Effects of the stress field induced by a running tyre on the soil pore system

Soil & Tillage Research 131 (2013) 36–46

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Effects of the stress field induced by a running tyre on the soil pore system F.E. Berisso a,*, P. Schjønning a, M. Lamande´ a, P. Weisskopf b, M. Stettler c, T. Keller b,d a

Aarhus University, Department of Agroecology, Blichers Alle´ 20, P.O. Box 50, DK-8830 Tjele, Denmark Agroscope Research Station ART, Department of Natural Resources & Agriculture, Reckenholzstrasse 191, CH-8046 Zu¨rich, Switzerland c Bern University of Applied Sciences, School of Agricultural, Forest and Food Sciences HAFL, La¨nggasse 85, CH-3052 Zollikofen, Switzerland d Swedish University of Agricultural Sciences, Department of Soil & Environment, Box 7014, SE-75007 Uppsala, Sweden b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 August 2012 Received in revised form 27 March 2013 Accepted 28 March 2013

This study investigated the impact of vehicle traffic on soil physical properties by systematically collecting samples in a transect running from the centreline to the outside of the wheel rut in a wheeling experiment conducted on a clay loam soil at Suberg near Bern, Switzerland, in 2010. Four repeated wheelings were performed by a forage harvester (wheel load 6100 kg; tyre width 80 cm). Mean normal and horizontal stresses were measured with Bolling probes (at 10, 20 and 40 cm depth) and load cells (at 40, 50, 60 cm lateral distance from the centreline of the wheel rut at 10, 30 and 50 cm depth), respectively. Intact soil cores of 100 cm3 sampled at 10, 30 and 50 cm depth in a soil transcet running from the centreline of the wheel rut to the unwheeled part of the field were used for measurements of water retention and air permeability (ka) at 30, 100 and 300 hPa matric potential. The complete stress state in the soil profile beneath the harvester tyre was calculated using the SoilFlex model. Pore continuity index (N) and blocked air-filled porosity (eb) were estimated from the relationship between ka and air-filled porosity (ea) for a range of matric potentials. Calculated and measured stresses agreed well at all depths. At 100 hPa, ea was consistently lower under the centreline of the wheel rut than at the lateral edge of the rut or outside the wheel rut, while ka was lowest at the lateral edge of the wheel rut and highest outside the wheel rut, with intermediate values under the centreline of the wheel rut. Simulations of the stress field in the soil beneath the tyre indicated that the trends in ka were determined by both the mean normal stress and the shear stress, while the trend in ea was determined by the mean normal stress only. At 10 cm depth, the index of pore continuity (N) supported the interpretation that soil pores under the centreline of the wheel rut are primarily reduced in size, while pore continuity is highly affected at the lateral edge of the wheel rut, as indicated by a higher value of eb than in other locations. These results indicate that sampling along the wheel track transect can provide better information about traffic-induced changes on soil physical properties than random sampling in lateral locations relative to the centreline of the wheel rut. ß 2013 Elsevier B.V. All rights reserved.

Keywords: Compression Distortion Soil transect Air permeability Air-filled porosity

1. Introduction The weight and efficiency of agricultural machinery is continually increasing in order to maximise the economic returns in agriculture (i.e. reduce production costs). However, the use of heavy machinery literally puts more stress on the soil and this may result in undesirable soil conditions that have a much wider impact than merely on crop yield. These may include negative effects on gas transport functions (Berisso et al., 2012; Gebhardt et al., 2009; Ku¨hne et al., 2012), water transport (Arvidsson, 2001; Zink et al., 2011), and a decline in environmental quality through a number of

* Corresponding author. Tel.: +45 8715 4756. E-mail address: [email protected] (F.E. Berisso). 0167-1987/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.still.2013.03.005

mechanisms (O‘Sullivan and Simota, 1995; Simojoki et al., 1991; Soane and van Ouwerkerk, 1995). Compression (i.e. volumetric strain) and distortion (i.e. shear deformation) are both due to stresses imposed by the tyres of agricultural machines and can affect many important soil ecological services and functions depending on the resulting soil pore system after deformation. Compression and distortion occur simultaneously during field operations, but many studies attribute observed effects entirely to compression. One reason for this could be an incomplete understanding of the mode of soil deformation and ultimate failure during machine-soil interaction as affected by different states of stress. Another reason may be that compression (or compaction) is often used as a composite term for ‘soil deformation due to vehicular traffic’. While there have been numerous studies on compression, only few studies address the effect of pure shear stress on soil porosity and its functions. An

F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

exception is the work of Dawidowski and Koolen (1987), who showed that shear stress, although causing fairly small changes in air-filled porosity, resulted in a significant reduction in air permeability. Some authors have attempted to investigate the combined effect of shear and compressive stresses on soil structure and associated transport properties in the laboratory. Kirby and Blunden (1991) investigated the effect of expansion or compression in combination with shear on the air permeability of soils with two contrasting structures (a soil dominated with macropores with a preferential orientation and a homogeneous soil). They found that the shear in the compression regime resulted in a reduction in air permeability in both soils, but the shear in the expansion regime yielded contrasting results: the air permeability increased in the soil with homogeneous structure, but decreased in the soil with preferential macropore orientation. They attributed the reduction in air permeability in the latter primarily to a reduction in pore continuity. O‘Sullivan et al. (1999) performed a similar experiment on a sandy silt loam soil at different water contents and recorded a reduction in air-filled porosity, air permeability and gas diffusivity, with more pronounced effects in wet than in dry soil. This kind of experiment can closely replicate the deformation that occurs under an agricultural tyre, but few field studies have investigated the complete stress field below tyres. The impact of traffic-induced changes on soil physical properties is usually quantified by collecting soil samples at random lateral locations relative to the tyre centreline. This is tantamount to assuming that passes by a vehicle would result in uniform changes in the soil physical properties at a certain depth within the soil profile below the wheel rut. Despite this assumption, modelling studies and some field measurements indicate a nonuniform stress distribution in the soil matrix across the wheel rut, which might induce different effects on soil physical properties. In some field wheeling experiments, track-by-track wheeling (to cover 100% of the surface of the experimental plots) is usually practised to minimise the non-homogeneous effect of wheeling, but this practice differs from actual field traffic. In other field experiments, only one track is wheeled and the samples are typically only taken under the centreline of the tyre. However, these types of experiments do not show wheeling effects as a function of the distance from the centreline to the outside of the wheel rut. The objectives of this study were thus to (i) investigate the impact of passage of an agricultural tyre on total porosity, airfilled porosity and air permeability in a transect running from the centreline to the outside of the wheel rut; and (ii) relate the changes in soil physical properties to the stress field induced by the tyre. This involved systematic collection of soil samples with high spatial resolution across the wheel rut at various depths.

