Effect of soil compaction and N fertilization on soil pore characteristics and physical quality of sandy loam soil under red clover/grass sward

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Soil & Tillage Research 144 (2014) 819Effect of soil compaction and N fertilization on soil pore characteristicsand physical quality of sandy loam soil under red clover/grass swardTomasz Gab *Institute of Machinery Exploitation, Ergonomics and Production Processes, University of Agriculture in Krakow, ul. Balicka 116B, Krakow 31-149, PolandA R T I C L E I N F OArticle history:Received 2 September 2013Received in revised form 28 April 2014Accepted 17 May 2014Keywords:Nitrogen fertilizationSoil compactionRed cloverGrassesSoil porositySoil water retentionA B S T R A C TDuring the 20th century grassland production systems were intensified with higher rates of nitrogenusage and increasing heavy vehicular activity. This tendency is reflected in the higher soil degradationrisk. However, in recent years European agri-environmental policies have promoted low-input grasslandmanagement practices. These changes in fertilization intensity may interact with the effects of machinetraffic in affecting soil physical properties. The objective of this study, was to investigate the influence ofsoil compaction caused by tractor traffic and N fertilization levels on a soil pore system under a clover/grass mixture during the period from 2010 to 2012. This experiment was established in a split-plot designwith fertilization as a main plot and soil compaction as a subplot. The N fertilizer treatments used were:untreated control (N0), 80 kg N ha1 (N80) and 160 kg N ha1 (N160). Four compaction treatments wereapplied using the following number of tractor passes: untreated control (P0), two passes (P2), four passes(P4) and six passes (P6). Undisturbed soil samples were collected in 20102012 in order to determine thewater retention parameters and morphometric characterization of soil pores. The soil watercharacteristic curve was determined using pressure plates. The soil macropore system was alsocharacterized using image analysis on the sections of soil samples hardened with polyester resin.The mineral fertilization did not significantly affect any physical parameters of soil at the trial. Theintensive wheeling resulted in a higher value of bulk density and penetration resistance and lower valuesof total porosity. The soil compaction has distinctly influenced the soil water retention characteristics inthe high matric potential range, which corresponds mainly with large pores (transmission pores andfissures) and storage pores. The result of changes in soil porosity was to increase the plant available watercapacity. On the other hand, the relative field capacity indicated that in compacted soil under grasslandplants, the biological activity was limited by insufficient soil aeration. This conclusion is in agreementwith the results in plant production, which showed decrease in root and above ground biomass as theresult of compaction. 2014 Elsevier B.V. All rights reserved.Contents lists available at ScienceDirectSoil & Tillage Researchjournal homepage: www.else vie r .com/locate /s t i l l1. IntroductionAgricultural production systems tend to increase the number ofpasses and the loads carried on agricultural vehicles, resulting in apotential for increased soil compaction (Newell-Price et al., 2013).According to the EU Commission, soil compaction is recognized asone of the main factors that can lower crop yields and thus hasbecome a serious agricultural problem (European Commission(EC), 2006). Compaction leads to soil structure degradation, whichresults in a deterioration in physical properties. Associated withthis bulk density and soil strength, measured as penetrationresistance, are increased (Richard et al., 2001; Pagliai et al., 2003;* Tel.: +48 12 662 45 51.E-mail address: rtglab@cyf-kr.edu.pl (T. Gab).http://dx.doi.org/10.1016/j.still.2014.05.0100167-1987/ 2014 Elsevier B.V. All rights reserved.Hamza and Anderson, 2005). To quantify soil structural changesfollowing agricultural activities, besides traditional measurementssuch as bulk density, penetration resistance, total porosity, andpore space measurements are being increasingly used (Pagliaiet al., 2004). The soil pore system is widely recognized as beingresponsible for available soil water content and aeration porosity,which affect root and crop growth. Changes in soil porosity arereflected in the water retention characteristics and are extremelyimportant in irrigation systems (Nawaz et al., 2013).The effect of soil compaction depends on the compaction effort,soil type, water status, landscape position, and cropping systeminvolved (Green et al., 2003; Sillon et al., 2003; Tarawally et al., 2004;Zhang et al., 2006). It is a serious problem for perennial crops, wherethesoil issubjectedtotraffickingwithoutanannual tillageoperation.Soil strength increased year-after-year due to machine traffic duringfield operations. The wheels of these machines cause direct planthttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.still.2014.05.010&domain=pdfmailto:rtglab@cyf-kr.edu.plhttp://dx.doi.org/10.1016/j.still.2014.05.010http://dx.doi.org/10.1016/j.still.2014.05.010http://www.sciencedirect.com/science/journal/01671987www.elsevier.com/locate/stillTable 2Average monthly temperature and total precipitation at the experimental siteduring the period 20102012 plus the long-term averages.2010 2011 2012 19611990Monthly average temperature (C)January 6.3 1.2 1.2 1.9February 2.2 2.6 6.6 0.8March 3.3 3.6 4.4 4.4April 9.0 10.3 9 4 7.2May 12.8 13.5 15.0 13.6June 17.5 18.2 17.4 15.1July 20.7 17.7 20.3 17.1August 18.4 19.0 18.9 16.6September 12.3 14.1 14.0 10.5October 8.7 8.6 8.6 5.9November 3.1 2.1 5.0 0.5December 2.3 1.6 2.9 2.4Annual mean 7.9 8.7 