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Effect of soil compaction on hydraulic properties of two loess soils in China
Soil & Tillage Research 90 (2006) 117–125 www.elsevier.com/locate/still
Effect of soil compaction on hydraulic properties of two loess soils in China Shulan Zhang a,b,*, Harald Grip b, Lars Lo¨vdahl b a
College of Resources and Environmental Sciences, Northwest Agriculture & Forestry University, Yangling 712100, Shaanxi, China b Department of Forest Ecology, Swedish University of Agricultural Sciences, Umea˚, SE-90183, Sweden Received 12 May 2005; received in revised form 12 August 2005; accepted 26 August 2005
Abstract Soil compaction affects hydraulic properties, and thus can lead to soil degradation and other adverse effects on environmental quality. This study evaluates the effects of three levels of compaction on the hydraulic properties of two silty loam soils from the Loess Plateau, China. Undisturbed soil cores were collected from the surface (0–5 cm) and subsurface (10–15 cm) layers at sites in Mizhi and Heyang in Shaanxi Province. The three levels of soil compaction were set by increasing soil bulk density by 0% (C0), 10% (C1) and 20% (C2) through compression and hammering in the laboratory. Soil water retention curves were then determined, and both saturated hydraulic conductivity (Ks) and unsaturated hydraulic conductivity were estimated for all of the samples using standard suction apparatus, a constant head method and the hot-air method, respectively. The high level of compaction (C2) significantly changed the water retention curves of both the surface and subsurface layers of the Heyang soil, and both levels of compaction (C1 and C2) changed the curves of the two layers from the Mizhi site. However, the effects of compaction on the two soils were only pronounced below water tensions of 100 kPa. Saturated hydraulic conductivities (Ks) were significantly reduced by the highest compaction level for both sampled layers of the Heyang soil, but no difference was observed in this respect between the C0 and C1 treatments. Ks values decreased with increasing soil compaction for both layers of the Mizhi soil. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range, and the values obtained were two to five orders of magnitude higher for the Mizhi soil than for the Heyang soil. The results indicate that soil compaction could strongly influence, in different ways, the hydraulic properties of the two soils. # 2005 Elsevier B.V. All rights reserved. Keywords: Compaction levels; Hydraulic conductivity; Soil water retention; Loess Plateau of China
1. Introduction Soil compaction caused by vehicular traffic is a global problem that may affect 68 Mha of land (Oldeman et al., 1991). The detrimental effects of soil compaction caused by traffic include increased bulk density, decreased porosity, and shifts in pore shapes
* Corresponding author. Tel.: +86 29 87088120. E-mail address: [email protected] (S. Zhang). 0167-1987/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2005.08.012
and size distributions (Flowers and Lal, 1998; Radford et al., 2000; Richard et al., 2001; Pagliai et al., 2003). Changes in these basic properties alter the soil’s water retention and hydraulic conductivity characteristics, which in turn affect the infiltration ability of the soil and its plant-available water storage capacity. Consequently, soil compaction can have serious effects on soil quality parameters and, hence, on crop growth and environmental quality. The effect of soil compaction depends on the compaction effort, soil type, water status, landscape
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position, and cropping system involved (Kirkegaard et al., 1993; Radford et al., 2000; Miller et al., 2002; Green et al., 2003; Sillon et al., 2003; Tarawally et al., 2004). The effects of traffic on soil hydraulic properties, which have been investigated by several authors, appear to depend on the prevailing conditions, as shown by the contrasting results of Hill and Sumner (1967), Hill and Meza-Montalvo (1990), Richard et al. (2001), Sillon et al. (2003), Stenitzer and Murer (2003) and Tarawally et al. (2004). Hill and Sumner (1967) measured soil water retention for a variety of soils artificially compacted to various bulk densities. Compactioninduced changes in the measured water retention curves varied by soil textural class. Radford et al. (2000) studied responses of soil properties in a clay soil (Vertisol) to harvester traffic under three axle loads (0, 10 and 12 Mg) in wet soil conditions. The applied compaction caused a statistically significant increase in the soil’s bulk density, and decreased its unsaturated hydraulic conductivity. Sillon et al. (2003) found that a calcareous soil had a higher hydraulic conductivity, across the whole range of water ratios tested, following a compaction treatment. However, the hydraulic conductivity of a loess soil was similar following all treatments with water ratios >0.3, and in drier conditions (i.e. water ratios <0.3), the hydraulic conductivity was lowest in a spring-tilled plot they examined and highest following the compaction treatment. Miller et al. (2002) reported soil water characteristic curves (SWCCs) to be more sensitive to changes in compaction effort than changes in water content when compaction occurred. In addition, SWCCs for soils compacted in the laboratory and the field showed similar changes in hydraulic properties. Results obtained by Tarawally et al. (2004), on the other hand, suggested that soil total porosity was not a good indicator of compaction effects, and that it should not, therefore, be used as a soil compaction index, as previously recommended by Al-Adawi and Reeder (1996). However, Green et al. (2003) noted that field traffic had significant effects on soil compaction and related hydraulic properties in some soils and climates, while in others, landscape and temporal variations were so strong that any effects of wheel tracks were relatively negligible. As Lipiec and Hatano (2003) stated, experimental data relating the effect of soil compaction to unsaturated flows are very limited. Thus, further studies are needed to accumulate a database for model applications and to extend our knowledge in this respect. With the development of modern agriculture in China the use of machinery is becoming more and
more frequent in field operations, and agricultural soils are increasingly likely to be subjected to the same kinds of compaction as those in developed countries. However, there is no information on the effects of compaction on soil hydraulic properties in the Loess Plateau, China. Therefore, the purpose of the study reported here was to determine changes in water retention characteristics and hydraulic conductivity at different compaction levels for two soils from this region. 2. Materials and methods 2.1. Soils and sampling Taking care not to compact the soil, undisturbed soil cores (5 cm high and 7 cm in diameter) of the silty loam soils were randomly collected at 0–5 cm and 10–15 cm depths from agricultural fields at two sites: one (Chromic Cambisols, FAO-Unesco Soil Map of the World, 1974) in Heyang (N 358200 E 110850 , 910 m a.s.l.) and the other (Calcic Cambisols) in Mizhi (N 378460 E 110870 , 1022 m a.s.l.), located in the southeastern and northern parts, respectively, of the Loess Plateau of China. From each layer 39 cores were taken from each of the two soils. Thus, in total 156 soil cores were taken. At the same time additional soil samples were taken for analyzing selected physical and chemical properties. Particle size distribution was determined by the hydrometer method (Gee and Bauder, 1986), particle density by the method of Blake and Hartge (1986) and organic carbon by a CHN elemental analyzer (PerkinElmer Model 2400). 2.2. Compaction A set of five cores was tested, for each permutation of soil layer and compaction level, to determine the effects of compaction on the soils’ water retention curves, and a further five or eight for determining its effects on their saturated and unsaturated hydraulic conductivity parameters. The soils were compacted by pressing and hammering one (C1) or two (C2) 5 mm thick PVC plate/s into the cylinders until the upper side of the plate was level with the cylinder rim. These treatments increased the bulk density for C1 and C2 samples by 10 and 20% and decreased the length of the soil cores by 5 and 10 mm, respectively. The water content of the soils at the time of compaction was between 15 and 20 volumetric percent for the Heyang soil, and somewhat lower for the Mizhi soil.
