parameters were signicantly differentfrom zero at P < 0.001. This indicates that the model was not over-parameterised. We also noted that estimates for the modelparameters were in the range reported by Dexter et al. (2008).In almost all ts, the DE model was exible enough to capture allpoints at the dryer and the wetter ends of the scale (Fig. 1ad). Inaddition, bias was of a random nature (i.e. no systematic erroracross the matric potentials) and lay within the range 0.00004 to0.0006 m3 m3 for all potentials.Averaged across the matric potentials in our study, we found aRMSE of 0.004 m3 m3 for the DE model, while the full vanGenuchten (1980) equation, with ve adjustable parameters,yielded a RMSE of 0.011 m3 m3. This indicates that the waterretention characteristic (pore size distribution) of the soil at theexperimental site is better expressed with a bimodal model thanwith a unimodal model. The RMSE of the DE model for eachcombination of treatment and depth was calculated across allmatric potentials, and the values are shown in Fig. 1ad. Thehighest (0.005 m3 m3) and the lowest (0.001 m3 m3) RMSEvalues were obtained from the t of the compacted soil at 0.5and 0.7 m depth, respectively. Generally, these values were alsolower than those obtained by tting the van Genuchten (1980)equation (data not shown).The bimodal nature of the pore system in the Brahmehem soil isalso illustrated in Fig. 2ad. The curves in the diagrams correspondto pore size distribution, derived from water retention data asdescribed above (see Section 3). In general, we observed that thepeak corresponding to structural porosity (A2) in the Brahmehemsoil was smaller than the peak corresponding to textural porosity(A1).Four repeated wheelings with the heavy sugar beet harvester(35 Mg) in 1995 reduced the peaks that correspond to structuraland textural pores at all four depths except the peak of texturalpores at 0.5 m depth (Table 1), and slightly shifted them to lowermatric potentials (shift of h1 and h2 from wetter to drier end;(a)Matric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5oisture content(m3m-3)0.20.30.4 (b)Matric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5oisture content(m3m-3)0.20.30.41RMSE =0.004RMSE+=0.002RMSE =0.004RMSE+=0.005d co samtentF.E. Berisso et al. / Soil & Tillage Research 122 (2012) 4251 45Volumetric m0.00.1(c)Matric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5Volumetric moisture content(m3m-3)0.00.10.20.30.4RMSE =0.002RMSE+=0.001Fig. 1. Measured volumetric moisture content for compacted (shaded circles) ancompacted; dashed line, control) as a function of matric potential (h, hPa) for soils(RMSE) and compacted soil (RMSE+) were calculated for the whole range of matric poVolumetric m0.00.1(d)Matric potential, h (- hPa)00 10 1 10 2 10 3 10 4 10 5Volumetric moisture content(m3m-3)0.00.10.20.30.4RMSE =0.003RMSE+=0.003ntrol (open circles) soils and ts of the double-exponential equation (solid line,pled at (a) 0.3 m, (b) 0.5 m, (c) 0.7 m and (d) 0.9 m depth. The RMSE for control soilials for each treatment and depth. Signicant differences are indicated by asterisks.(a)Matric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5 10 6d/d(log h)0.000.040.080.120.16(b)Matric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5 10 6d/d(log h)0.000.040.080.120.16(d)/d(log h)0.080.120.16F.E. Berisso et al. / Soil & Tillage Research 122 (2012) 425146(c)d/d(log h)0.040.080.120.16Fig. 2ad). However, the compaction effect was found to bestatistically signicant only for the A2 parameter (Table 1). Thisindicates that the compaction treatment at the experimental sitereduced the soil structural porosity in particular.The effect of compaction on the A2 parameter can also beassessed from the regression analysis of A2 and BD. As expected, anincrease in bulk density of the soil due to compaction resulted in adecrease in A2 (Fig. 3) and the relationship can be given by:A2 0:4090:082 0:1990:0488BD;R2 0:35; P 0:0003 (4)Matric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5 10 60.00Fig. 2. Pore size distribution (du/d(log h)) as a function of matric potential at (a) 0.3, (b) 0.5soils. The pore size distribution equation, du/d(log h), was obtained by differentiating tTable 1Estimates of parameters of the double-exponential model (Eq. (3)) and probabilityvalues for tests of differences between treatments.Depth(m)Treatment