8.5 7.4Sum of monthly precipitation (mm)January 44 26 52 34February 32 8 28 32March 31 15 17 34April 40 78 49 48May 299 48 18 83June 135 33 144 97July 105 186 71 85August 128 73 55 87September 113 14 44 54October 14 32 96 46November 27 0 22 45December 43 38 27 41Annual sum 1010 552 622 681Table 1Basic soil physical and chemical properties of Mollic Fluvisol from the trial location(020 cm layer).pH(KCl) 6.5Organic C g kg1 12.5Total N g kg1 1.39C:N ratio 9.0P mg kg1 107.2K mg kg1 138.0Mg mg kg1 67.9Solid particle density Mg m3 2.65Sand g kg1 560Silt g kg1 270Clay g kg1 170Texture Sandy loamT. Gab / Soil & Tillage Research 144 (2014) 819 9damage, which are reported to be as important in terms ofcontributing to decreased plant yield as soil compaction. Moreover,perennial forage crop production demands a very intense vehicularactivity, especially during crop harvesting operations (Jorajuria andDraghi,1997). Inrecentyearsmorepowerful and heaviertractors andmachinery have been used on farms.On the other hand the extensification in livestock productionthat is promoted by the EU results in a reduction in stocking rateand fertilizer application (European Union, 1998). The design andimplementation of the resulting national agri-environmentalschemes radically modified the direction of grassland systemdevelopment in the EU. One of the most important changes was inthe regulation and incentives designed to limit grassland fertilizerapplication in order to reduce nutrient losses and mitigate soil andwater pollution (Gibon, 2005).The soil physical properties can be modified by mineralfertilization in two ways. The most direct impact is achieved bymodifying the chemical composition of the soil solution, its ionicstrength, pH and soil aggregate stability (Schroder et al., 2011;Bronick and Lal, 2005). The major benefits of fertilizers on soilproperties are improved crop yields, by increasing the root andresidue biomass in soil, thereby resulting in higher organic mattercontent. Organic matter has been shown to have beneficial effects onsoil structure and leads to higher hydraulic conductivity (Gab andKulig, 2008; Hargreaves et al., 2008). The increase in soil organiccarbon reduces bulk density and increases water holding capacityand soil aggregate stability (Celik et al., 2004; Herencia et al., 2011).Researchers have also reported an increase in the field capacity andwilting point (Chang et al., 1983). These changes in the physicalproperties are ascribed to the mixing of soil with less dense organicmaterial (Khaleel et al., 1981). These effects are most clearlyidentified in relatively compacted, fine-textured soils (Aggelidesand Londra, 2000; Celik et al., 2004) or in coarse-textured soils(Turner et al., 1994). Many parameters affect the physical propertiesof soil, so that a specific impact derived from NPK treatment issometimes hard to demonstrate. It is usually low, especially whencompared to the impact of crop residues or management practices(Simansky et al., 2008). Most of the available information in theliterature deals with annual crops. However, the interaction betweenthe effect of soil compaction and the effect of fertilization used ongrasslands has not, so far, been studied.In this study, we assessed the effect of different N fertilizationrates and trafficking intensity on the soil quality. The objective of thestudy was to evaluate the water retention characteristics and themorphometric characterization of soil pores with a wide diameterrange running from 0.005 to >2000 mm with a special focus onmacropores investigated using image analysis of soil sections.2. Material and methods2.1. Site, location and climateThis study was conducted as a field experiment located inMydlniki near Krakow, Poland (50040N,19510E, 211 m a.s.l.) over athree-year period (20102012). The field experiment was locatedon sandy loam Mollic Fluvisol (IUSS Working Group WRB, 2007).Table 1 details some of the soil characteristics. The climate of theexperiment site is temperate-continental. Average annual precipi-tation reaches 681 mm per year, and the mean daily temperature isaround 7.4 C (Table 2).2.2. Field trial design and treatmentsThe soil before trafficking was plowed in the autumn 2009 andharrowed in March 2010 for seedbed preparation, after whichseeds of red clover/grass mixture were sown. Experimental plots(9 m2) were established with four replications as a randomizedcomplete block in split-plot design with fertilization as the mainplot and compaction as the sub plot. Each plot was sown with amixture of 10 kg ha1 of perennial ryegrass (Lolium perenne L.) cv.Diament, 13 kg ha1 of meadow fescue (Festuca pratensis Huds.) cv.Skra, 3 kg ha1 of Timothy (Phleum pratense L.) cv. Skala, 2 kg ha1of Kentucky bluegrass (Poa pratensis L.) cv. Skiz and 2 kg ha1 of redclover (Trifolium pratense L.) cv. Nike.The N fertilizer (ammonium nitrate, 34% N) treatments used inthe main plots were: untreated control (N0), 80 kg N ha1 (N80)and 160 kg N ha1 (N160). The doses of nitrogen were applied threetimes a year, first in March and after the first and second harvests ina ratio of 50, 25 and 25%, respectively. The phosphorus andpotassium fertilization was always at the annual rate of 80 kgP2O5ha1 (triple super phosphate, 46% P2O5) and 160 kg K2O ha1(potassium chloride, 60% K2O), respectively. The P and K fertilizerswere applied every year in two terms, first in March and then afterthe second harvest, in equal rates of 50%. No organic fertilizer wasapplied on the experimental plots.Four compaction treatments in sub plots were applied using thefollowing numbers of tractor passes, namely: untreated control10 T. Gab / Soil & Tillage Research 144 (2014) 819(P0), two passes (P2), four passes (P4) and six passes (P6). URSUS C-360 tractor of 2056 kg weight was used for traffic simulation. Theinflation pressure