2. Materials and methods 2.1. Site and soil A wheeling experiment was conducted during autumn 2010 on an arable field in Suberg (47.068 N, 7.338 E) near Bern, Switzerland. The field has been under conservation tillage for many years. A forage stand was established four years before the experiments and grass silage was harvested in several cuts annually. The soil has a clay loam texture and developed on alluvial sediments. According to the WRB system the Suberg soil is classified as a Gleyic Cambisol (WRB, 2006). Some characteristics of the soil are shown in Table 1 and methods used to determine these characteristics are briefly described below. The soil matric

37

Table 1 Selected soil properties of the experimental soil and penetration resistance (measured before the experiment) and matric potential (measured using tensiometer) during the field experiment. Property

Soil depth (cm) 10a

Physical Clayb (g g1) Siltb (g g1) Sandb (g g1) Organic matter (g g1) Mechanical propertiesc Isotropic precompression stress (kPa) Cohesion (kPa) Angle of internal friction (deg) Field measurements Matric potential (hPa) Penetration resistance (MPa)f a b c d e f

30a 0.32 0.40 0.28 0.04

50a 0.34 0.39 0.27 0.02

45.3 67.7 33.2

43.5 68.4 33.2

100d 2.06

160e 1.81

0.32 0.38 0.30 0.01 56.9 76.4 29.3

1.65

The soil depth refers to the midpoint depth of a sample of 4.2 cm height. Size range: sand 50–2000 mm; silt 2–50 mm; clay < 2 mm. Measured at field water content. 20 cm depth. 40 cm depth. Average values of four replicate measurements.

potentials recorded during the experiment using tensiometers are also listed in Table 1. 2.2. Traffic experiment and machinery The wheeling experiment had four replicate plots and comprised 5–6 sampling positions (corresponding to the lateral distance from the tyre centreline) in a soil transcet running from the centreline of the wheel rut to the unwheeled part of the plots (Fig. 1a). The wheel tracks were made by four repeated wheelings with a Claas Jaguar 890 Field Shuttle self-propelled forage harvester, equipped with an eight-row maize cutter, offset steering and a bunker that is hitched to the harvester with hydraulic connections. Wheeling was done only with the front right wheel in the same track by driving the harvester forward and back twice, i.e. four passes of the front right tyre. The wheel load was 6100 kg, the tyre was a Goodyear DT 820 (800/65R32) inflated to 200 kPa (the recommended tyre inflation pressure at a speed of 30 km h1 is 190 kPa). The activated offset steering of the harvester prevented overlap between the tracks of the front and rear wheels. 2.3. Field measurements and soil sampling 2.3.1. Stress measurements and tyre footprint Normal stresses from the harvester wheelings were measured using two different kinds of sensors: Bolling probes (Berli et al., 2006; Bolling, 1987) and load cells (Lamande´ et al., 2007) (Fig. 1b). Two to four different insertions of the Bolling probes were made in all plots at 10, 20 and 40 cm depths (relative to the undisturbed soil surface), at the centreline of the wheel rut, and readings of the maximum Bolling probe pressure were taken manually after each wheeling. The probes were inserted by boring inclined holes from the side of the wheel rut as shown in Fig. 1b. The mean normal stress (sm) was calculated from inclusion pressure according to Berli et al. (2006)

sm ¼

PI 2ð1  vm Þ

(1)

where PI is the measured Bolling pressure and vm is the Poisson’s ratio in the soil matrix. Assuming a Poisson’s ratio of 0.3 (e.g. Berli

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20

20

10

38

20

20 10 10

(a)

300

1

4 3

20

10

5

20

2

(b)

40

10 10

Fig. 1. Rear view of the tyre and soil showing (a) the soil core sampling locations, and (b) the location of load cells and Bolling probes for stress measurements. 1 = tyre; 2 = load cells; 3 = data logger; 4 = computer; 5 = Bolling probes. All dimensions are in cm.

et al., 2006), Eq. (1) yields sm = PI/1.4, suggesting that sm is overestimated by a factor of 1.4 by the inclusion pressure. The load cells were installed at various lateral locations relative to the wheel rut, namely at 40, 50 and 60 cm lateral distance from the centreline of the wheel rut, and used to measure the horizontal stresses perpendicular to the driving direction (measurements were made at 10, 30 and 50 cm depth (Fig. 1b) with three replicates, i.e. insertions in three of four experimental plots). The load cells were implanted by drilling vertical holes using an auger of the same size as the load cell housing. Good contact was created between the soil and the sensors by the wedging action of the load cell housing (see Lamande´ et al., 2007 for detailed description). Data from the transducers were automatically recorded at 50 Hz by a data logger and loaded to a PC (Fig. 1b) for subsequent processing. The periphery of the contact patch of the front right tyre (i.e. the tyre footprint) was marked with sand while the harvester was stationary, and then photographed after the machinery had left the spot. The geometry and area of the tyre footprint was determined by image analysis and used as input for modelling (as described below). 2.3.2. Soil core sampling After wheeling, access pits of 200 cm  200 cm  100 cm (width  length  depth) were carefully opened in each plot, and horizontal planes were sequentially prepared for sampling

at 10, 30 and 50 cm depth. Intact cores of 100 cm3 were sampled at 0, 20, 40, 50 and 300 cm lateral distance from the centreline of the wheel rut at all three sampling depths (10, 30 and 50 cm, see Fig. 1b). Additional cores were collected at 60 cm lateral distance from the centreline of the wheel rut at 30 and 50 cm depth. At each lateral position, three replicate cores were collected for each plot and sampling depth. At 300 cm lateral distance, cores of 250 and 580 cm3 were also sampled. All soil cores were sampled in the vertical direction by hammering metal cylinders into the soil and carefully retrieving them from the bulk soil.