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2.3. Determination of soil water retention curves Soil cores from the different treatments (five replicates) were first gradually saturated from the bottom using tap water, and then soil water retention was measured after equilibration to a series of soil water tensions (0, 0.25, 0.75, 2.0, 5.0, 8.0, 20.0, 30.0, 50.0, 100.0 and 300.0 kPa) by the standard ceramic tension plates. The wilting point (1500 kPa) was determined on disturbed samples by ceramic tension plate. The soil moisture content was expressed as the water volume ratio (W, volume of water per unit volume of solid phase) by r #¼ w (1) rw where r is the soil particle density, rw is the water density (assumed to be 1 g cm3), and w is the gravimetric water content. Thus, W does not depend on the changes in soil bulk density and is the preferred variable to use for swelling soil (Hillel, 2004, p.15). Soil water retention data were fitted using the model of van Genuchten (1980) Se ¼
1 ½1 þ ðacÞn m
(2)
showed that the required moisture distribution curves for the Heyang and Mizhi soils could be produced by equilibrating the soil cores to tensions of 100 and 20 kPa followed by drying times of 20 and 15 min, respectively. The hot air was blown against one end of each core, causing the sample to dry out quickly from that end. By cutting such cores into thin slices (2–4 mm) and determining the moisture content of each slice, a moisture distribution curve W(z) can be established. If the water content in the bottom of the soil core has not changed from the initial water content, and the evaporation rate is proportional to the square root of the drying time, the diffusivity D(W) can be calculated from: Z 1 dz #i Dð#Þ ¼ zd# (3) 2t d# # where t is the total drying time, z is the distance from the evaporating surface, and Wi is the initial water volume ratio. The calculation of diffusivity followed the procedure described by Gieske and De Vries (1990). The unsaturated hydraulic conductivity was subsequently obtained by multiplying D(W) by dW/dc, the slope of the soil water retention curve at the point being considered, i.e. Kð#Þ ¼ Dð#Þ
where Se ¼
u ur # #r ¼ ; us ur #s #r
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d# dc
(4)
2.5. Statistical analysis
s and r indicate saturated and residual values of the volumetric moisture content u or water volume ratio W, respectively, a, m and n are parameters where m = 1 1/n, and c is the soil water tension (kPa). The same fixed values of residual water content were used for all samples of the same soil, and Ws, a and m was fitted by the software Origin (Origin, Vers. 7, 2003, OriginLab Corp., USA).
Mean values, standard deviations and standard errors are reported for each of the measurements. ANOVA was used to assess the effects of compaction on the measured variables. When ANOVA indicated a significant F-value, multiple comparisons of mean values were performed by the least significant difference method (LSD). The SPSS software package (2003) was used for all of the statistical analyses.
2.4. Determination of soil hydraulic conductivity
3. Results
Soil cores from different treatments (five replicates) were saturated to measure saturated hydraulic conductivity (Ks) by the constant head method (Klute and Dirksen, 1986). Then these soil cores plus cores without running for Ks were equilibrated in a pressure chamber to determine their diffusivity D (W) by the hot-air method (Arya et al., 1975). Due to high hydraulic conductivity at high water content it was difficult to keep the water content constant at the bottom of cylinder during the drying process. Preliminary tests
3.1. Soil characteristics According to the USDA soil texture classification system, both soils were silty loams (Table 1). Nevertheless, they were quite different in terms of their particle size distribution and organic carbon content, although they had similar bulk density, and same particle density. The soil in Heyang contained twice as much clay and much less sand than that in Mizhi, but had similar silt contents. Furthermore, the organic
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Table 1 Selected physical properties of the soils Site Heyang
Mizhi
a b
Silt 0.002–0.05 mm (%)
Clay <0.002 mm (%)
Organic carbon (g kg1)
2.8
76.5
20.6
13.5b
2.65
1.6
73.6
24.8
1.30 (0.02) 1.45 (0.03) 1.61 (0.03)
2.65
27.7
60.6
11.7
1.34 (0.03) 1.47 (0.01) 1.69 (0.04)
2.65
29.5
60.3
10.2
Compaction level
Bulk density (g cm3)
Particle density (g cm3)
0–5
C0 C1 C2
1.27 (0.09)a 1.37 (0.10) 1.60 (0.12)
2.65
10–15
C0 C1 C2
1.29 (0.11) 1.45 (0.16) 1.65 (0.14)
0–5
C0 C1 C2
10–15
C0 C1 C2
Depth (cm)
Sand 0.05–2 mm (%)
5.5
Numbers in parentheses are standard deviations. Organic carbon was determined for 0–20 cm soil depth.
carbon content in Heyang soil was more than double that in the soil at Mizhi. On the whole, there are significant differences in physical properties between the two soils. 3.2. Soil water retention curves The soil water retention curves obtained at different compaction levels for the two soils are shown in Fig. 1 and the fitted parameters in van Genuchten’s equation (Eq. (2)) are given in Table 2. Soil from the Heyang site that was subjected to the low level of compaction (C1)
retained less water than the non-compacted soil (C0) for water tensions 0.75 kPa, but there were no significant differences across the whole measured tension range between C0 and C1 for either soil depth. In contrast, the high level of compaction (C2) significantly decreased the water content of the surface layer (0–5 cm) at tensions <2 kPa and that of the subsurface layer (10– 15 cm) at tensions 5 kPa (P < 0.05). The surface layer soil water volume ratio of soil from the Mizhi site significantly decreased with increased compaction levels at tensions 8 kPa (P < 0.01), but no differences were found among the treatments at tensions >8 kPa.