of the front tractor tires (6.00-16 6PR) of thetractor was 150 kPa and that of the rear tires (14.928 8PR) 100 kPa.The process of multiple passes was applied three times a year, afterevery harvest. The wheel passes covered the whole area of the plotin a wheel-beside-wheel design. During the trial period the sameexperimental sub plots were compacted through a set of passeswithout rotating the plots. The trafficking was applied when soilmoisture content was of approximately 0.30 cm3 cm3 (0.03 cm3cm3). We used a portable probe with an ECH2O EC5 sensor(Decagon Devices, Pullman, Washington, USA) to measure soilmoisture during the traffic phase. At the beginning of theexperiment the probe was calibrated for the trial site usinggravimetric samples. The soil moisture during trafficking was inthe plastic range. The plastic limits were determined in accordancewith the Casagrande method (McBride, 2008). The lower plasticlimit was of 28.88 cm3 cm3 and the upper plastic limit was39.65 cm3 cm3.2.3. MeasurementsSoil samples were collected after the third harvest in September20102012, from two soil layers, at 010 and 020 cm. Theundisturbed soil samples were taken with the use of steel coresampler (volume of 100 cm3, 5.02 cm diameter and 5.05 cm high)in six replications for every plot. We used these samples todetermine soil water retention characteristics and bulk density.The soil water retention curve (SWRC) was determined usingpressure plates (Soil Moisture Equipment Corp., Santa Barbara CA,USA), according to Richards method (Klute and Dirksen, 1986).After saturation, suction was successively applied to establishseven matric potentials, namely 4, 10, 33, 100, 200, 500,and 1500 kPa. van Genuchten (1980) parameters were fitted tothe SWRC experimental data with the Mualem constraint(m = 1 1/n) (Mualem, 1986) (Eq. (1)):u ur us ur1 ah n 11=n (1)where u is the soil water content (cm3 cm3), h represents thematric potential (kPa), us is the saturated water content (assumingequivalence with total porosity), ur is the residual soil watercontent, whilst a and n represent the model parameters. ur isassociated with the immobile water present within a dry soil (ath = 1). It was found that the value of the residual soil water contentdoes not appear to greatly affect the goodness of fit of the SWRC(Fayer and Simmons, 1995; Leij et al., 2005; Haverkamp et al.,2005), therefore in this study it was set ur = 0. The SWRC models(Eq. (1)) were fitted to the experimental water retention data usingthe non-linear least-squares procedure in the statistical softwarepackage Statistica v. 9.0 (StatSoft Inc. Tulsa, OK, USA). Based on thisfunction, the following soil quality parameters were calculated:(i) Field capacity (FC), defined as the equilibrium volumetric soilwater content at 10 kPa matric potential,(ii) Permanent wilting point (PWP), volumetric soil water contentat 1500 kPa matric potential,(iii) Relative field capacity (RFC), defined by Reynolds et al. (2008)(Eq. (2)):RFC FCuS(2)(iv) The slope (S) at the inflection point of the SWRC according tothe S-theory by Dexter (2004); with the Mualem constraint(Eq. (3)):S nus ur 2n 1n 1 1=n2 (3)(v) The available water content (AWC), calculated as the differencebetween the FC and PWP.The SWRC models were also used to estimate the pore sizedistribution (Ahuja et al., 1998). The volume of different porecategories was determined according to the pore classificationdeveloped by Greenland (1981), which characterizes pores as abonding space (500 mm).The samples were weighed and dried at the temperature of105 C to determine the bulk density (BD). Total porosity (TP) wascalculated from the soil particle density and dry bulk density of thesamples. The soil particle density was determined using thepycnometer method. The mean value of the particle density was2.65 Mg m3 in the 020 cm soil layer.Penetration resistance (PR) was measured using a chartrecording cone penetrograph (Stiboka, Eijkelkamp AgrisearchEquipments, Giesbeek, The Netherlands) with a base area of100 mm2 and 60 cone angle. Penetrograph readings were taken at1 cm intervals down to 20 cm depth and averaged for two soillayers, 010 and 1020 cm. The measurements were made at sixpoints in each plot, in a randomized position on the same day as thetractor traffic was applied. The PR was measured at a soil moisturecorresponding to field capacity.Due to the more detailed characterization of pores in high matricpotential ranges, the macropore system was characterized usingimage analysis on sections prepared from undisturbed soil samples(Jongerius and Heintzberger,1975; Murphy,1989). The samples weretaken at a soil moisture corresponding to field capacity in a verticalposition using metal boxes (80 mm 90 mm 40 mm) in threereplications on every plot, in two soil layers, at 010 and 1020 cm.They were then dried at room temperature and saturated withpolyester resin (Polimal 109 32K). After hardening of the resin, thesamples were cut in slices and placed into a water solutioncontaining sodium hypochlorite (with 20 g dm3 available Clcontent) in order to brighten the solid phase of the soil (Gab,2007a). The surfaces of the samples (80 mm 90 mm) were scannedat a resolution of 1200 dpi (images with the dimension of1890 2126 pixels) using an Epson Perfection 4870 Photo scannerand the images (a total number of 216) were saved as tiff files. TheAphelion v. 3.2 software (ADCIS, Saint-Contest, France) was used forimage analysis. The final result was a set of pores grouped in sixdiameter classes according to a method described by Gab (2007b),namely: 50100 mm, 100200 mm, 200500 mm, 5001000 mm,10002000 mm and >2000 mm. The pores of every fractionwere alsodivided into three classes according to their shape and expressed bythe shape factor (4p area/perimeter2) according to Pagliai et al.