2.4. Laboratory analyses 2.4.1. Initial mechanical and basic physical properties of the soil The 250 and 580 cm3 soil cores were used to determine the initial mechanical properties of the soil. The precompression stress and other mechanical properties shown in Table 2 (except cohesion and angle of internal friction) were derived by fitting the Gompertz equation (Gregory et al., 2006) to the stress–void ratio curve obtained from uniaxial confined compression tests (stepwise sequential loading with 0.5 h loading time for each stress level). The vertical stresses applied were 20, 30, 40, 50, 60, 80, 100, 125, 150, 200, 250, 400, 600, 800 and 1000 kPa. The shear strength parameters (i.e. cohesion and angle of internal friction; Table 1)

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Table 2 Soil parameters used for the simulations shown in Figs. 2 and 3. Parameter

Symbol (unit)

Specific volumea at p = 1 kPa Compression indexb Swelling indexa Slope of the ‘steeper recompression line’a Separation between yield line and virgin compression linea Cohesion Angle of internal friction Initial bulk density Particle density

n (–) ln (ln (kPa1)) k (ln (kPa1)) k0 (ln (kPa1)) m (kPa) c (kPa) w (deg) r (Mg m3) rs (Mg m3)

a b

Soil deptha (cm) 10

30

50

2.41 0.113 0.005 0.024 0.8 67.7 33.2 1.31 2.59

2.44 0.125 0.004 0.022 1.4 68.4 32.3 1.33 2.62

2.44 0.124 0.002 0.016 1.8 76.4 29.3 1.36 2.63

The soil depth refers to the midpoint depth of a sample of 4.2 cm height. Calculated from oedometer tests by assuming s2 = s3 = 0.5 s1 (Koolen and Kuipers, 1983; Keller et al., 2007).

were derived from linear regression between the measured shear strength and the applied vertical stresses (normal loads of 30, 60, 90, 120, 150 and 180 kPa) on data obtained from direct shear tests (horizontal displacement rate 0.165 mm s1). Both measurements were conducted at field water content. Basic soil physical properties were determined on bulk soil. The soil texture was analysed using the standard sieve-hydrometer method. Total carbon content was determined using a LECO CNS1000 elemental analyser coupled to an infrared CO2 detector (LECO Corporation, St. Joseph Michigan, USA). A water pycnometer was used to determine soil particle density (PD). 2.4.2. Water retention and air permeability Soil cores of 100 cm3 were used to determine the effect of wheeling across the soil transcet. Prior to the measurements, the soil cores were carefully trimmed with a sharp-edged knife, covered with fine nylon cloth and placed in a trough, and then slowly (by increasing the level of water in steps of approximately 1/3 of the core height over 24 h) wetted from below to saturation. The samples were then saturated in distilled water with 0.01 M CaCl2 for a week and finally transferred to sandboxes, where they were sequentially drained to 30, 100 and 300 hPa matric potential. The height of each sample was measured at each matric potential and used to calculate total volume of the soil cores at the respective matric potential. Prior to the measurements of air permeability (ka), the soil was carefully pressed to the edge of the cylinder walls in order to minimise leakage of air between the soil and the cylinder wall (Ball and Schjønning, 2002). Air at a pressure of 2 hPa was then applied to the top of the core and the volumetric flow rate was recorded. Air permeability was then calculated from the volumetric flow rate and the applied pressure head by using Darcy’s equation: ka ¼

Qls h D pAs

(2)

where Q is the volumetric flow rate (m3 s1), ls is the height of the soil sample (m), h is the dynamic air viscosity (mPa s), Dp is the difference in air pressure (Pa), and As is the cross-sectional area of the soil sample (m2). The cores were weighed at each matric potential and after oven drying at 105 8C for 24 h. Bulk density (BD) was calculated from the weight of the oven-dried soil and the total volume of the soil cores. Total porosity (us) was calculated from BD and PD. Gravimetric water content (w) was calculated as the difference between the weight of the sample at a given matric potential and that at ovendry conditions. Volumetric water content at a given matric potential (u) was calculated from w and BD. Air-filled porosity (ea) was calculated as the difference between us and u.

2.5. Modelling and calculations 2.5.1. Stress in the soil profile The purpose of simulating stress transmission due to passage of the tyre was to obtain the complete stress field, which was then related to the measured changes in soil physical properties (us, ea and ka). Measured total porosity and soil stress (see Sections 3.2 and 3.3, respectively) were used to validate the simulations. The complete stress state in the soil profile beneath the harvester tyre was predicted using the SoilFlex model presented by Keller et al. (2007). Soil deformation (change in soil volume) was calculated according to the O‘Sullivan and Robertson (1996) model, which is included in SoilFlex. Model parameters used for calculations are summarised in Table 2. 2.5.2. Pore continuity index Pore continuity index over a range of matric potentials (in this study from 30 to 300 hPa) can be assessed by combining measurements of air-filled porosity and air permeability as proposed by Ball et al. (1988): ka ¼ MeN a

(3)

where M and N are empirical parameters that are generally determined by fitting a straight line to the log–log plot of ka versus ea: logðka Þ ¼ logðMÞ þ N logðea Þ:

(4)

A number of studies (Ball et al., 1988; Berisso et al., 2013; Do¨rner et al., 2010; Schjønning et al., 2002) have interpreted N as a pore continuity index that reflects an increase in ka with increasing ea or a decrease in pore tortuosity with an increasing fraction of pores available for flow. Ball et al. (1988) regarded soil with ka  1 mm2 as impermeable soil. Accordingly, the intercept of Eq. (4) on the ea-axis where ka = 1 mm2 can be regarded as an estimate of blocked porosity, eb (Berisso et al., 2013; Peng and Horn, 2008; Schjønning et al., 2002). Here we also calculated a pore functional index at a given matric potential from the ratio of ka to ea. This was first suggested by Groenevelt et al. (1984) as an index of pore continuity and later named ‘pore organisation’ (PO) by Blackwell et al. (1990): PO ¼

ka

ea

(5)

2.6. Statistics Checks on the normality of total porosity, air-filled porosity and air permeability data showed that the first two passed the test for normality, whereas air permeability values were found to be

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rare, and our work showed a possibility to gain more insight into details of stress propagation by taking measurements in a nonvertical direction with load cells. In the past few decades, load cells have only been used for measurements of vertical stress (e.g. Arvidsson and Keller, 2007; Keller, 2005; Lamande´ and Schjønning, 2008, 2011; Schjønning et al., 2008). However, horizontal normal stresses have been measured using stress state transducers (e.g. by Bailey et al., 1988; Way et al., 1996; Wiermann et al., 1999).

positively skewed and not normally distributed. Thus, air permeability data were log-transformed for further analysis. The statistical analysis was performed with the Mixed procedure in SAS (SAS Institute Inc., Cary, NC, USA) with lateral distance as fixed effect, and plot and interaction between plot and lateral distance as random effects using the model: y ¼ m þ lateral distance þ plot þ plot  lateral distance

(6)

where y is the observation at a given lateral distance and plot, and m is an intercept. The normality of residuals was checked after fitting the statistical model (Eq. (6)) to the data in order to ensure that the normality assumption of the model was validated. The Kenward and Roger (1997) method was used for calculation of degrees of freedom. An autoregressive, AR(1), covariance structure was used to account for correlation between samples from the same plot at different lateral distances.