C(m3m3)A1(m3m3)A2(m3m3)h1 (m) h2 (m)0.3 Control 0.136 0.156 0.067 57 0.64Compacted 0.142 0.149 0.044 58 0.83P-value 0.620 0.745 0.002 0.794 0.2340.5 Control 0.153 0.131 0.084 29 0.46Compacted 0.121 0.149 0.058 48 0.57P-value 0.202 0.538 0.039 0.196 0.1620.7 Control 0.147 0.137 0.083 30 0.56Compacted 0.133 0.122 0.067 40 0.60P-value 0.501 0.506 0.009 0.156 0.5400.9 Control 0.108 0.153 0.082 47 0.69Compacted 0.133 0.122 0.058 50 0.72P-value 0.135 0.412 0.072 0.173 0.382d0.04Here, and in the following equation, the gures in brackets indicatethe standard error.From regression equation (4), we predicted the critical value ofBD at which the A2 parameter would reach zero, i.e. the BD at whichMatric potential, h (- hPa)100 10 1 10 2 10 3 10 4 10 5 10 60.00, (c) 0.7 and (d) 0.9 m depth in the control (dashed lines) and compacted (solid lines)he double exponential equation (DE).Bulk density (Mg m-3 )1.55 1.60 1.65 1.70 1.75 1.80 1.85A1+C, A2 (m3m-3)0.000.050.100.150.200.250.300.35Fig. 3. Estimates for the DE model parameters (Dexter et al., 2008; Eq. (3)), A2 (opencircles) and A1 + C (shaded circles), as a function of bulk density.all structural pores would be destroyed. Densities higher than thisvalue could only be attained through loss of textural porosity. Forthe Brahmehem soil, the critical BD was 2.055 (0.27) Mg m3. Thisvalue was higher than the critical BD values of 1.58 and 1.89 Mg m3reported by Dexter et al. (2008) for two soils with an average BDof 1.39 Mg m3 (Rogow soil; Dexter and Richard, 2009) and1.68 Mg m3 (Babrowko soil; Dexter and Richard, 2009) respectively.In Fig. 3, the A1 + C term, which we refer to here as texturalporosity, is also given as a function of bulk density, calculated as:A1 C 0:5240:096 0:1440:059BD;R2 0:13; P 0:022 (5)Eq. (5) indicates a weak relationship between A1 + C and BD, asreected by the low R2 value. However, the A1 + C term wassignicantly decreased (P = 0.022; Eq. (5)) with increased BD. Thisresult conrms ndings by Coulon and Bruand (1989), whoreported a reduction in textural porosity in a sandy soil due tocompaction. However, we found that the effect of BD on A1 + C(Eq. (5)) was smaller than its effect on A2 (Eq. (4)), suggesting thatBD (compaction) has a stronger effect on structural pores than ontextural pores.A comparison between the A2 parameter and the air-lledporosity at 100 hPa was also made. In the following, the air-lledporosity at 100 hPa is taken to represent the volume of pores>30 mm tube-equivalent diameter, in accordance with textbookson soil physics (e.g. Hillel, 1982). This comparison helps to relateF.E. Berisso et al. / Soil & Tillage Research 122 (2012) 4251 47the A2 parameter to the classical categorisation of pore sizeaccording to subjectively dened xed boundary values (e.g. poreslarger than 30 mm are often referred to as macropores). From thecomparison, we observed a positive linear relationship (R2 = 0.67)and a uniform scatter of points around a 1:1 line (Fig. 4). A similarcomparison (not shown) illustrated that the A2 parameter had alinear relationship with the volume of pores >6 mm. However, allthe points ended up further below the 1:1 line when the A2parameter was plotted as a function of the volume of pores >6 mm.Kutlek et al. (2006) proposed a minimum equivalent radius of thestructural pore domain in the range 1935 mm for sandy clay loamA2 (m3m-3 )0.00 0.02 0.04 0.06 0.08 0.10 0.12Pores > 30m (m3m-3)0.000.020.040.060.080.100.12Fig. 4. Volume of pores >30 mm tube-equivalent diameter as a function of the A2parameter of the double-exponential equation (Dexter et al., 2008; Eq. (3)) at alldepths (0.30.9 m) in control (open circles) and compacted (shaded circles) soils.soil under different levels of uniaxial load. Our modelled dataconrmed the validity of this proposal.4.2. Total porosity and functioning of soil pores at 100 hPa matricpotentialThe four repeated wheelings in 1995 reduced the total porosityat all four soil depths studied (Fig. 5a). This reduction wasstatistically signicant at P < 0.1, except at 0.5 m depth. Thereduction in total porosity observed here conrms the increase inbulk density reported by Arvidsson (2001) in the same experiment35 years after the compaction event. Hence, there has been noincrease in total porosity of the compacted soil during the past 10years, indicating the persistency of subsoil compaction. Similarresults were reported by Alakukku (1996) for a clay and an organicsoil over the three successive years following experimental trafcin Finland. In another study, Ishaq et al. (2003) reported aconsiderable reduction in total porosity below 0.15 m depth fouryears after a compaction event on a sandy clay loam soil in atropical region of Pakistan.The air-lled porosity values, ea, measured at 100 hPa weresignicantly lower in the compacted treatment than in the controltreatment at 0.3 m depth, and the same trend was found for theother depths studied (Fig. 5b). Our observed reductions in volumeof ea at 100 hPa (0.031, 0.020, 0.032 and 0.027 m3 m3,respectively at 0.3, 0.5, 0.7 and 0.9 m depth) are similar to ndingsby Alakukku (1996), who reported 0.03 and 0.02 m3 m3 reduc-tions at 0.30.4 and 0.40.5 m depth, respectively, in a clay soil.However, that study found no reduction in total porosity at 0.50.6 m depth, possibly due to the smaller wheel load of 4.5 Mg usedin that study, compared with 10.4 Mg in the present study.For most combinations of depth and experimental treatment,the ea values were below 10% (0.1 m3 m3), the value suggested tobe the critical lower limit for plant growth (Grable and Siemer,1968). The low ea values in both control and compacted soilsindicate that the soil at the site was generally dense prior to thestart of the compaction experiment.Diffusion is the main process taking gases to and from respiringroots and microbes in the soil prole. Stepniewski (1981)combined measurements of gas diffusion with literature reportsof lower thresholds of air-lled pore space for satisfactory plantgrowth and identied a critical Ds/Do band between 0.005 and 0.02,based on a range of soils. Schjnning et al. (2003) conrmed this foraerobic microbial activity and found the lower threshold to be validfor loamy soils, while sandy soils seemed to demand a higher Ds/Do.It can be seen from Fig. 5c that even the control soil at ourexperimental site exhibited diffusivity values close to or below theDs/Do threshold of 0.005. The compaction treatment decreased thediffusivity further for all four depths studied (Fig. 5c; statisticallysignicant at 0.3 m depth). A matric potential of 100 hPa isconsidered eld capacity (e.g. Al Majou et al., 2008; Schjnning andRasmussen, 2000). Our results thus indicate that the compactedsoil at the experimental site is likely to experience critically lowoxygen concentrations in the soil prole, at least at water contentshigher than eld capacity.The changes in gas concentrations in the soil prole withgradients in soil respiration and with changes in soil water contentare dynamic and complex (e.g. Sierra and Renault, 1998;Bartholomeus et al., 2008). The diffusivity thresholds discussedabove are dependent on the oxygen consumption, which is likely tobe lower for subsoil layers than for topsoil. We applied a simplesteady-state model for calculation of oxygen concentration in thesoil prole based on soil respiration and the soil oxygen diffusioncoefcient (Glinski and Stepniewski, 1985; Schjnning, 1989).Because we had no knowledge of the respiration rates for theexperimental soil, we used data from Schjnning et al. (2003)F.E. Berisso et al. / Soil & Tillage Research 122 (2012) 425148Total porosity (m3m-3 )0.00 0.10 0.20 0.30 0.40Soil depth (m)0.30.50.70.90.0Soil depth (m)0.30.50.70.0P=0.011P=0.547P=0.099P=0.054P=0.029P=0.188P=0.172(a)(c)(topsoil) and Sierra and Renault (1998) (subsoil layers), while weused the oxygen diffusion coefcients obtained in this study. Thesesimulations indicated a decline in oxygen concentration from theatmospheric 0.209 m3 m3 to 0.11 m3 m3 at 1 m depth for thecontrol soil and 0.04 m3 m3 at 1 m depth for the compacted soil(calculations and data not shown). Although recent research showsthat many factors affect root oxygen stress (Bartholomeus et al.,2008), our simulations indicate that the compaction-inducedreduction in the volume of air-lled macropores and gas diffusionmost probably had an important inuence on aeration of the soilprole at our experimental site. Compaction-induced effects on theoxygen concentration in a soil prole have been reportedpreviously (e.g. McAfee et al., 1989).The compaction event 14 years prior to soil sampling reducedair permeability, ka, at 100 hPa matric potential at all four soildepths studied, although the trend was only statistically signi-cant at 0.3 and 0.9 m depths (Fig. 5d). While