(1983): regular (shape factor 0.51.0), irregular (0.20.5) andelongated (0.00.2).The methodology established by Ringrose-Voase and Bullock(1984) was used for detection of biopores. The biopores wereselected on the basis of their convex shape and shape factor. Theconvex shape measures the perimeter of a convex figure aroundthe object. The shape factor was calculated from the area and totalperimeter of the object. The decision on pore classification wasbased on a definition described by Ringrose-Voase and Bullock(1984) and modified by VandenBygaart et al. (2000). The bioporeswere divided into three size classes: 10002000, 20004000 and>4000 mm. The minimum diameter (1000 mm) was acceptedaccording to Lee (1985) who stated that earthworms are generally>1000 mm in diameter. The isolation and measurement of thebiopores >1000 mm in diameter using morphometric analysis isT. Gab / Soil & Tillage Research 144 (2014) 819 11sufficient for an estimation of the influence of earthworms on soilstructure formation (VandenBygaart et al., 2000).2.4. StatisticsAnalysis of variance for a split-plot design was performed inorder to evaluate the significance of soil compaction, depth and Nfertilization on soil physical parameters using the statisticalsoftware package Statistica v. 9.0 (StatSoft Inc. Tulsa, OK, USA).Means were compared using Tukeys test with a level ofsignificance of P < 0.05. The ANOVA indicated no interactions(P > 0.05) across different seasons. Therefore, the data presentedhere represent the average of the years studied.3. Results3.1. Bulk density, total porosity, and penetration resistanceThe mean bulk density of the investigated soil was 1.630 Mg m3in the 020 cm soil layer. The mean value for soil total porosity was0.385 cm3 cm3. The BD values of the soil treated with different ratesof fertilizers, N0, N80 and N160, were not significantly different fromTable 3Soil physical quality parameters of the investigated soil under different treatments. Resporosity; PR, penetration resistance; FC, field capacity; PWP, permanent wilting point; AWof the SWRC. P0, P2, P4, P6 soil compaction levels; N0, N80, N160 nitrogen fertilizSoil depth Nitrogen level Compaction treatment BD (Mg m3) TP (cm3 cm010 cm N0 P0 1.498 0.435 P2 1.622 0.388 P4 1.661 0.373 P6 1.631 0.385 N80 P0 1.486 0.439 P2 1.608 0.393 P4 1.621 0.388 P6 1.606 0.394 N160 P0 1.520 0.426 P2 1.644 0.380 P4 1.659 0.374 P6 1.630 0.385 1020 cm N0 P0 1.510 0.430 P2 1.648 0.378 P4 1.701 0.358 P6 1.678 0.367 N80 P0 1.498 0.434 P2 1.634 0.383 P4 1.659 0.374 P6 1.652 0.376 N160 P0 1.533 0.421 P2 1.670 0.369 P4 1.698 0.359 P6 1.677 0.367 Means for soil compaction treatmentP0 1.508 0.431 P2 1.638 0.382 P4 1.667 0.371 P6 1.646 0.379 Means for soil depth010 cm 1.599 0.397 1020 cm 1.630 0.385 LSD (0.05)Compaction 0.121 0.042 Nitrogen ns ns Depth ns ns C N ns ns C D ns ns N D ns ns C N D ns ns each other (Table 3). However, the tractor passes affect the BD. Thelowest value was noticed at the non-compacted P0 control(1.508 Mg m3). For the P2 treatment the BD increased by 9%(1.638 Mg m3). All the compacted plots (P2, P4, and P6), were notstatistically different (1.650 Mg m3 on average). The inverse effectwas recorded for the TP (Table 3). The highest value was noticed forthe control P0 object (0.431 cm3 cm3) and it significantly decreasedwhen the soil was compacted (0.377 cm3cm3 on average). The TPwas not significantly affected by the fertilization. The differencesbetween the BD and TP values in both investigated soil layers werenot significantly different. At the control P0 the penetrationresistance was 1.493 MPa (Table 3) and this increased proportionallywith the number of tractor passes at the P2, P4 and P6 treatments,recording the values of 2.060, 2.165 and 2.421 MPa, respectively. ThePR was the only parameter which changed with soil depth. In thelower soil layer, 1020 cm, the PR was higher by 13% compared withthe upper layer.3.2. SWRC characteristicsWater retention curves are presented in Fig. 1. Estimatedparameters (us, a and n) of the van Genuchten model andults are the averages of three years, 20102012. BD, soil bulk density; TP, soil totalC, available water content; RFC, relative field capacity; S, slope at the inflection pointation rates.3) PR (MPa) FC (cm3 cm3) PWP(cm3 cm3)AWC(cm3 cm3)RFC S1.244 0.272 0.128 0.144 0.626 0.04331.856 0.304 0.146 0.158 0.785 0.03871.928 0.274 0.119 0.155 0.735 0.04062.311 0.294 0.136 0.158 0.765 0.03971.324 0.286 0.144 0.142 0.652 0.04091.911 0.285 0.135 0.150 0.723 0.03932.158 0.306 0.150 0.155 0.788 0.03772.553 0.315 0.151 0.164 0.800 0.03941.417 0.272 0.131 0.141 0.638 0.04161.819 0.276 0.133 0.143 0.727 0.03732.220 0.283 0.124 0.159 0.757 0.04062.231 0.291 0.136 0.155 0.755 0.03921.717 0.269 0.127 0.142 0.625 0.04282.250 0.281 0.135 0.146 0.744 0.03742.022 0.285 0.136 0.150 0.796 0.03622.322 0.287 0.130 0.157 0.782 0.03881.671 0.274 0.138 0.136 0.631 0.04032.303 0.283 0.133 0.150 0.738 0.03862.355 0.306 0.150 0.157 0.819 0.03692.631 0.287 0.134 0.154 0.764 0.03861.585 0.269 0.129 0.140 0.640 0.04122.221 0.290 0.136 0.154 0.786 0.03782.305 0.296 0.137 0.158 0.823 0.03752.475 0.291 0.133 0.158 0.792 0.03861.493 0.274 0.133 0.141 0.635 0.04172.060 0.287 0.136 0.150 0.751 0.03822.165 0.292 0.136 0.156 0.786 0.03832.421 0.294 0.137 0.158 0.776 0.03901.914 0.288 0.136 0.152 0.729 0.03992.155 0.285 0.135 0.150 0.745 0.03870.132 0.015 ns 0.009 0.083 0.0030ns ns ns ns ns ns0.129 ns ns ns ns nsns ns ns ns ns nsns ns ns ns ns nsns ns ns ns ns nsns ns ns ns ns ns0. 1 10 10 0 1 00 0 10 00 0P0P2P4P6matric po tential h (-kPa)water content (cm3cm-3)N160 , 0-10 cm0. 1 10 10 0 1 00 0 10 00 0P0P2P4P6matric po tential h (-kPa)water content (cm3cm-3)N160, 10-20 cm0. 1 10 100 1 00 0 10 000P0P2P4P6matric po ten tia l h (- kPa)water content (cm3cm-3)N0, 0-10 cm0. 1 10 10 0 1 00 0 10 00 0P0P2P4P6matric po ten tial h (-kPa)water content (cm3cm-3)N0, 10 -20 cm0. 