3.2. Measured and predicted total porosity Calculated and measured total porosity as a function of lateral distance from the centreline of the wheel rut is shown in Fig. 3. In general, total porosity increased with lateral distance from the rut centreline, i.e. was smallest under the rut centreline, and was highest in the unwheeled soil, with intermediate values at the lateral edge of the wheel rut. However, these differences were statistically significant (p < 0.05) only at 10 cm depth. Calculated total porosity agreed well with measured total porosity in all sampling points. The shape of the calculated curve, i.e. the change in total porosity with lateral distance, was also satisfactorily captured, and clearly indicated that passage of the harvester wheel resulted in non-homogeneous (non-uniform) changes in total porosity (at any depth) in the soil transect under the wheel rut. As can be seen in Fig. 3, soil close to the centreline of the wheel rut was compressed more than soil close to the lateral edge of the wheel rut.

3. Results and discussion 3.1. Stress transmission The stress measured by Bolling probes is a good indicator of the mean normal stress (Gysi et al., 2000) and thus the corrected values at different depths were compared with predicted mean normal stresses from SoilFlex. There was good agreement between predicted and measured stresses at 20 and 40 cm depth (Fig. 2a), and the model was able to capture the basic shape of the stress distribution. However, the mean stress at 10 cm was considerably over-estimated by the model. This discrepancy can mainly be attributed to uncertainties in the assumption of stress distribution at the soil–tyre contact area (e.g. Keller and Lamande, 2010). The predicted horizontal stresses and the measured values (by means of load cells at 40, 50 and 60 cm lateral distance from the centreline of the wheel rut at 10, 30 and 50 cm depth) are shown in Fig. 2b. In all positions, there was good agreement between the predicted value and the mean of the measured values. However, there was high variation in the measured values, as indicated by the large error bars (Fig. 2b), possibly due to soil heterogeneity and to the limited number of replicate measurements. In this study we made three true replicate measurements, i.e. insertions of the load cells at three different locations (plots), which is the typical number of replicates used for vertical stress measurements (e.g. Keller et al., 2012). Attempts to measure horizontal stresses are

Mean normal stress, 0

40

80

m,

3.3. The stress field and the soil pore system During field traffic, soil is generally subjected to a complicated stress field, whereas it is subjected to simple stress states during soil mechanical tests (e.g. uniaxial compression, direct shear tests in the laboratory). The intensity and direction of these stresses govern the deformation of the soil and thus changes in the pore size distribution, tortuosity and connectivity of the soil pore system, which in turn has a dramatic effect on air permeability. Generally, any deformation can be divided into a part of volume change (compression) and a part of distortion (deformation at constant volume), which can be related to the mean normal stress (sm) and the shear stresses (tij), respectively. Consequently, a change in porosity is expected to be solely related to sm. However, a change in ka may be due to pure compression (pores

(kPa) 120

Lateral stress, y, (kPa) 160

0

20

40

60

80

100

120

0.0

Depth (cm)

0.2

0.4

0.6

0.8

1.0

0.4 m simulated 0.5 m simulated 0.6 m simlated 0.4 m measured 0.5 m measured 0.6 m measured

Fig. 2. (a) Calculated mean normal stress from Bolling probes and (b) horizontal stress measured using load cells. Bolling probes were installed under the centre of the wheel rut at 10, 20 and 40 cm depth. Load cells were installed at 40, 50 and 60 cm horizontal distance from the centre of the wheel rut at 10, 30, and 50 cm depth. Measurements were made with three replicates (i.e. insertions in three of the four experimental plots). Error bars indicate 1 standard error.

F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

41

0.52 (b)

3

-3

Total porosity (m m )

(a)

0.48

0.44

0.00 -80

0.52

-40

0

40

300

Distance from the centerline of the wheel rut (cm)

3

-3

Total porosity (m m )

(c)

0.48

0.44

0.00 -80

-40

0

40

300

Distance from the centerline of the wheel rut (cm) Fig. 3. Comparison of measured (points with error bars) and predicted (line) total porosity after four repeated wheelings at (a) 10 cm, (b) 30 cm, and (c) 50 cm depth. Error bars indicate 1 standard error.

become smaller) or distortion (pores become more tortuous or disconnected). Therefore, we postulated that:

ea ¼ f ð s m Þ

(7)

and ka ¼ gðs m Þ þ hðt i j Þ

(8)

where f, g and h are functions. Because tij = tji, and since txy = tyx = 0 and tyz = tzy = 0 in the vertical x–z plane below the centreline of the tyre, tij in Eq. (8) reduced to txz. In this section x represents the direction of driving, y represents lateral distance with the positive y-axis pointing to the right, and z represents soil depth with the positive z-axis pointing downward. Analysis of Eqs. (7) and (8) based on our data for the 100 hPa matric potential resulted in:

ea ¼ 0:0003s m ð0:00009Þ þ 0:1326ð0:0005Þ; ¼ 0:47;

R2

p ¼ 0:001

(9)

and logðka Þ ¼ 0:0041s m ð0:0009Þ  0:0122t xz ð0:0035Þ þ 2:1799ð0:07Þ; ¼ 0:69;

R2

p ¼ 0:0003

(10)

The values within brackets in Eqs. (9) and (10) are the standard error. As expected, there was no significant effect of txz on ea (p = 0.85; analysis not shown). The mean normal stress decreased with depth and, at a given depth, decreased with increasing lateral distance from the

centreline of wheel rut (Fig. 4a), while the opposite trend was observed for ea (Fig. 4c). This implies that the highest value of sm indeed induced the highest reduction in ea. Interestingly, contours (pressure bulbs) of predicted sm and isolines of measured ea displayed an almost similar shape. The mean normal stress from the forage harvester tyre significantly reduced ka (Eq. (10)). However, unlike the case of ea, the highest value of sm did not correspond to the highest reduction in ka (Fig. 4d). This was probably due to (1) the resistance of vertical pores to the axial loading (Blackwell et al., 1990) beneath the centreline of the tyre. These pores were subjected to mainly compressive stress with little or no shear stress; and (2) shear stresses resulting in pore distortion that was largest at the lateral edge of the rut of the tyre (Fig. 4d). There was a strong negative relationship between txz and ka (Eq. (10)). Interestingly, isolines for both txz (Fig. 4b) and ka (Fig. 4d) displayed a ‘dual-peak’ distribution across the wheel rut. As already pointed out, the highest reduction in ka (Fig. 4d; observed close to the lateral edge of the wheel rut) can be explained by the effect of txz, indicating a detrimental effect of shear stress on ka without affecting the magnitude of ea. This is in agreement with findings by Dawidowski and Koolen (1987), who reported an important reduction in air permeability of a clay soil under nearly pure distortion (deformation without a change in soil volume). Using regression analysis, we found that sm explained 42.5% of the variability observed in ka (data not shown), whereas txz, and sm together explained 69% (Eq. (10)). This could indicate that the shear stress which occurred in locations with high compression (e.g. at 20 cm lateral distance) was not large enough to cause distortion of soil pores and thereby to induce a change in ka. High