Fig. 1. Soil water retention curves at different levels of compaction (C0, 0%; C1, 10%; and C2, 20%; VG, fitted by the van Genuchten model) of soil from the Heyang (left panel) and Mihzi (right panel) sites at 0–5 cm (upper) and 10–15 cm (lower) soil depths. The same symbols mean replicates.
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Table 2 Parameters from the van Genuchten model fit (m = 1 1/n) Site
Soil depth (cm)
Compaction level
Wsa (cm3 cm3)
Wr (cm3 cm3)
a (kPa1)
n
R2
Heyang
0–5 0–5 0–5 10–15 10–15 10–15
C0 C1 C2 C0 C1 C2
1.165 0.967 0.757 1.154 1.054 0.708
0.125 0.125 0.125 0.125 0.125 0.125
3.221 0.405 0.125 2.552 0.992 0.134
1.203 1.221 1.264 1.180 1.195 1.176
0.926 0.926 0.926 0.924 0.924 0.924
Mizhi
0–5 0–5 0–5 10–15 10–15 10–15
C0 C1 C2 C0 C1 C2
1.005 0.919 0.832 1.001 0.897 0.764
0.080 0.080 0.080 0.080 0.080 0.080
0.089 0.080 0.074 0.095 0.082 0.071
2.173 2.190 2.061 2.204 2.214 2.072
0.986 0.986 0.986 0.982 0.982 0.982
a Ws is the saturated water volume ratio, Wr is the residual water volume ratio, a and n are shape factors of the VG model. R2 is determination coefficient of fitness and optimized simultaneously for all treatments of each depth, respectively.
For the subsurface layer, the soil water volume ratio significantly decreased with increased compaction levels at tensions 5 kPa (P < 0.01). No difference was observed in this respect between C0 and C1 at tensions 8 kPa, but the C2 treatment resulted in significantly lower water ratios than the C0 and C1 treatments at a tension of 8 kPa (P < 0.01), and a significantly higher water ratio than the C0 treatment at a tension of 20 kPa (P < 0.05). The saturated water volume ratio decreased with increasing compaction levels regardless of soil type and soil depth (Table 2). In comparison with C0, the saturated
water volume ratio following the C1 and C2 treatments was 13 and 37% lower for the Heyang soil, and it was 9 and 20% lower for the Mizhi site, respectively. For both soils, the parameter a values decreased with increasing compaction levels, reflecting the associated increases in air entry tension. In contrast, n values were similar among compaction levels, indicating that the compaction treatments had not affected the shape of the soil water retention curves. The saturated water volume ratios for the C0 and C1 treatments were higher for the Heyang soil than for the Mizhi soil, as were the residual water ratio and a values. Conversely, the C2 treatment resulted in
Fig. 2. Saturated hydraulic conductivity at different levels of compaction (C0, 0%; C1, 10%; and C2, 20%) of soil from two layers (0–5 cm and 10– 15 cm) from Heyang (left panel) and Mihzi (right panel). Error bars indicate standard errors.
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Fig. 3. Unsaturated hydraulic conductivity determined by the hot-air method as affected by soil compaction (C0, 0%; C1, 10%; and C2, 20%) of soil from the Heyang (left panel) and Mihzi (right panel) sites at 0–5 cm (upper) and 10–15 cm (lower) soil depths.
lower saturated water volume ratios when applied to the Heyang soil than when applied to the Mizhi soil. 3.3. Saturated conductivity The effect of soil compaction on saturated hydraulic conductivity (Ks) is shown in Fig. 2. Generally, for the Heyang soil Ks values were higher in the surface layer than in the subsurface layer (Fig. 2, left panel). The C2 treatment resulted in significantly lower Ks values than the C0 and C1 treatments (P 0.05), but there was no significant difference between the C0 and C1 treatments in this respect. In contrast, Ks values decreased significantly when the compaction level increased in the Mizhi soil (Fig. 2, right panel) (P < 0.01). Furthermore, the variations between measurements (standard errors) for each treatment of the Mizhi soil were small in comparison with corresponding variations for the Heyang soil. The Ks values following the C2 treatment were only 18 and 8% of the corresponding values for the C0-treated surface and subsurface layers from Heyang, respectively. In contrast Ks values following the C2 treatment of the Mizhi soil were equivalent to 36 and 28% of the C0 treatment values for the respective soil layers. 3.4. Unsaturated conductivity The hydraulic conductivity, obtained by the hot-air method, is shown in Fig. 3 as a function of the water volume ratio. There were no major differences among
treatments for either of the soil depths at either site. Soil from the Mizhi site showed higher hydraulic conductivity than soil at the Heyang site. Under similar water ratios, the hydraulic conductivity of the two soils differed by more than five orders of magnitude in dry conditions and by two to three orders of magnitude in wet conditions. The hydraulic conductivity changed more rapidly with changes in the soil water ratio in soil from Heyang than in soil from Mizhi. 4. Discussion The water retention curves of the Heyang soil became flatter with increasing compaction levels when soil water was expressed as volumetric water content (data not shown), in accordance with previous observations (Assouline et al., 1997; Miller et al., 2002; Stenitzer and Murer, 2003). The relationship between volumetric water content and water tension is dependent on bulk density, and thus might give misleading indications concerning the effects of compaction on soil water retention. The relationship between soil water volume ratio and water tension might provide greater insight into compaction effects on soil water retention. Hence, we discuss water volume ratios below, unless otherwise stated. The low level of compaction did not significantly affect the water retention of the Heyang soil (from either depth), due to the large variations between replicates, caused by natural field variations and soil management
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(Fig. 1). Similar results have been found for several soils in different landscapes by Green et al. (2003). This implies that our low compaction level (10%) was within the range of normal field variation. However, the high level of compaction (C2) significantly decreased pore volumes with equivalent pore diameters >150 mm in the surface layer, and with equivalent pore diameters 60 mm in the subsurface layer (Fig. 1), which directly correlated to saturated flow (Pagliai et al., 2003). Tarawally et al. (2004) reported that compaction significantly reduced the pore volume with equivalent pore diameter >50 mm in a Rhodic Ferralsol. The significant reduction in large pores due to compaction (C2) would influence air exchange and root development since the growth of feeding roots requires pores ranging from 100 to 200 mm in diameter (Tippkotter, 1983). The water retention of the Mizhi soil was significantly influenced by compaction across a wider tension range than the Heyang soil, for both depths, and there was a