1 10 10 0 1 00 0 10 00 0P0P2P4P6matric po tential h (-kPa)water content (cm3cm-3)N80 , 0-10 cm0. 1 10 100 1 000 10 000P0P2P4P6matric po tentia l h (-kPa)water content (cm3cm-3)N80, 10-20 cmFig. 1. The measured data and soil water retention curves based on van Genuchten equation for the investigated soil under different treatments at two soil layers, 010 and1020 cm. P0, P2, P4, P6 compaction levels; N0, N80, N160 nitrogen fertilization rates. Vertical bars represent standard deviations.12 T. Gab / Soil & Tillage Research 144 (2014) 819coefficients of determination are shown in Table 4. The differencesbetween the SWRC of treated soils appeared mainly within thehigh matric potential range and they did not depend on soil depth,which is reflected in changes in some water retention character-istics. The mineral fertilization did not play a significant role in theSWRC characteristics but the compaction treatments changed thewater retention properties of the soil (Table 3). The FC increasedwhen the soil was subjected to compaction treatment. However,the PWP was not affected by any treatment applied. Hencecompaction resulted in an increase in plant available water contentin the soil, which was proportional to changes in the FC. The lowestAWC was recorded for the untreated P0 (0.141 cm3 cm3). The AWCfor soil at the P2 treatment was at the same statistical level. Asignificantly higher AWC was recorded for the soil at the P4 and P6Table 4Parameters values (us, a and n) of the van Genuchten model fitted for the treated soil. P0, P2, P4, P6 soil compaction levels; N0, N80, N160 nitrogen fertilization rates.Soil depth Nitrogen level Compaction treatment us(cm3 cm3)a (kPa1) n R2 a010 cm N0 P0 0.435 2.32 1.15 0.949P2 0.388 0.45 1.15 0.976P4 0.373 0.58 1.17 0.947P6 0.385 0.50 1.16 0.956N80 P0 0.439 2.29 1.14 0.932P2 0.393 0.82 1.15 0.943P4 0.388 0.46 1.14 0.941P6 0.394 0.38 1.15 0.936N160 P0 0.426 2.19 1.15 0.940P2 0.380 0.84 1.15 0.942P4 0.374 0.47 1.17 0.958P6 0.385 0.57 1.15 0.9651020 cm N0 P0 0.430 2.32 1.15 0.948P2 0.378 0.69 1.15 0.945P4 0.358 0.38 1.15 0.947P6 0.367 0.40 1.16 0.954N80 P0 0.434 2.95 1.14 0.950P2 0.383 0.69 1.15 0.944P4 0.374 0.32 1.15 0.943P6 0.376 0.51 1.15 0.945N160 P0 0.421 2.14 1.15 0.938P2 0.369 0.41 1.15 0.941P4 0.359 0.27 1.16 0.962P6 0.367 0.37 1.16 0.956a Coefficient of determination between measured and fitted u(h) data for estimation of a and n.T. Gab / Soil & Tillage Research 144 (2014) 819 13treatments (0.157 cm3 cm3 on average). The regression model forthe relationship between tractor traffic intensity and AWC ispresented in Fig. 2.The soil compaction affected the soil quality indexes, S-slopeand relative field capacity. The RFC increased after traffic treat-ments, P2, P4 and P6 (0.771 in average) when compare withuntreated P0 (0.635). The S decreased after compaction. Thedifferences between S values were pronounced between P0 andother compaction treatments. It was found that S and RFC weresignificantly correlated with the soil BD. The regression functionsfor these relationships are presented in Fig. 3.y = -0.0002 x2 + 0.0031 x + 0.152 2R = 0.8390. 2 4 6AWC (cm3cm-3)Number of pass esFig. 2. The relationship between the available water content (AWC) and the numberof tractor passes with fitted regression function.3.3. Differential porosityThe soil at the trial location was characterized by storage pores(0.550 mm in diameter) as the most frequent pore fraction(Table 5). The soil pore distribution was not affected by nitrogenfertilization nor soil depth. Only soil compaction used as anexperimental factor resulted in significant changes in differentialporosity. Tractor multiple passes affected all pores with a diameterabove 0.5 mm, storage pores, transmission pores and fissures. Thesoil compaction increased the volume of the storage pores. Thelowest value was noticed at the control P0 (0.141 cm3 cm3),whereas the highest was recorded for P4 and P6 (0.153 cm3 cm3on average). These pores store water for plants and for micro-organisms and they are strictly related to the water retentionproperties of the investigated soil (available and productive watercontent). The opposite relationship was observed for larger poreswith a diameter above 50 mm. The transmission pores (50500 mm) and those above 500 mm in diameter (fissures) play animportant role in water movement and root growth. The highestvolume of transmission pores was noticed at the P0 object(0.091 cm3 cm3). All compacted treatments (P2, P4 and P6), werecharacterized by lower volumes of these pores (0.057 cm3 cm3 onaverage) on the same statistical level. A similar relationship wasnoticed for the larges pores with a diameter above 500 mm. In theinvestigated soil, both bonding space and residual pores were notaffected by any treatment applied.3.4. Soil macroporosityIn accordance with Fitzpatricks (Fitzpatrick, 1980) classifica-tion, the structure of the investigated soil can be described as acomposite structure because it consists of (i) a massive structurewith continuous solid phases representing the dominant type ofstructure at the site and (ii) a granular structure, locally present,and typical for conventionally tilled crops.The average total macroporosity (percentage of area occupiedby pores larger than 50 mm in diameter) in the trial was 3.18%. They = -0.0214x + 0.073 9R = 0.623 10.03 40.03 60.03 80.04 00.04 20.04 41.4 1.5 1.6 1.7 1.8SBD (Mg m-3)y = 0.8757x - 0.676 9R = 0.818 1.5 1.6 1.7 1.8RFCBD (Mg m-3)Fig. 3. Relationship between slope S, relative field capacity (RFC) and bulk density (BD) of treated soil with fitted regression lines.Table 5Pore size distribution of the investigated soil under different treatments. Results are the averages of three years, 20102012. P0, P2, P4, P6 soil compaction levels; N0, N80,N160 nitrogen fertilization rates.Soil