the differencesbetween treatments were not signicant at 0.5 and 0.7 m depth,the estimated values of ka were reduced by 25 and 30%,respectively. Fish and Koppi (1994) classied soil into vepermeability classes based on eld ka measurements and a scorefrom the visual assessment of soil morphological properties relatedto ka (e.g. abundance of biopores). According to this classication,all four depths of soil from our experimental site can be groupedinto the low permeability class (ka 20 mm2). These low values ofka could reect the nature of this soil, while our results indicatethat high mechanical stresses aggravated the problem.Relative diffusivity (-)0.000 0.005 0.010 0.015 0.0200.9 P=0.131Fig. 5. (a) Total porosity, (b) air-lled pore space, (c) relative diffusivity and (d) air permea(open circles). The values shown are least squares means of medians observed in four differences between control and compacted treatments.Air filled pore space (m3m-3 )0.00 0.04 0.08 0.12 0.16Soil depth (m)0.30.50.70.90.0Soil depth (m)0.30.50.70.0P=0.004P=0.398P=0.182P=0.112P=0.007P=0.180P=0.188(b)(d)The individual soil cores collected at the experimental site canbe regarded as representative volumes of local soil in the soilprole. Hence the distribution of Ds/Do and ka values for controland compacted soil adds an extra dimension to the estimatespresented in Fig. 5c and d. Fig. 6a shows Ds/Do measured onindividual cores at 100 hPa matric potential. The dottedhorizontal line (at Ds/Do = 0.005) indicates the lower limit foraerobic microbial activity in loamy soil (Schjnning et al., 2003;Stepniewski, 1981). For our control soils, Ds/Do exceeded thecritical value in 29, 46, 71 and 22% of cores taken at 0.3, 0.5, 0.7 and0.9 m depth, respectively. The corresponding values for compactedsoil were 0, 25, 58 and 25% of cores, i.e. the Ds/Do value was lowerthan this critical value for all compacted cores at 0.3 m depth.These results suggest that at eld capacity, soil from 0.5 and 0.7 mdepth in control plots would be able to maintain the minimum gasdiffusion requirement for adequate soil aeration. However, most ofthe soil from 0.3, 0.5 and 0.9 m depth in compacted plots and 0.3and 0.9 m depth in control plots would need to be drained furtherto attain this minimum gas diffusion value for aerobic microbialactivity.From Fig. 6a, it can also be seen that the Ds/Do values for thecores collected in the sorted, sandy soil of the plots with deviatingtexture (yellow/grey symbols) were generally higher than theother observations. At 0.7 and 0.9 m depth, when the values fromthe compacted plot with a different texture were ignored, thepercentage of cores with Ds/Do higher than the critical valuedecreased to 33 and 0%, respectively.log (Permeability, ( m2))0.0 0.4 0.8 1.20.9P=0.031bility measured at 100 hPa for compacted (shaded circles) and control treatmentsreplicate blocks. P-values show the results of the linear mixed model tests on the0.3 0.5 0.7 0.9Depth (m)log (Permeability ,m2)-1.0-0.50.00.51.01.52.02.5(b) the compacted (shaded) and the control soils (open circles) at all four depths studied.dotted line in (a) indicates the lower limit for aerobic microbial activity for loamy soila, mm2) = 1.3; ka = 20 mm2) are in the low permeability class according to Fish and Koppisidered impermeable according to Ball et al. (1988). (For interpretation of the referencescle.)F.E. Berisso et al. / Soil & Tillage Research 122 (2012) 4251 49Fig. 6b shows log-transformed values of ka for individual coresat 100 hPa matric potential. The dotted horizontal line (at log(ka,mm2) = 0; ka = 1 mm2) shows the limit for impermeable soil (Ballet al., 1988), while the dashed line (at log(ka, mm2) = 1.3;ka = 20 mm2) shows the limit for low permeability soil (Fish andKoppi, 1994). Most of the cores from control and compactedtreatments lie in the region between these two limits. However,30% of the soil cores from 0.3 m depth in compacted treatmentswere classied as impermeable soil. In addition, about 12% of soilcores at 0.5 and 0.9 m depth fell into this category.This detailed analysis of gas transport measurements clearlyreveals the existence of local areas with anoxic conditions in thesoil. This was the case even for control plots, but the problemwas signicantly aggravated in the compacted plots. It isobvious that the soil at the experimental site is generally poorlyaerated.4.3. Functioning