significant difference between the two compaction treatments (Fig. 1). Nevertheless, effects of soil compaction for the two soils were still only pronounced below tensions of 100 kPa. This is consistent with expectations, since the amount of water retained at low matric suctions (0–100 kPa) depends on capillarity and the pore size distribution, which are both strongly affected by soil structure at low suctions. At high suctions (100–1500 kPa) water retention is more influenced by soil texture and specific area (Hillel, 2004). This means that compaction levels in the present study did not affect the textural pores, but significantly changed the structural pores, which form the main functional environment for plant roots. Saturated hydraulic conductivity has been used to evaluate the effect of soil compaction on water flow, since Ks values are predominantly governed by the abundance of relatively large pores and their continuity (Pagliai et al., 2003 and Lipiec and Hatano, 2003). Therefore, changes in this group of pores tend to have a strong influence on Ks values. Our results showed that Ks values were heavily reduced by the two soil compaction levels in both Heyang and Mizhi soil (Fig. 2), correlating well with the changes in the water retention curves discussed above. The C1 treatment decreased the saturated hydraulic conductivity relative to C0, but no significant differences were detected between these treatments for the Heyang soil, due to the large variation among replicates. Similar findings were reported by Alakukku (1996) and Green et al. (2003). At the highest compaction level the effect of the compaction on the soil from Heyang was sufficiently strong, in relation to field variations, to significantly reduce Ks values. Because the soil at Mizhi was very homogenous, even the
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low level of compaction (as well as the high level) caused significant changes in soil water retention and Ks values. However, the extents to which the compaction levels reduced Ks and saturated water volume ratios (Fig. 2 and Table 2) imply that the Mizhi soil was more sensitive to low levels of compaction than the Heyang soil, but less affected by high levels of compaction. The topography of the Loess Plateau typically consists of ridges and gullies (as exemplified by the Mizhi site), where soil erosion is more severe than anywhere else in the world (Zhang et al., 1998), and plateaux (as exemplified by the Heyang site). At the Mizhi site, soil surface runoff will increase following compaction, further increasing soil erosion. At the Heyang site, compaction of the soil by even fairly moderate pressures (for example, C2 levels) could reduce Ks to very low values and thus raise the possibility of floods if heavy rain falls. Unsaturated hydraulic conductivities were not affected by the soil compaction levels tested, for either the Mizhi or Heyang soils. However, the unsaturated hydraulic conductivity of the Mizhi soil from both depths tended to be lower following the C2 treatment than following the C0 treatment. Stenitzer and Murer (2003) reported similar changes in hydraulic conductivity between compacted and non-compacted soil at tensions >10 kPa in tests with a loamy silt soil. Sillon et al. (2003), however, found that the compaction treatment gave a higher value in the dry soil moisture range (water volume ratio <0.3 cm3 cm3) and no difference in the wet range for a loess soil. It is difficult to compare results from different experiments due to variations in the soils and compaction efforts used. However, unsaturated hydraulic conductivity depends on the continuity of the small pores within soil fragments under certain moisture conditions (Gue´rif et al., 2001). Thus, it seems logical that the treatments did not affect the unsaturated hydraulic conductivities at either site in our study because there were no significant differences among treatments at pore sizes <60 mm for the Heyang soil and at pore sizes <15 mm for the Mizhi soil. The water volume ratio corresponding to our measured unsaturated hydraulic conductivity was beyond the range where compaction had significant effects on pore sizes and spaces (Figs. 1 and 3). Therefore, hydraulic conductivity among treatments should be the same at the same water volume ratio. 5. Conclusions Hydraulic conductivities of two soils from the Loess Plateau region, China, responded in somewhat different
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ways to different levels of soil compaction. Water retention curves for both the surface and subsurface layers from the two sites were significantly changed by the tested levels of soil compaction, but the tension range affected was wider for the Mizhi soil than the Heyang soil. Differences in the properties (physical and chemical) of the two soils resulted in different field variations. Thus, effects of the low compaction level were masked by large field variations at Heyang, but not at Mizhi. Saturated conductivities were significantly reduced by the high compaction level for both soil layers in Heyang soil, but due to large field variations no significant differences were found between the noncompacted treatment and the C1 treatment. In Mizhi soil Ks values significantly decreased with increasing soil compaction levels for both soil layers. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range. Soil from Mizhi had two to five orders of magnitude higher conductivity values than soil from Heyang. In conclusion, the results indicate that soil compaction could greatly influence hydraulic properties, depending on the compaction effort, soil type and field variability. Acknowledgements The study was supported by a cooperative project between Swedish University of Agricultural Sciences and Northwest Agriculture & Forestry University (INEC-KTS/453/01), project of NSFC, China (No. 30370822) and the Natural Science Foundation of Shaanxi Province, China (No. 2004D03). References Al-Adawi, S.S., Reeder, R.C., 1996. Compaction and subsoiling effects on corn and soybean yields and soil physical properties. Trans. ASAE 39, 1641–1649. Alakukku, L., 1996. Persistence of soil compaction due to high axle load traffic. II. Long-term effects on the properties of fine-textured organic soils. Soil Till. Res. 37, 223–238. Arya, L.M., Farrell, D.A., Blake, G.R., 1975. A field study of soil water depletion pattern in presence of growing soybean roots. I. Determination of hydraulic properties of the soil. Soil Sci. Soc. Am. J. 39, 424–430. Assouline, S., Tavares-Filho, J., Tessier, D., 1997. Effect of compaction on soil physical and hydraulic properties: experimental results and modelling. Soil Sci. Soc. Am. J. 61, 390–398. Blake, G.R., Hartge, K.H., 1986. Particle density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods. Madison, WI, USA, pp. 377–378.