depth Nitrogen level Compaction treatment Volume of pores (cm3 cm3) for diameter classes500 mm010 cm N0 P0 0.0743 0.0735 0.1443 0.0950 0.0475P2 0.0844 0.0838 0.1549 0.0549 0.0097P4 0.0645 0.0753 0.1543 0.0657 0.0132P6 0.0765 0.0810 0.1560 0.0598 0.0112N80 P0 0.0872 0.0765 0.1417 0.0894 0.0442P2 0.0778 0.0775 0.1487 0.0711 0.0182P4 0.0887 0.0837 0.1515 0.0541 0.0099P6 0.0873 0.0871 0.1595 0.0516 0.0084N160 P0 0.0766 0.0733 0.1414 0.0909 0.0438P2 0.0778 0.0749 0.1417 0.0677 0.0176P4 0.0670 0.0781 0.1577 0.0603 0.0108P6 0.0772 0.0797 0.1530 0.0622 0.01261020 cm N0 P0 0.0735 0.0727 0.1427 0.0941 0.0471P2 0.0783 0.0767 0.1447 0.0636 0.0146P4 0.0777 0.0789 0.1459 0.0479 0.0078P6 0.0720 0.0795 0.1538 0.0527 0.0087N80 P0 0.0839 0.0731 0.1356 0.0896 0.0517P2 0.0766 0.0773 0.1483 0.0660 0.0151P4 0.0872 0.0849 0.1511 0.0441 0.0066P6 0.0757 0.0791 0.1515 0.0586 0.0111N160 P0 0.0760 0.0727 0.1402 0.0897 0.0425P2 0.0771 0.0801 0.1510 0.0520 0.0088P4 0.0768 0.0829 0.1527 0.0410 0.0056P6 0.0738 0.0807 0.1543 0.0503 0.0079Means for soil compaction treatmentP0 0.079 0.074 0.141 0.091 0.046P2 0.079 0.078 0.148 0.063 0.014P4 0.077 0.081 0.152 0.052 0.009P6 0.077 0.081 0.155 0.056 0.010Means for soil depth010 cm 0.078 0.079 0.150 0.069 0.0211020 cm 0.077 0.078 0.148 0.062 0.019LSD (0.05)Compaction ns ns 0.008 0.028 0.025Nitrogen ns ns ns ns nsDepth ns ns ns ns nsC N ns ns ns ns nsC D ns ns ns ns nsN D ns ns ns ns nsC N D ns ns ns ns ns14 T. Gab / Soil & Tillage Research 144 (2014) 8190. 0 100 -20 0 200 -50 0 500 -100 0 1000 -200 0 >200 0P0P2P4P6pore diameter clas ses (m)porosity (%)0-10 cm0. 100-200 200 -500 500 -100 0 100 0-2000 >2000P0P2P4P6pore diameter clas ses (m)porosity (%)10-20 cmFig. 4. Effects of soil compaction on soil macroporosity, according to the pore size. Results are the percentages of macropore fractions relative to total soil volume and theaverages of the three years 20102012. Vertical bars represent standard errors. P0, P2, P4, P6 compaction levels.T. Gab / Soil & Tillage Research 144 (2014) 819 15distribution of macropores depends on soil depth. The macro-porosity of the upper soil layer (4.07%) was approximately twicethat in the 1020 cm layer (2.30%). According to the Pagliai et al.(2004) classification, the soil can be classified as dense (compact)since the total macroporosity is below 10%. For a detailedcharacterization of soil macropores, the pore size distributionand pore shape were also considered. The less frequent size classwas that above 2000 mm in diameter (0.015% on average) and thedominant fraction was the size range of 50100 mm in diameter(1.512% on average) (Fig. 4). The nitrogen fertilization did notsignificantly affect the soil macropore volume. Soil compactiondecreased macroporosity in the range of 502000 mm. However,the significant differences appeared only between the control P0and the other compacted treatments, P2, P4 and P6 and they weremore pronounced in the upper 010 cm soil layer. In the 1020 cmzone the compaction reduced only pore volume in the range of 50100 mm. There were no interactions between the size and shape ofthe pores. For all size classes, the proportions of elongated, P2 P4 P6regularirr egularelongatedporosity (%)0-10 cmFig. 5. Effects of soil compaction on soil macroporosity, according to the pore shape. Resaverages of the three years 20102012. Vertical bars represent standard errors. P0, P2,irregular, and regular pores were similar. The macropores weremostly of regular shape (Fig. 5). The differences between treat-ments were noticed only in this shape category in the 010 cm soillayer. Below 10 cm the soil compaction did not affect the poreshape. The higher volume of regular macropores was noticed in theupper soil layer soil at the P0, above 1%, whereas for any othercompacted treatments it was reduced, on average, by up to 0.7%.Any other macropores with different shapes namely, irregular orelongated, were not significantly affected. The mean number ofbiopores developed by the soil fauna and roots was 1659 m2 withthe dominant fraction being small pores with a diameter between1000 and 2000 mm (Table 6). The number of biopores was reducedby soil compaction for a wide range of pore diameters. However,this effect depended on soil depth. Tractor traffic decreased thenumber of small biopores from 3825 m2 at the P0 to below600 m2 at the P4 and P6 in the upper soil layer. Below 10 cm thebiopore volume was significantly reduced. At the P0 object this was1015 m2 and decreased when the compaction treatment was0. P2 P4 P6porosity (%)10-20 cmults are the percentages of macropore fractions relative to total soil volume and the P4, P6 compaction levels.Table 6Number of biopores in the investigated soil under different treatments. Results are the averages of three years, 20102012. P0, P2, P4, P6 soil compaction levels; N0, N80,N160 nitrogen fertilization rates.Soil depth Nitrogen level Compaction treatment Number of biopores (m2) for diameter classes10002000 mm 20004000 mm >4000 mm010 cm N0 P0 3804 958 189P2 1281 395 86P4 1466 494 80P6 1379 299 66N80 P0 3895 972 187P2 1282 385 84P4 1441 482 107P6 1378 291 66N160 P0 3777 945 177P2 1301 386 82P4 1395 485 70P6 1405 285 661020 cm N0 P0 1006 84 0P2 726 87 0P4 445 91 0P6 500 93 0N80 P0 1042 89 0P2 749 86 0P4 456 83 0P6 600 89 0N160 P0 997 88 0P2 716 82 0P4 435 76 0P6 600 96 0Means for soil compaction treatmentP0 2420 483 93P2 1009 197 43P4 940 245 43P6 977 149 35Means for soil depth010 cm 1983 531 1051020 cm 689 87 0Means for compaction soil depth interaction010 cm P0 3825 958 184.4P2 1288 389 84.2P4 1434 487 85.7P6 1387 292 66.01020 cm P0 1015 87 0.0P2 730 85 0.0P4 445 83 0.0P6 567 93 0.0LSD (0.05)Compaction 215 124 11Nitrogen ns ns nsDepth 325 153 16C N ns ns nsC D 265 132 13N D ns ns nsC N D ns ns ns16 T. Gab / Soil & Tillage Research 144 (2014) 819applied. The effect of compaction