F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

42

(a) 10

140

20

10

120 40

20

Depth (cm)

20

60

20

-40

-20

0

20

40

60

10

30

10

20 20

20

10

-60

-40

-20

0

20

40

60

Distance from the centre of the wheel rut (cm)

Distance from the centre of the wheel rut (cm)

(c)

(d)

10

10 0.09 0.10

0.12

20 0.12

0.13

0.13

30

0.11

0.12

Depth (cm)

0.12

20 Depth (cm)

40

20

30

30

10

50 -60

40

30

40

50

20

30 40

20

80

30

40

10

20

40

100

20

Depth (cm)

(b) 10

240 200 160 120

40

40

30

120

120 80

80

0.13

40

0.12 0.11

80

160

50 -60

240 200 160

0

0

-40

-20

0

20

40

60

Distance from the centre of the wheel rut (cm)

50 -60

-40

-20

0

20

40

60

Distance from the centre of the wheel rut (cm)

Fig. 4. Predicted (a) mean normal stress and (b) shear stress (t) in the x–z or z–x plane (txz = tzx), and (c) interpolated measured air-filled porosity and (d) air permeability at 100 hPa as a function of horizontal distance and depth. The numbers placed along the isolines are given in kPa for the mean normal stress and the shear stress, in m3 m3 for the air-filled porosity, and in mm2 for the air permeability. Note that measurements took place only in one of the two half-spaces from the centre of the wheel rut (the plots include a ‘mirror’ of these data).

normal loads resulted in high shearing strength and suppressed the effect of shear stress. These locations appeared as ‘plateaus’ in the ka isolines (Fig. 4d, at 20 cm lateral distance). The effects of the stress field on the volume and function of airfilled pores at 100 hPa matric potential are described below. Measurements at 100 hPa can be taken as indication of water content during the field experiment (see Table 1) and the resulting effect of traffic on soil porosity and air transport across the wheel rut. Air-filled porosity at 100 hPa was generally lower under the centreline of the wheel rut (0–20 cm lateral distance from the centreline of the wheel rut) than at the lateral edge of the wheel rut (40 cm) or outside the wheel rut (50–300 cm lateral distance from the centreline of the wheel rut) (Fig. 5). This trend applied at all depths, and the results were significant at 10 and 30 cm depth. For soil at 50 cm depth, despite the decreasing impact of wheeling with depth, the ea was higher at 300 cm lateral distance than at the other locations. Within the wheel rut, differences in ea values were also noted in all three soil depths (Fig. 5). However, statistically significant differences were only observed at 10 cm depth in the following order: ea was lowest at the centreline of the wheel rut and highest at the lateral edge of the rut, with an intermediate value at 20 cm laterally away from the centreline of the wheel rut. The nonsignificant response within the wheel rut at the two deeper depths

could be attributed to low stress levels at these depths, which could be counteracted by the soil internal strength. Despite four repeated wheelings with 6100 kg wheel load, the ea values for almost all sampling positions were higher than the value of 0.1 m3 m3 proposed as a critical lower limit for plant growth by Grable and Siemer (1968). One reason could be that the water content during our experiment was slightly drier than field capacity (100 hPa; Table 1), so that the soil was able to bear the stresses from the harvester wheel. Another reason could be the relatively high soil strength because of the good structure and the grass roots present. In most field experiments, wheeling is conducted when the soil is in slightly wetter condition (e.g. Arvidsson, 2001; Peth et al., 2006), and consequently the reported ea values are lower than those found in this study. Air permeability at 30 and 50 cm soil depth showed a reducing trend after four repeated wheelings. However, no significant effect was observed except for the soil at the lateral edge of the wheel rut at 30 cm depth, where ka was reduced to a lower level than in soil at 300 cm distance (Fig. 5). At 10 cm depth, there was a significant effect on ka for different lateral distances underneath the wheel, as compared with the values measured laterally outside the wheel rut (Fig. 5). Air permeability was lowest at the lateral edge of the wheel rut. However, at this lateral distance, ea was not affected (Fig. 5). A possible mechanism to explain this observation can be the suggested differences in the effects of the stress field

F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

Air-filled porosity a

b

c

43

Air permeability a

c

c

0.16

a

b

d

c

100 40

0.12

10 cm

10

0.08 ab

ab c

bc

ab

c Air permeability ( m2 )

Air-filled porosity (m3 m-3 )

a 0.16

0.12

0.08

0.16

ab

b

ab ab

a

100 40 30 cm

10

100 40

0.12

50 cm

10

0.08 a

a

a

0

20

40

a

a

b

60 300

a

a

a

0

20

40

a

a

c

60 300

Distance from the centreline of the wheel rut (cm) Fig. 5. Air-filled porosity (left) and air permeability (right) measured at 100 hPa as a function of horizontal distance from the centre of the wheel rut at a depth of 10 cm (upper two figures), 30 cm (middle two figures), and 50 cm (lower two figures). The depth refers to the midpoint depth of a sample of 4.2 cm height. Figures within the same depth with different letters are statistically different at p < 0.05. Error bars indicate 1 standard error.