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Effect of soil compaction on hydraulic properties of two loess soils in China Shulan Zhang a,b,*, Harald Grip b, Lars Lo¨vdahl b a
College of Resources and Environmental Sciences, Northwest Agriculture & Forestry University, Yangling 712100, Shaanxi, China b Department of Forest Ecology, Swedish University of Agricultural Sciences, Umea˚, SE-90183, Sweden Received 12 May 2005; received in revised form 12 August 2005; accepted 26 August 2005
Abstract Soil compaction affects hydraulic properties, and thus can lead to soil degradation and other adverse effects on environmental quality. This study evaluates the effects of three levels of compaction on the hydraulic properties of two silty loam soils from the Loess Plateau, China. Undisturbed soil cores were collected from the surface (0–5 cm) and subsurface (10–15 cm) layers at sites in Mizhi and Heyang in Shaanxi Province. The three levels of soil compaction were set by increasing soil bulk density by 0% (C0), 10% (C1) and 20% (C2) through compression and hammering in the laboratory. Soil water retention curves were then determined, and both saturated hydraulic conductivity (Ks) and unsaturated hydraulic conductivity were estimated for all of the samples using standard suction apparatus, a constant head method and the hot-air method, respectively. The high level of compaction (C2) significantly changed the water retention curves of both the surface and subsurface layers of the Heyang soil, and both levels of compaction (C1 and C2) changed the curves of the two layers from the Mizhi site. However, the effects of compaction on the two soils were only pronounced below water tensions of 100 kPa. Saturated hydraulic conductivities (Ks) were significantly reduced by the highest compaction level for both sampled layers of the Heyang soil, but no difference was observed in this respect between the C0 and C1 treatments. Ks values decreased with increasing soil compaction for both layers of the Mizhi soil. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range, and the values obtained were two to five orders of magnitude higher for the Mizhi soil than for the Heyang soil. The results indicate that soil compaction could strongly influence, in different ways, the hydraulic properties of the two soils. # 2005 Elsevier B.V. All rights reserved. Keywords: Compaction levels; Hydraulic conductivity; Soil water retention; Loess Plateau of China
1. Introduction Soil compaction caused by vehicular traffic is a global problem that may affect 68 Mha of land (Oldeman et al., 1991). The detrimental effects of soil compaction caused by traffic include increased bulk density, decreased porosity, and shifts in pore shapes
* Corresponding author. Tel.: +86 29 87088120. E-mail address: [email protected] (S. Zhang). 0167-1987/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2005.08.012
and size distributions (Flowers and Lal, 1998; Radford et al., 2000; Richard et al., 2001; Pagliai et al., 2003). Changes in these basic properties alter the soil’s water retention and hydraulic conductivity characteristics, which in turn affect the infiltration ability of the soil and its plant-available water storage capacity. Consequently, soil compaction can have serious effects on soil quality parameters and, hence, on crop growth and environmental quality. The effect of soil compaction depends on the compaction effort, soil type, water status, landscape
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position, and cropping system involved (Kirkegaard et al., 1993; Radford et al., 2000; Miller et al., 2002; Green et al., 2003; Sillon et al., 2003; Tarawally et al., 2004). The effects of traffic on soil hydraulic properties, which have been investigated by several authors, appear to depend on the prevailing conditions, as shown by the contrasting results of Hill and Sumner (1967), Hill and Meza-Montalvo (1990), Richard et al. (2001), Sillon et al. (2003), Stenitzer and Murer (2003) and Tarawally et al. (2004). Hill and Sumner (1967) measured soil water retention for a variety of soils artificially compacted to various bulk densities. Compactioninduced changes in the measured water retention curves varied by soil textural class. Radford et al. (2000) studied responses of soil properties in a clay soil (Vertisol) to harvester traffic under three axle loads (0, 10 and 12 Mg) in wet soil conditions. The applied compaction caused a statistically significant increase in the soil’s bulk density, and decreased its unsaturated hydraulic conductivity. Sillon et al. (2003) found that a calcareous soil had a higher hydraulic conductivity, across the whole range of water ratios tested, following a compaction treatment. However, the hydraulic conductivity of a loess soil was similar following all treatments with water ratios >0.3, and in drier conditions (i.e. water ratios <0.3), the hydraulic conductivity was lowest in a spring-tilled plot they examined and highest following the compaction treatment. Miller et al. (2002) reported soil water characteristic curves (SWCCs) to be more sensitive to changes in compaction effort than changes in water content when compaction occurred. In addition, SWCCs for soils compacted in the laboratory and the field showed similar changes in hydraulic properties. Results obtained by Tarawally et al. (2004), on the other hand, suggested that soil total porosity was not a good indicator of compaction effects, and that it should not, therefore, be used as a soil compaction index, as previously recommended by Al-Adawi and Reeder (1996). However, Green et al. (2003) noted that field traffic had significant effects on soil compaction and related hydraulic properties in some soils and climates, while in others, landscape and temporal variations were so strong that any effects of wheel tracks were relatively negligible. As Lipiec and Hatano (2003) stated, experimental data relating the effect of soil compaction to unsaturated flows are very limited. Thus, further studies are needed to accumulate a database for model applications and to extend our knowledge in this respect. With the development of modern agriculture in China the use of machinery is becoming more and
more frequent in field operations, and agricultural soils are increasingly likely to be subjected to the same kinds of compaction as those in developed countries. However, there is no information on the effects of compaction on soil hydraulic properties in the Loess Plateau, China. Therefore, the purpose of the study reported here was to determine changes in water retention characteristics and hydraulic conductivity at different compaction levels for two soils from this region. 2. Materials and methods 2.1. Soils and sampling Taking care not to compact the soil, undisturbed soil cores (5 cm high and 7 cm in diameter) of the silty loam soils were randomly collected at 0–5 cm and 10–15 cm depths from agricultural fields at two sites: one (Chromic Cambisols, FAO-Unesco Soil Map of the World, 1974) in Heyang (N 358200 E 110850 , 910 m a.s.l.) and the other (Calcic Cambisols) in Mizhi (N 378460 E 110870 , 1022 m a.s.l.), located in the southeastern and northern parts, respectively, of the Loess Plateau of China. From each layer 39 cores were taken from each of the two soils. Thus, in total 156 soil cores were taken. At the same time additional soil samples were taken for analyzing selected physical and chemical properties. Particle size distribution was determined by the hydrometer method (Gee and Bauder, 1986), particle density by the method of Blake and Hartge (1986) and organic carbon by a CHN elemental analyzer (PerkinElmer Model 2400). 2.2. Compaction A set of five cores was tested, for each permutation of soil layer and compaction level, to determine the effects of compaction on the soils’ water retention curves, and a further five or eight for determining its effects on their saturated and unsaturated hydraulic conductivity parameters. The soils were compacted by pressing and hammering one (C1) or two (C2) 5 mm thick PVC plate/s into the cylinders until the upper side of the plate was level with the cylinder rim. These treatments increased the bulk density for C1 and C2 samples by 10 and 20% and decreased the length of the soil cores by 5 and 10 mm, respectively. The water content of the soils at the time of compaction was between 15 and 20 volumetric percent for the Heyang soil, and somewhat lower for the Mizhi soil.