on medium biopores wasrecorded only in the upper soil layer. For large biopores, in the 010 cm layer, the difference was only observed between P0 andother compacted treatments. This biopore fraction was not noticedin the 1020 cm soil layer. No influence of mineral fertilization onthe appearance of biopores was observed.4. DiscussionIt is widely recognized that soil compaction caused by tractorwheels results in deterioration of the physical quality of soil andparticularly its pore system (Pagliai et al., 2003; Raper, 2005). Soilcompaction leads to soil structure degradation, which is stronglyassociated with changes in the physical properties of soil likeporosity, bulk density and penetration resistance (Coelho et al.,2000). These findings were confirmed by results obtained in thecurrent study. The intensive wheeling resulted in higher values ofbulk density and penetration resistance and lower values of totalporosity.Changes in basic soil parameters were related to modificationsin the pores system and water retention characteristics. Soilcompaction resulted in a decrease in the volume of large pores butalso led to an increase in volume of some smaller pore fractions.Tarawally et al. (2004) reported that compaction significantlyreduced the pore volume with an equivalent pore diameter>50 mm in a Rhodic Ferralsol. Zhang et al. (2006) noted that theT. Gab / Soil & Tillage Research 144 (2014) 819 17effect of compaction on silty loam soil was only pronounced in theSWRC below water tensions of 100 kPa. Similar results were alsoobtained by other authors (van Dijck and van Asch, 2002; Kutileket al., 2006). The most important as water resources for plants arepores of 0.550 mm in diameter (Greenland, 1981). In the presentstudy this pore fraction increased when tractor traffic was applied.Inorganic fertilizers may indirectly benefit the earthwormpopulations by increasing plant production. The plant yields, ascrop residues and roots, affect the soil fauna by supplying themwith dead organic matter as their food supply (Brussard, 1998).Edwards and Lofty (1982) showed that there was a strong positivecorrelation between the amounts of inorganic N applied and thesize of the earthworm population. Based on these findings it wasexpected that mineral fertilization can affect soil porosity in therange of biopores. However, this effect was not significant in thecurrent trial.Soil compaction in the trial increased the volume of waterretained in the soil. Suwardji and Eberbach (1998) noticed twice asmuch soil water in a relatively compacted soil than in a lesscompacted medium, coinciding with Tormena et al. (1999) whoreported a higher soil water content at the plant wilting point in azero tilled soil than in a looser soil. Results obtained by Diaz-Zoritaand Grosso (2000) on the other hand, indicated greater soil watercontent in a relatively less compacted soil than in a morecompacted medium. Tarawally et al. (2004) also reported thenegative effects of soil compaction on soil hydro-physicalproperties, denoted by an increased volume of 50 mm pore size fractions,followed the same trend. These ambiguous effects could beascribed to different load applied during compaction and soilconditions like water content, texture etc.The increase in AWC due to compaction was in contradictedresults in the soil quality indicator namely, the RFC and S-slope.According to Reynolds et al. (2008) the optimal balance betweenroot-zone soil water capacity and soil air capacity may be achievedwhen 0.6 RFC 0.7. Lower RFC values (below 0.6) result inreduced microbial activity because of insufficient soil water.Greater RFC values (above 0.7) reduced microbial activity becauseof insufficient soil air (Skopp et al., 1990; Reynolds et al., 2009).Pranagal and Podstawka-Chmielewska (2012) concluded that theoptimum equilibrium in airwater relations take places whenRFC = 0.66.The results obtained in the present study show that nitrogenfertilization has not influenced the RFC. This parameter dependedonly on soil compaction. Untreated control was characterized bythe RFC value in the optimum range. When traffic compaction wasapplied the RFC exceeded 0.7, which indicated that in compactedsoil under grassland plants, the biological activity was limited byinsufficient soil aeration.The slope S is another index of soil quality based on the waterretention model. The S-theory by Dexter (2004) is based on theinflection point of the SWRC. The inflection point is the pointwhere the curvature is zero or where it changes from convex toconcave. In most soils, larger values of S are consistent with thepresence of a better-defined microstructure. Dexter and Richard(2009) suggested the following categories of this index of soilphysical quality: S < 0.020, very poor; 0.020 < S < 0.035, poor;0.035 < S < 0.050, good; S > 0.050, very good. The presented soilwith the S ranged from 0.362 to 0.433 fitted into the good soilquality class. The study showed that soil compaction decreased theS value, thus leading to a deterioration in soil quality. A similareffect was also observed by Dexter (2004).The results in the soil quality indicators, namely RFC and S, arein contradiction with the results of plant available water content.The AWC indicated an improvement in soil quality which shouldresult in higher biomass production. However, in this trial the rootand above ground production was decreased when more intensivecompaction treatments were applied (Gab, 2013b). It can beconcluded that the RFC and S parameters are found as the indiceswhich are well correlated with the productive effect of soilcompaction.These changes in soil physical properties affect root growth andplant yields. A slight compaction may have a positive effect onplant growth and could be due to a better root-soil contactallowing higher nutrient transport rates (Arvidsson, 1999). Thiseffect could be also ascribed to the fact that an increase in bulkdensity is related to an increase in the amount of nutrients pervolume unit (Alameda and Villar, 2012). However, a higherincrease in soil compaction does not increase nutrient availability,but it has a negative effect on the growth and morphology of roots.It means that there is a critical value of soil compaction whendeleterious effects start to appear. The soil penetration resistancevalue, assumed to be a limiting factor for plant growth, wasrecognized at 2.0 MPa (Silva et al., 1994). In the present trialpenetration resistance was higher than the critical value only in thestrongly compacted treatments, P4 and P6. The critical soil bulkdensity was strongly correlated with the soil texture. Alameda et al.