of soil pores in wet soil conditionsDepth (m)Relative diffusivity (-)0.0000.0050.0100.0150.0200.025(a)0.3 0.5 0.7 0.9Fig. 6. (a) Gas diffusivity and (b) air permeability measured on individual cores fromCores from a compacted plot with a different texture are shaded yellow/grey. The (Stepniewski, 1981; Schjnning et al., 2003). In (b), soils below the dashed line (log(k(1994), while soils with ka below the dotted line (log(ka, mm2) = 0; ka = 1 mm2) are conto colour in this gure legend, the reader is referred to the web version of the artiThe climate in most parts of the Nordic countries ischaracterised by wet subsoil conditions in autumn, winter andspring. During these periods, pores with large diameter arecritically important in improving the aeration of the generally wetsoils. At 6 hPa matric potential, where pores with equivalentdiameter >500 mm are air-lled, there was a signicant reductionin ea due to compaction at 0.3 m depth, with the estimate for thecontrol treatment being about threefold higher than for thecompacted treatment (Table 2). No statistically signicantcompaction effect on ea was found at 0.5, 0.7 and 0.9 m depth.For the soil at 0.3 and 0.7 m depth, reduced ea due tocompaction resulted in a signicant reduction in ka at 6 hPamatric potential (P < 0.05, Table 2). Although we measured thehighest values of ea at 0.9 m depth (both in control and compactedsoils), the ka was relatively low. Except for control soils at 0.5 and0.7 m depth, the ka values of soil at 6 hPa were below 1 mm2 andthey may be regarded as effectively impermeable (Ball et al., 1988)and completely limiting for plant growth according to McQueenand Shepherd (2002).For all four soil depths studied, the general trend observed for eaand ka at 6 hPa matric potential was also observed when thesamples were further drained at 30 hPa matric potential (Table 2).At 30 hPa there were no signicant compaction effects on ea and kaat 0.5 and 0.7 m depth, while the values were signicantly reducedat 0.3 m depth (P < 0.05 for both ea and ka) and 0.9 m depth (P < 0.1and P < 0.05 for ea and ka, respectively). At this matric potential,although the ea values at all four depths were lower than the criticallimit, the ka values were higher than the completely limiting valueaccording to McQueen and Shepherd (2002), except for the soil from0.3 m depth in the compacted plots.4.4. Compaction effects averaged for the subsoil proleTo test the persistency of compaction in the whole subsoil layer(0.30.9 m depth), we included all depths in a repeated measure-ment analysis (see Section 3.2). The results showed that thecompaction that was inicted in 1995 continues to signicantlyreduce total porosity, ea and ka at 6, 30 and 100 hPa and Ds/Doat 100 hPa 14 years after the event (Table 3). These resultssummarise our ndings and unequivocally show that (i) thecompaction treatment performed in 1995 induced plastic soildeformation and (ii) subsoil compaction in this soil has persistedTable 2Air-lled pore space (ea) and air permeability (ka), and probability values for tests ofdifferences between treatments. In the column headings, all variables that havesubscript 6 or 30 are measurements for 6 and 30 hPa matric potential,respectively.Soil depth (m) Treatment Air-lledporosity (m3m3)Air permeability(mm2)ea 6 ea 30 ka 6a ka 30a0.3 Control 0.039 0.060 0.450 5.916Compacted 0.014 0.032 0.002 0.040P-value 0.021 0.003 0.009 0.0070.5 Control 0.040 0.069 1.030 6.063Compacted 0.030 0.051 0.050 3.528P-value 0.138 0.187 0.105 0.1520.7 Control 0.045 0.081 1.155 9.204Compacted 0.032 0.061 0.037 3.999P-value 0.354 0.328 0.009 0.1700.9 Control 0.056 0.086 0.008 5.054Compacted 0.044 0.071 0.003 1.338P-value 0.143 0.084 0.624 0.040a Geometric means.escrF.E. Berisso et al. / Soil & Tillage Research 122 (2012) 425150for 14 years. Previous reports on persistent subsoil compaction inthe humid Nordic countries exist, but have focused on bulk densityand penetration resistance alone. For instance in Denmark,Schjnning and Rasmussen (1994) found signicantly higherpenetration resistance in the compacted subsoil of a coarse sandyand a loamy soil ve to six years after trafc with 5 Mg wheel loads.In a clay soil in Finland, Alakukku (1996) found persistent effects ofcompaction on total porosity/bulk density for nine years. Onexamining nine different eld experiments in Sweden on soils withclay contents ranging from 