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2.3. Determination of soil water retention curves Soil cores from the different treatments (five replicates) were first gradually saturated from the bottom using tap water, and then soil water retention was measured after equilibration to a series of soil water tensions (0, 0.25, 0.75, 2.0, 5.0, 8.0, 20.0, 30.0, 50.0, 100.0 and 300.0 kPa) by the standard ceramic tension plates. The wilting point (1500 kPa) was determined on disturbed samples by ceramic tension plate. The soil moisture content was expressed as the water volume ratio (W, volume of water per unit volume of solid phase) by r #¼ w (1) rw where r is the soil particle density, rw is the water density (assumed to be 1 g cm3), and w is the gravimetric water content. Thus, W does not depend on the changes in soil bulk density and is the preferred variable to use for swelling soil (Hillel, 2004, p.15). Soil water retention data were fitted using the model of van Genuchten (1980) Se ¼
1 ½1 þ ðacÞn m
(2)
showed that the required moisture distribution curves for the Heyang and Mizhi soils could be produced by equilibrating the soil cores to tensions of 100 and 20 kPa followed by drying times of 20 and 15 min, respectively. The hot air was blown against one end of each core, causing the sample to dry out quickly from that end. By cutting such cores into thin slices (2–4 mm) and determining the moisture content of each slice, a moisture distribution curve W(z) can be established. If the water content in the bottom of the soil core has not changed from the initial water content, and the evaporation rate is proportional to the square root of the drying time, the diffusivity D(W) can be calculated from: Z 1 dz #i Dð#Þ ¼ zd# (3) 2t d# # where t is the total drying time, z is the distance from the evaporating surface, and Wi is the initial water volume ratio. The calculation of diffusivity followed the procedure described by Gieske and De Vries (1990). The unsaturated hydraulic conductivity was subsequently obtained by multiplying D(W) by dW/dc, the slope of the soil water retention curve at the point being considered, i.e. Kð#Þ ¼ Dð#Þ
where Se ¼
u ur # #r ¼ ; us ur #s #r
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d# dc
(4)
2.5. Statistical analysis
s and r indicate saturated and residual values of the volumetric moisture content u or water volume ratio W, respectively, a, m and n are parameters where m = 1 1/n, and c is the soil water tension (kPa). The same fixed values of residual water content were used for all samples of the same soil, and Ws, a and m was fitted by the software Origin (Origin, Vers. 7, 2003, OriginLab Corp., USA).
Mean values, standard deviations and standard errors are reported for each of the measurements. ANOVA was used to assess the effects of compaction on the measured variables. When ANOVA indicated a significant F-value, multiple comparisons of mean values were performed by the least significant difference method (LSD). The SPSS software package (2003) was used for all of the statistical analyses.
2.4. Determination of soil hydraulic conductivity
3. Results
Soil cores from different treatments (five replicates) were saturated to measure saturated hydraulic conductivity (Ks) by the constant head method (Klute and Dirksen, 1986). Then these soil cores plus cores without running for Ks were equilibrated in a pressure chamber to determine their diffusivity D (W) by the hot-air method (Arya et al., 1975). Due to high hydraulic conductivity at high water content it was difficult to keep the water content constant at the bottom of cylinder during the drying process. Preliminary tests
3.1. Soil characteristics According to the USDA soil texture classification system, both soils were silty loams (Table 1). Nevertheless, they were quite different in terms of their particle size distribution and organic carbon content, although they had similar bulk density, and same particle density. The soil in Heyang contained twice as much clay and much less sand than that in Mizhi, but had similar silt contents. Furthermore, the organic
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Table 1 Selected physical properties of the soils Site Heyang
Mizhi
a b
Silt 0.002–0.05 mm (%)
Clay <0.002 mm (%)
Organic carbon (g kg1)
2.8
76.5
20.6
13.5b
2.65
1.6
73.6
24.8
1.30 (0.02) 1.45 (0.03) 1.61 (0.03)
2.65
27.7
60.6
11.7
1.34 (0.03) 1.47 (0.01) 1.69 (0.04)
2.65
29.5
60.3
10.2
Compaction level
Bulk density (g cm3)
Particle density (g cm3)
0–5
C0 C1 C2
1.27 (0.09)a 1.37 (0.10) 1.60 (0.12)
2.65
10–15
C0 C1 C2
1.29 (0.11) 1.45 (0.16) 1.65 (0.14)
0–5
C0 C1 C2
10–15
C0 C1 C2
Depth (cm)
Sand 0.05–2 mm (%)
5.5
Numbers in parentheses are standard deviations. Organic carbon was determined for 0–20 cm soil depth.