components on the volume and distortion of soil pores (Fig. 4). This explanation agrees with the finding by Kirby and Blunden (1991) of a significant reduction in ka in structured soil after shearing. A reduction in ka due to field traffic has been reported previously (e.g. Arvidsson and Ha˚kansson, 1996; Peng and Horn, 2008). However, it is difficult to make a quantitative comparison between the findings in the present study and those in the literature. In most previous studies, the areas of compacted plots were usually wheeled by driving track-by-track (to cover 100% of the plot surface) and the soil samples were collected randomly from the plots. In such studies, the various effects of wheeling across the wheel rut cannot be assessed. In the present study, we attempted to elucidate the influence of loading by an agricultural tyre on soil physical properties by systematically analysing the properties at various locations in a 300 cm transect (vertical plane perpendicular to the driving direction) starting from the centreline of the wheel rut. Only few similar studies are found in the literature. Exceptions are studies by Way et al. (1997, 2009), who conducted wheeling experiments in a soil bin facility and reported significantly higher soil bulk density, cone penetration resistance

and mean normal stress beneath the centreline than beneath the edge of the tyres at 40 cm depth. 3.4. Pore continuity indices The log–log plots of ka as a function of ea together with the regression lines (Eq. (2)) are shown in Fig. 6 for some selected lateral distances from the centreline of the wheel rut (i.e. 0, 40 and 300 cm). As expected, there was a strong positive linear relationship (R2 ranging from 0.84 to 0.99; p values mostly <0.001) between ka and ea. Similar relationships have been reported in previous studies (Berisso et al., 2013; Do¨rner and Horn, 2009; Peng and Horn, 2008; Schjønning et al., 2002). Soil under the wheel rut at 10 cm depth presented remarkable differences between the sampling locations with respect to the relationship between ka and ea. The log ka versus log ea line fell above (soil at the tyre centreline) and below (soil at the lateral edge of the rut) that for the soil at 300 cm lateral distance from the centreline of the wheel rut (Fig. 6a). As discussed in the previous section, soil under the centreline of the wheel rut was exposed mainly to compressive forces (Fig. 4a), which

F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

44

(a)

(b)

2

Air permeability (log ( m ))

3

2

1

0

-1 -1.2 3

(c)

-1.0

-0.8 3

-0.6 -3

Air-filled porosity (log (m m ))

2 0 cm 40 cm 300 cm

1

0

-1 -1.2

-1.0

-0.8 3

-0.6 -3

Air-filled porosity (log (m m )) Fig. 6. Relationship between air permeability and air-filled porosity at the centre (0 cm) and the periphery (40 cm) of the wheel rut, and at 300 cm horizontal distance from the centre of the wheel rut (square) at (a) 10 cm, (b) 30 cm, and (c) 50 cm depth. The depth refers to the midpoint depth of a sample of 4.2 cm height. Error bars indicate 1 standard error.

induced a significant reduction in ea (for all three matric potentials, i.e. 30, 100 and 300 hPa) and thereby ka as shown in Fig. 5a. In contrast, soil at the lateral edge of the wheel rut was subjected to a stress field inducing relatively strong shear stresses (Fig. 4b), which resulted in a significant reduction in ka. This reduction in ka with little or no change in ea indicates considerable distortion of the structure (Dawidowski et al., 1990). At the wetter end (i.e. at 30 hPa), the distorted air passageways might easily be blocked by water bridges (Hamamoto et al., 2009), which progressively disappear as the soil drains (the lines in Fig. 6 seem to converge with decreasing matric potential) and eventually result in a dramatic increase in ka. The slope N (which shows the rate of opening of continuous pore paths with decreasing matric potential) and the blocked airfilled porosity, eb, as derived from Eq. (4) are shown in Table 3. At 10 and 30 cm depth, N was highest for soil samples from the lateral edge of the wheel rut, but there was no significant difference in N between soil at 0 and 300 cm from the centreline of the wheel rut (Table 3). This could reflect the fact that the soil at the centreline of

wheel rut was mainly compressed (the pores were primarily reduced in size, but not destroyed). In contrast, the value of N for the soil at the lateral edge of the wheel rut was significantly higher, which can be ascribed to the distortion discussed above. No significant differences in N were observed at 50 cm depth, reflecting low stress levels or high soil strength, or both, and associated low changes both in ea and ka. The eb values at 30 and 50 cm depth were obtained by extrapolating beyond measured values, and have to be interpreted with care. The extrapolation yielded much lower estimates for eb than typical values found in the literature for soils with similar texture. Hence, the results for these two lower depths (30 and 50 cm) are not addressed further in this study. For soil at 10 cm depth, as expected, the highest eb for the soil occurred at the lateral edge of the wheel rut (pores were not significantly reduced in size, but distorted or sheared off). It is interesting to note that no significant difference in eb was observed between soil at 0 (i.e. below the centreline of the wheel rut) and 300 cm lateral distance from the centreline of the wheel rut.

Table 3 Regression parameters for air permeability as a function of air-filled porosity. The value eb = 10log (M)/N gives the model prediction of blocked air-filled porosity (eb) at ka = 1 mm2. Values for soil from the same depth followed by different letters are significantly different (p < 0.05). eb values for 30 and 50 cm depths are uncertain due to extrapolation. Deptha (cm)

Blocked air-filled porosity eb = 10log (M)/N (m3 m3)

Slope = N Distance from the centre of the rut (cm)

10 30 50 a

Distance from the centre of the rut (cm)

0

40 (periphery)

70

0

40 (periphery)

70

7.65a 1.68a 2.15a

14.89b 3.05b 1.4a

8.04a 2.1ab 2.8a

0.049a 0.007a 0.013a

0.106b 0.03b 0.006a

0.065a 0.013ab 0.031a

The depth refers to the midpoint depth of a sample of 4.2 cm height.

F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

Pore Organisation

m2)

(a)

45

(b)

1000

100

10

1

0.00

Pore Organisation

m2)

(c)

0.05

0.10

0.15

0.20

0.25

Air-filled porosity (m3m-3)

1000

0 cm 40 cm 300 cm

100

ka = 25 m2 ka = 2.5 m2 ka = 250 m2

10

1

0.00

0.05

0.10

0.15

0.20

0.25

Air-filled porosity (m3m-3) Fig. 7. Pore organisation as a function of air-filled porosity at the centre (circle), the periphery (40 cm from the centre of the wheel rut) and 300 cm horizontal distance from the centre of the wheel rut at (a) 10 cm, (b) 30 cm, and (c) 50 cm depth (the depth refers to the midpoint depth of a sample of 4.2 cm height). Error bars indicate 1 standard error.