(2012) found that, in the absence of additional limiting stressessuch as water and nutrient supplies, soil compaction enhancedgrowth at bulk densities up to 1.4 Mg m3 for a sandy soil. Tracyet al. (2013) observed that plants grown on clay loam exhibitedgreater growth at the higher bulk density (1.6 Mg m3), whereasthose grown on loamy sand showed optimum growth at 1.3 Mgm3. Czy _z (2004) found the optimum bulk density of sandy loamsoil to be in the range from 1.51 to 1.63 Mg m3 in a trial withbarley. Jones (1983) developed regression relationships to distin-guish for various soil textures a critical bulk density below whichroot growth is effectively unimpeded. According to the Jonessmodel the critical bulk density for the studied soil (sandy loam,170 g kg1 clay content) was 1.74 Mg m3. A determined bulkdensity at the trial was in the range between 1.49 and 1.70 Mg m3and did not exceed the critical value for sandy loam textured soil.The effect of soil compaction and N fertilization on roots of redclover/grass mixture was reported in Gab (2013b). The soilcompaction reduced the biomass and length of roots. However, thedifferences between particular treatments were observed only inthe 515 cm soil layer. Previous researches conducted by theauthor also confirmed that tractor traffic intensities significantlyaffected the root system development of perennial plants,Medicago sativa and Poa pratensis (Gab, 2011, 2013a).Many authors reported that increased N supply fosters growth ofgrasses (Malhi et al., 2004; Hogh-Jensen and Schjoerring, 2010). Thehigh yields for the above ground part of plants are strictly connectedwith root matter. This hypothesis is mainly confirmed for speciescultivated in monoculture (Monk, 1966). However, Gab andKacorzyk (2011) also observed the relationship between fertilizationand root system development for perennial plants. It could beexpected that increasing below-ground root matter could bereflected in soil physical properties. The author stated in his previouswork (Gab, 2005) that the roots of perennial legumes reduced soilcompaction and improved the water retention of soil. It was alsostated that plant roots played a favorable role in the recovery ofstrongly compacted soil (Milleret et al., 2009). However, Zaleski andKopec (2003) reported the unfavorable effect of mineral NPKfertilization on the soil water retention. They observed decrease inplant available water content in the soil under mountain meadowwhat was ascribed to changes in botanical composition during the 30years of the trial. The results obtained in the present study did notconfirm any of the expectations expressed above. The mineralfertilization did not significantly affect any physical parameters ofsoil in the trial. In previous studies (Gab and Szewczyk, 2004; Gabet al., 2009) a similar result was observed for grasslands exhibiting18 T. Gab / Soil & Tillage Research 144 (2014) 819good structural quality and relatively high porosities. The influenceof fertilization, whether mineral or organic, was found to beinsignificant in these studies.5. ConclusionsThe mineral nitrogen fertilization of perennial grass/red clovermixture did not affect any physical properties of the soil. It also didnot interact with the soil compaction treatment. Soil compactioncaused by multiple tractor traffic significantly modified the airwater properties of the soil. The intensive wheeling resulted in ahigher value of bulk density and penetration resistance and lowervaluesof total porosity. Changes in basic soilparameters were relatedto a modification in the pore systems and water retentioncharacteristics. The soil compaction distinctly influenced the soilwater retention characteristics in the high matric potential range,which corresponds mainly with the decrease in volume of largepores. It also led to an increase in volume of some fractions of smallerpores, resulting in a higher plant available water capacity. Some ofthe soil quality indicators showed the harmful effects of soilcompaction. The relative field capacity indicates that in compactedsoil under grassland plants, the biological activity is limited byinsufficient soil aeration. This conclusion is in agreement with theresults found for plant yields, which showed a decrease in root andabove ground biomass as a result of compaction.AcknowledgementFinancial support for this study was provided by the Ministry ofScience and Higher Education in Poland (grant no. N N313 146738).ReferencesAggelides, S.M., Londra, P.A., 2000. Effects of compost produced from town wastesand sewage sludge on the physical properties of a loamy and a clay soil.Bioresour. Technol. 71 (3), 253259.Ahuja, L.R., Fiedler, F., Dunn, G.H., Benjamin, J.G., Garrison, A., 1998. Changes in soilwater retention curves due to tillage and natural reconsolidation. Soil Sci. Soc.Am. J. 62, 12281233.Alameda, D., Villar, R., 2012. Linking root traits to plant physiology and growth inFraxinus angustifolia Vahl seedlings under soil compaction conditions. 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