carbon content in Heyang soil was more than double that in the soil at Mizhi. On the whole, there are significant differences in physical properties between the two soils. 3.2. Soil water retention curves The soil water retention curves obtained at different compaction levels for the two soils are shown in Fig. 1 and the fitted parameters in van Genuchten’s equation (Eq. (2)) are given in Table 2. Soil from the Heyang site that was subjected to the low level of compaction (C1)
retained less water than the non-compacted soil (C0) for water tensions 0.75 kPa, but there were no significant differences across the whole measured tension range between C0 and C1 for either soil depth. In contrast, the high level of compaction (C2) significantly decreased the water content of the surface layer (0–5 cm) at tensions <2 kPa and that of the subsurface layer (10– 15 cm) at tensions 5 kPa (P < 0.05). The surface layer soil water volume ratio of soil from the Mizhi site significantly decreased with increased compaction levels at tensions 8 kPa (P < 0.01), but no differences were found among the treatments at tensions >8 kPa.
Fig. 1. Soil water retention curves at different levels of compaction (C0, 0%; C1, 10%; and C2, 20%; VG, fitted by the van Genuchten model) of soil from the Heyang (left panel) and Mihzi (right panel) sites at 0–5 cm (upper) and 10–15 cm (lower) soil depths. The same symbols mean replicates.
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Table 2 Parameters from the van Genuchten model fit (m = 1 1/n) Site
Soil depth (cm)
Compaction level
Wsa (cm3 cm3)
Wr (cm3 cm3)
a (kPa1)
n
R2
Heyang
0–5 0–5 0–5 10–15 10–15 10–15
C0 C1 C2 C0 C1 C2
1.165 0.967 0.757 1.154 1.054 0.708
0.125 0.125 0.125 0.125 0.125 0.125
3.221 0.405 0.125 2.552 0.992 0.134
1.203 1.221 1.264 1.180 1.195 1.176
0.926 0.926 0.926 0.924 0.924 0.924
Mizhi
0–5 0–5 0–5 10–15 10–15 10–15
C0 C1 C2 C0 C1 C2
1.005 0.919 0.832 1.001 0.897 0.764
0.080 0.080 0.080 0.080 0.080 0.080
0.089 0.080 0.074 0.095 0.082 0.071
2.173 2.190 2.061 2.204 2.214 2.072
0.986 0.986 0.986 0.982 0.982 0.982
a Ws is the saturated water volume ratio, Wr is the residual water volume ratio, a and n are shape factors of the VG model. R2 is determination coefficient of fitness and optimized simultaneously for all treatments of each depth, respectively.
For the subsurface layer, the soil water volume ratio significantly decreased with increased compaction levels at tensions 5 kPa (P < 0.01). No difference was observed in this respect between C0 and C1 at tensions 8 kPa, but the C2 treatment resulted in significantly lower water ratios than the C0 and C1 treatments at a tension of 8 kPa (P < 0.01), and a significantly higher water ratio than the C0 treatment at a tension of 20 kPa (P < 0.05). The saturated water volume ratio decreased with increasing compaction levels regardless of soil type and soil depth (Table 2). In comparison with C0, the saturated
water volume ratio following the C1 and C2 treatments was 13 and 37% lower for the Heyang soil, and it was 9 and 20% lower for the Mizhi site, respectively. For both soils, the parameter a values decreased with increasing compaction levels, reflecting the associated increases in air entry tension. In contrast, n values were similar among compaction levels, indicating that the compaction treatments had not affected the shape of the soil water retention curves. The saturated water volume ratios for the C0 and C1 treatments were higher for the Heyang soil than for the Mizhi soil, as were the residual water ratio and a values. Conversely, the C2 treatment resulted in
Fig. 2. Saturated hydraulic conductivity at different levels of compaction (C0, 0%; C1, 10%; and C2, 20%) of soil from two layers (0–5 cm and 10– 15 cm) from Heyang (left panel) and Mihzi (right panel). Error bars indicate standard errors.
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Fig. 3. Unsaturated hydraulic conductivity determined by the hot-air method as affected by soil compaction (C0, 0%; C1, 10%; and C2, 20%) of soil from the Heyang (left panel) and Mihzi (right panel) sites at 0–5 cm (upper) and 10–15 cm (lower) soil depths.
lower saturated water volume ratios when applied to the Heyang soil than when applied to the Mizhi soil. 3.3. Saturated conductivity The effect of soil compaction on saturated hydraulic conductivity (Ks) is shown in Fig. 2. Generally, for the Heyang soil Ks values were higher in the surface layer than in the subsurface layer (Fig. 2, left panel). The C2 treatment resulted in significantly lower Ks values than the C0 and C1 treatments (P 0.05), but there was no significant difference between the C0 and C1 treatments in this respect. In contrast, Ks values decreased significantly when the compaction level increased in the Mizhi soil (Fig. 2, right panel) (P < 0.01). Furthermore, the variations between measurements (standard errors) for each treatment of the Mizhi soil were small in comparison with corresponding variations for the Heyang soil. The Ks values following the C2 treatment were only 18 and 8% of the corresponding values for the C0-treated surface and subsurface layers from Heyang, respectively. In contrast Ks values following the C2 treatment of the Mizhi soil were equivalent to 36 and 28% of the C0 treatment values for the respective soil layers. 3.4. Unsaturated conductivity The hydraulic conductivity, obtained by the hot-air method, is shown in Fig. 3 as a function of the water volume ratio. There were no major differences among
treatments for either of the soil depths at either site. Soil from the Mizhi site showed higher hydraulic conductivity than soil at the Heyang site. Under similar water ratios, the hydraulic conductivity of the two soils differed by more than five orders of magnitude in dry conditions and by two to three orders of magnitude in wet conditions. The hydraulic conductivity changed more rapidly with changes in the soil water ratio in soil from Heyang than in soil from Mizhi. 4. Discussion The water retention curves of the Heyang soil became flatter with increasing compaction levels when soil water was expressed as volumetric water content (data not shown), in accordance with previous observations (Assouline et al., 1997; Miller et al., 2002; Stenitzer and Murer, 2003). The relationship between volumetric water content and water tension is dependent on bulk density, and thus might give misleading indications concerning the effects of compaction on soil water retention. The relationship between soil water volume ratio and water tension might provide greater insight into compaction effects on soil water retention. Hence, we discuss water volume ratios below, unless otherwise stated. The low level of compaction did not significantly affect the water retention of the Heyang soil (from either depth), due to the large variations between replicates, caused by natural field variations and soil management