Pore organisation as a function of ea is shown in Fig. 7 for some selected lateral distances from the centreline of the wheel rut (i.e. 0, 40 and 300 cm). Air permeability isolines (2.5, 25 and 250 mm2) are also included to illustrate the trend of increasing ka with increasing ea observed in Fig. 6a–c. There were no significant differences in PO among lateral sampling locations at 30 and 50 cm depth (except at 30 hPa matric potential and at 30 cm depth between soil at the lateral edge of the wheel rut and at 300 cm lateral distance from the centreline of the wheel rut; Fig. 7b and c). However, there were significant effects of the wheeling on PO at all three matric potentials at 10 cm depth: PO was greatest for the soil at 300 cm lateral distance from the centreline of the wheel rut and least for the soil at the lateral edge of the wheel rut, with intermediate values for the soil at the centreline of the wheel rut (Fig. 7a). When soil drains, the associated increase in ka can be attributed solely to an increase in ea or it can be attributed to an increase in pore connectivity (a decrease in tortuosity), expressed here by PO, or a combination of both. The effects of these two possible mechanisms can be distinguished by inspecting the change in ka with PO and ea in Fig. 7. At 30 and 50 cm depth, almost all values for PO lay in the region between ka = 25 mm2 and ka = 250 mm2. This implies that the increase in ka was smaller than one order of magnitude when the soil drained from 30 to 300 hPa, and it occurred mainly due to an increase in ea rather than an increase in PO, as indicated by the small slope of PO as a function of ea. In contrast, for soil at 10 cm depth the increase in ka was mainly caused by an increase in PO (steep slope of PO versus ea), as can be

seen from Fig. 6a. At this depth, data points for each matric potential occurred at different levels of ka, which implies an increase in ka higher than one order of magnitude between two consecutive matric potentials. 4. Conclusions Measured (Bolling probe) mean normal stress agreed reasonably well with values simulated with the SoilFlex model, but the stress was over-estimated by the model at 10 cm depth. Measured horizontal stress also agreed well with simulated values, despite high variation in measured values. Measurement of horizontal stress using load cells provided more potential insights into details of stress propagation by taking measurements in a non-vertical direction. This study produced strong evidence of non-homogeneous compression, shear or a combination of compression and shear effects across the wheel rut, which was especially pronounced at 10 cm depth. At the centreline of the wheel rut both ea and ka were significantly reduced, while at the lateral edge of the wheel rut only ka was significantly affected. The latter can be attributed to distortion due to shear stresses. The results show that shear stresses can cause great damage to soil structure, air permeability and pore continuity as expressed by the relationship between ka and ea (indices quantified by N and eb). Sampling in a soil transect running from centreline of wheel rut to un-wheel part of wheel also provided better information about traffic-induced changes on soil physical properties than random sampling in lateral locations.

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F.E. Berisso et al. / Soil & Tillage Research 131 (2013) 36–46

The data presented here may be useful for modelling gas and water transport in soil, where the assumption of a homogeneous wheeling effect is clearly an inadequate oversimplification of real compaction effects. Acknowledgements The technical assistance of Marlies Sommer (Agroscope ART) during field experimentation, sample collection and laboratory analyses is highly acknowledged. We also would like to thank Peter Sta¨hli (Firma Sta¨hli, Suberg BE, Switzerland) for allowing us to use his field and for providing the harvester. Urs Zihlmann (Agroscope ART) is thanked for the soil profile description. This work is part of a Scandinavian cooperation on the effects of subsoil compaction on soil functions (www.poseidon-nordic.dk). The study reported here took place during a study stay by F.E. Berisso at Agroscope ART, Zu¨rich, Switzerland, as part of his PhD studies. The work was partly funded by the Danish Ministry of Food, Agriculture and Fisheries, and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas). F.E. Berisso wishes to thank Agroscope ART for hosting him during the study stay and the Graduate School of the Faculty of Science and Technology at Aarhus University (GSST) for financial support for the PhD study. References Arvidsson, J., 2001. Subsoil compaction caused by heavy sugarbeet harvesters in southern Sweden: I. Soil physical properties and crop yield in six field experiments. Soil and Tillage Research 60, 67–78. Arvidsson, J., Ha˚kansson, I., 1996. Do effects of soil compaction persist after ploughing? Results from 21 long-term field experiments in Sweden. Soil and Tillage Research 39, 175–197. Arvidsson, J., Keller, T., 2007. Soil stress as affected by wheel load and tyre inflation pressure. Soil and Tillage Research 96, 284–291. Bailey, A.C., Nichols, T.A., Johnson, C.E., 1988. Soil stress state determination under wheel loads. Transactions of the ASAE 31, 1309–1314. Ball, B.C., Osullivan, M.F., Hunter, R., 1988. Gas-diffusion, fluid-flow and derived pore continuity indexes in relation to vehicle traffic and tillage. Journal of Soil Science 39, 327–339. Ball, B.C., Schjønning, P., 2002. Air permeability. In: Dane, J.H.,Topp, G.C. (Eds.),Methods of Soil Analysis: Part 1. 3rd ed. Agronomy Monograph, ASA and SSSA, Madison, pp. 1141–1158. Berisso, F.E., Schjønning, P., Keller, T., Lamande´, M., Etana, A., de Jonge, L.W., Iversen, B.V., Arvidsson, J., Forkman, J., 2012. Persistent effects of subsoil compaction on pore size distribution and gas transport in a loamy soil. Soil and Tillage Research 122, 42–51. Berisso, F.E., Schjønning, P., Keller, T., Lamande´, M., Simojoki, A., Iversen, B.V., Alakukku, L., Forkman, J., 2013. Gas transport and subsoil pore characteristics: anisotropy and long-term effects of compaction. Geoderma 195–196, 184–191. Berli, M., Eggers, C.G., Accorsi, M.L., Or, D., 2006. Theoretical analysis of fluid inclusions for in situ soil stress and deformation measurements. Soil Science Society of America Journal 70, 1441–1452. Blackwell, P.S., Green, T.W., Mason, W.K., 1990. Responses of biopore channels from roots to compression by vertical stresses. Soil Science Society of America Journal 54, 1088–1091. Bolling, I., 1987. Bodenverdichtung und Triebkraftverhalten bei Reifen – Neue Mess- und Rechenmethoden (In German). PhD Thesis. Technische Universita¨ t Mu¨nchen, Mu¨nhen. Dawidowski, J.B., Koolen, A.J., 1987. Changes of soil–water suction, conductivity and dry strength during deformation of wet undisturbed samples. Soil and Tillage Research 9, 169–180. Dawidowski, J.B., Lerink, P., Koolen, A.J., 1990. Controlled distortion of soil samples with reference to soil physical effects. Soil and Tillage Research 17, 15–30. Do¨rner, J., Horn, R., 2009. Direction-dependent behaviour of hydraulic and mechanical properties in structured soils under conventional and conservation tillage. Soil and Tillage Research 102, 225–232. Do¨rner, J., Sandoval, P., Dec, D., 2010. The role of soil structure on the pore functionality of an ultisol. Journal of Soil Science and Plant Nutrition 10, 495–508. Gebhardt, S., Fleige, H., Horn, R., 2009. Effect of compaction on pore functions of soils in a Saalean moraine landscape in North Germany. Journal of Plant Nutrition and Soil Science-Zeitschrift Fur Pflanzenernahrung Und Bodenkunde 172, 688–695. Grable, A.R., Siemer, E.G., 1968. Effects of bulk density, aggregate size, and soil water suction on oxygen diffusion, redox potentials, and elongation of corn roots. Soil Science Society of America Journal 32, 180–186.