6 to 85%, Etana and Hakansson (1994)observed persistence of subsoil compaction in terms of bulkdensity and penetration resistance by comparing results recordedone year and 11 years after experimental trafc.5. Conclusions and perspectivesThe upper 0.9 m of the agricultural soil studied here wasmechanically compacted by trafc with heavy machinery and thateffect had persisted for at least 14 years, with negative effects onsoil porosity and gas transport properties. The conditions foraeration of the soil prole were reduced to levels consideredcritical to aerobic microbial activity. This may promote increasedproduction and potential emissions of greenhouse gases such asN2O. The low permeabilities recorded in the compacted soil mayincrease the risk of preferential convective ow of water inperiods with high precipitation. This may affect even deeperlayers of the vadose zone and carry contaminants to receivingwater bodies.The mechanisms responsible for the natural amelioration of soilcompaction are nearly absent in subsoil layers. There is reason tobelieve that the compaction effects documented here will persistfor decades or even longer. This demonstrates the urgent need toavoid subsoil compaction. It is generally accepted that the level ofvertical stress in deep soil layers is determined primarily by thewheel load. Therefore, to avoid subsoil compaction, wheel loadsneed to be reduced.AcknowledgementsThe technical assistance of B.B. Christensen, M. Koppelgaard,J.M. Nielsen, S.T. Rasmussen and C. Ohman is highly acknowl-edged. We thank A. Westlin, L. Evasdotter, E. Petersson and L.Borjesson for the assistance during eld experimentation andsample collection. We also would like to thank G. Persson forTable 3Soil total porosity (utot), air-lled pore space (ea), relative diffusivity (Ds/Do) and air pdifferences between treatments. In the column headings, all variables that have subTreatment Porosity and air-lled pore space (m3m3) utot ea 6 ea 30 ea 100Control 0.3827 0.045 0.074 0.101 Compacted 0.3571 0.030 0.054 0.077 P-value 0.033 0.017 0.011 0.020 a Geometric means.allowing us to use his eld for the experiment and Dr. SrenHjsgaard and Dr. Ulrich Halekoh for their support in the use of theR software for statistics. This work is part of a Scandinaviancooperation on the effects of subsoil compaction on soil functions(www.poseidon-nordic.dk). The study reported here was fundedby the Danish Ministry of Food, Agriculture and Fisheries and theSwedish Research Council for Environment, Agricultural Sciencesand Spatial Planning (Formas) via the Nordic Joint Committee forAgricultural Research (NKJ). The PhD School SAFE at the Faculty ofAgricultural Sciences, Aarhus University, supported the PhD studyof the rst author.ReferencesAl Majou, H., Bruand, A., Duval, O., 2008. The use of in situ volumetric water contentat eld capacity to improve prediction of soil water retention properties. Can. J.Soil Sci. 88, 533541.Alakukku, L., 1996. Persistence of soil compaction due to high axle load trafc. 1.Short-term effects on the properties of clay and organic soils. Soil Till. Res. 37,211222.Alakukku, L., 1999. Subsoil compaction due to wheel trafc. Agric. Food Sci. Finland8, 333351.Alakukku, L., 2000. Response of annual crops to subsoil compaction in a eldexperiment on clay soil lasting 17 years. 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Load risks of subsoil compaction and depths ofstress propagation in arable luvisols. Soil Sci. Soc. Am. J. 74, 17331742.Zhang, S.L., Grip, H., Lovdahl, L., 2006. Effect of soil compaction on hydraulicproperties of two loess soils in China. Soil Till. Res. 90 (12), 117125.Persistent effects of subsoil compaction on pore size distribution and gas transport in a loamy soilIntroductionMaterials and methodsSoilCompaction experimentVertical stress in the soil profile at the time of compactionSample collectionLaboratory measurements and calculationsModel and statisticsWater retention characteristics and pore size distributionStatistical analysisResults and discussionEffect of compaction on soil water retention and pore size distributionTotal porosity and functioning of soil pores at -100hPa matric potentialFunctioning of soil pores in wet soil conditionsCompaction effects averaged for the subsoil profileConclusions and perspectivesAcknowledgementsReferences

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