Gregory, A.S., Whalley, W.R., Watts, C.W., Bird, N.R.A., Hallett, P.D., Whitmore, A.P., 2006. Calculation of the compression index and precompression stress from soil compression test data. Soil and Tillage Research 89, 45–57. Groenevelt, P.H., Kay, B.D., Grant, C.D., 1984. Physical assessment of a soil with respect to rooting potentia. Geoderma 34, 101–114. Gysi, M., Klubertanz, G., Vulliet, L., 2000. Compaction of an Eutric Cambisol under heavy wheel traffic in Switzerland – field data and modelling. Soil and Tillage Research 56, 117–129. Hamamoto, S., Moldrup, P., Kawamoto, K., Komatsu, T., 2009. Effect of particle size and soil compaction on gas transport parameters in variably saturated, sandy soils. Vadose Zone Journal 8, 987–995. Keller, T., 2005. A model for the prediction of the contact area and the distribution of vertical stress below agricultural tyres from readily available tyre parameters. Biosystems Engineering 92, 85–96. Keller, T., Arvidsson, J., Schjønning, P., Lamande´, M., Stettler, M., Weisskopf, P., 2012. In situ subsoil stress–strain behavior in relation to soil precompression stress. Soil Science 177, 490–497. Keller, T., De´fossez, P., Weisskopf, P., Arvidsson, J., Richard, G., 2007. SoilFlex: a model for prediction of soil stresses and soil compaction due to agricultural field traffic including a synthesis of analytical approaches. Soil and Tillage Research 93, 391–411. Keller, T., Lamande, M., 2010. Challenges in the development of analytical soil compaction models. Soil and Tillage Research 111, 54–64. Kenward, M.G., Roger, J.H., 1997. Small sample inference for fixed effects from restricted maximum likelihood. Biometrics 53, 983–997. Kirby, J.M., Blunden, B.G., 1991. Interaction of soil deformations, structure and permeability. Australian Journal of Soil Research 29, 891–904. Koolen, A.J., Kuipers, H., 1983. Agricultural Soil Mechanics. Springer-Verlag, Berlin. Ku¨hne, A., Schack-Kirchner, H., Hildebrand, E.E., 2012. Gas diffusivity in soils compared to ideal isotropic porous media. Journal of Plant Nutrition and Soil Science 175, 34–45. Lamande´, M., Schjønning, P., 2008. The ability of agricultural tyres to distribute the wheel load at the soil–tyre interface. Journal of Terramechanics 45, 109– 120. Lamande´, M., Schjønning, P., 2011. Transmission of vertical stress in a real profile. Part I: site description, evaluation of the So¨hne model, and the effect of topsoil tillage. Soil and Tillage Research 117, 57–70. Lamande´, M., Schjønning, P., Togersen, F.A., 2007. Mechanical behaviour of an undisturbed soil subjected to loadings: effects of load and contact area. Soil and Tillage Research 97, 91–106. O‘Sullivan, M.F., Robertson, E.A.G., 1996. Critical state parameters from intact samples of two agricultural topsoils. Soil and Tillage Research 39, 161–173. O‘Sullivan, M.F., Robertson, E.A.G., Henshall, J.K., 1999. Shear effects on gas transport in soil. Soil and Tillage Research 50, 73–83. O‘Sullivan, M.F., Simota, C., 1995. Modeling the environmental impacts of soil compaction – a review. Soil and Tillage Research 35, 69–84. Peng, X.H., Horn, R., 2008. Time-dependent, anisotropic pore structure and soil strength in a 10-year period after intensive tractor wheeling under conservation and conventional tillage. Journal of Plant Nutrition and Soil ScienceZeitschrift Fur Pflanzenernahrung Und Bodenkunde 171, 936–944. Peth, S., Horn, R., Fazekas, O., Richards, G.B., 2006. Heavy soil loading and its consequence for soil structure, strength, and deformation of arable soils. Journal of Plant Nutrition and Soil Science 169, 775–783. Schjønning, P., Lamande´, M., Tøgersen, F.A., Arvidsson, J., Keller, T., 2008. Modelling effects of tyre inflation pressure on the stress distribution near the soil–tyre interface. Biosystems Engineering 99, 119–133. Schjønning, P., Munkholm, L.J., Moldrup, P., Jacobsen, O.H., 2002. Modelling soil pore characteristics from measurements of air exchange: the long-term effects of fertilization and crop rotation. European Journal of Soil Science 53, 331–339. Simojoki, A., Jaakkola, A., Alakukku, L., 1991. Effect of compaction on soil air in a pot experiment and in the field. Soil and Tillage Research 19, 175–186. Soane, B.D., van Ouwerkerk, C., 1995. Implications of soil compaction in crop production for the quality of the environment. Soil and Tillage Research 35, 5–22. Way, T.R., Johnson, C.E., Bailey, A.C., Raper, R.L., Burt, E.C., 1996. Soil stress state orientation beneath a tire at various loads and inflation pressures. Journal of Terramechanics 33, 185–194. Way, T.R., Kishimoto, T., Burt, E.C., Bailey, A.C., 1997. Tractor tire aspect ratio effects on soil stresses and rut depths. Transactions of the ASAE 40, 871–881. Way, T.R., Kishimoto, T., Torbert, H.A., Burt, E.C., Bailey, A.C., 2009. Tractor tire aspect ratio effects on soil bulk density and cone index. Journal of Terramechanics 46, 27–34. Wiermann, C., Way, T.R., Horn, R., Bailey, A.C., Burt, E.C., 1999. Effect of various dynamic loads on stress and strain behavior of a Norfolk sandy loam. Soil and Tillage Research 50, 127–135. WRB, I.W.G., 2006.World reference base for soil resources—a framework for International classification, correlation and communication. World Soil Resources Reports 103. International Union of Soil Sciences, ISRIC—World Soil Information and Food and Agriculture Organization of the United Nations, Rome. Zink, A., Fleige, H., Horn, R., 2011. Verification of harmful subsoil compaction in loess soils. Soil and Tillage Research 114, 127–134.