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(Fig. 1). Similar results have been found for several soils in different landscapes by Green et al. (2003). This implies that our low compaction level (10%) was within the range of normal field variation. However, the high level of compaction (C2) significantly decreased pore volumes with equivalent pore diameters >150 mm in the surface layer, and with equivalent pore diameters 60 mm in the subsurface layer (Fig. 1), which directly correlated to saturated flow (Pagliai et al., 2003). Tarawally et al. (2004) reported that compaction significantly reduced the pore volume with equivalent pore diameter >50 mm in a Rhodic Ferralsol. The significant reduction in large pores due to compaction (C2) would influence air exchange and root development since the growth of feeding roots requires pores ranging from 100 to 200 mm in diameter (Tippkotter, 1983). The water retention of the Mizhi soil was significantly influenced by compaction across a wider tension range than the Heyang soil, for both depths, and there was a significant difference between the two compaction treatments (Fig. 1). Nevertheless, effects of soil compaction for the two soils were still only pronounced below tensions of 100 kPa. This is consistent with expectations, since the amount of water retained at low matric suctions (0–100 kPa) depends on capillarity and the pore size distribution, which are both strongly affected by soil structure at low suctions. At high suctions (100–1500 kPa) water retention is more influenced by soil texture and specific area (Hillel, 2004). This means that compaction levels in the present study did not affect the textural pores, but significantly changed the structural pores, which form the main functional environment for plant roots. Saturated hydraulic conductivity has been used to evaluate the effect of soil compaction on water flow, since Ks values are predominantly governed by the abundance of relatively large pores and their continuity (Pagliai et al., 2003 and Lipiec and Hatano, 2003). Therefore, changes in this group of pores tend to have a strong influence on Ks values. Our results showed that Ks values were heavily reduced by the two soil compaction levels in both Heyang and Mizhi soil (Fig. 2), correlating well with the changes in the water retention curves discussed above. The C1 treatment decreased the saturated hydraulic conductivity relative to C0, but no significant differences were detected between these treatments for the Heyang soil, due to the large variation among replicates. Similar findings were reported by Alakukku (1996) and Green et al. (2003). At the highest compaction level the effect of the compaction on the soil from Heyang was sufficiently strong, in relation to field variations, to significantly reduce Ks values. Because the soil at Mizhi was very homogenous, even the
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low level of compaction (as well as the high level) caused significant changes in soil water retention and Ks values. However, the extents to which the compaction levels reduced Ks and saturated water volume ratios (Fig. 2 and Table 2) imply that the Mizhi soil was more sensitive to low levels of compaction than the Heyang soil, but less affected by high levels of compaction. The topography of the Loess Plateau typically consists of ridges and gullies (as exemplified by the Mizhi site), where soil erosion is more severe than anywhere else in the world (Zhang et al., 1998), and plateaux (as exemplified by the Heyang site). At the Mizhi site, soil surface runoff will increase following compaction, further increasing soil erosion. At the Heyang site, compaction of the soil by even fairly moderate pressures (for example, C2 levels) could reduce Ks to very low values and thus raise the possibility of floods if heavy rain falls. Unsaturated hydraulic conductivities were not affected by the soil compaction levels tested, for either the Mizhi or Heyang soils. However, the unsaturated hydraulic conductivity of the Mizhi soil from both depths tended to be lower following the C2 treatment than following the C0 treatment. Stenitzer and Murer (2003) reported similar changes in hydraulic conductivity between compacted and non-compacted soil at tensions >10 kPa in tests with a loamy silt soil. Sillon et al. (2003), however, found that the compaction treatment gave a higher value in the dry soil moisture range (water volume ratio <0.3 cm3 cm3) and no difference in the wet range for a loess soil. It is difficult to compare results from different experiments due to variations in the soils and compaction efforts used. However, unsaturated hydraulic conductivity depends on the continuity of the small pores within soil fragments under certain moisture conditions (Gue´rif et al., 2001). Thus, it seems logical that the treatments did not affect the unsaturated hydraulic conductivities at either site in our study because there were no significant differences among treatments at pore sizes <60 mm for the Heyang soil and at pore sizes <15 mm for the Mizhi soil. The water volume ratio corresponding to our measured unsaturated hydraulic conductivity was beyond the range where compaction had significant effects on pore sizes and spaces (Figs. 1 and 3). Therefore, hydraulic conductivity among treatments should be the same at the same water volume ratio. 5. Conclusions Hydraulic conductivities of two soils from the Loess Plateau region, China, responded in somewhat different
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ways to different levels of soil compaction. Water retention curves for both the surface and subsurface layers from the two sites were significantly changed by the tested levels of soil compaction, but the tension range affected was wider for the Mizhi soil than the Heyang soil. Differences in the properties (physical and chemical) of the two soils resulted in different field variations. Thus, effects of the low compaction level were masked by large field variations at Heyang, but not at Mizhi. Saturated conductivities were significantly reduced by the high compaction level for both soil layers in Heyang soil, but due to large field variations no significant differences were found between the noncompacted treatment and the C1 treatment. In Mizhi soil Ks values significantly decreased with increasing soil compaction levels for both soil layers. Unsaturated hydraulic conductivities were not affected by soil compaction levels in the measured water volume ratio range. Soil from Mizhi had two to five orders of magnitude higher conductivity values than soil from Heyang. In conclusion, the results indicate that soil compaction could greatly influence hydraulic properties, depending on the compaction effort, soil type and field variability. Acknowledgements The study was supported by a cooperative project between Swedish University of Agricultural Sciences and Northwest Agriculture & Forestry University (INEC-KTS/453/01), project of NSFC, China (No. 30370822) and the Natural Science Foundation of Shaanxi Province, China (No. 2004D03). References Al-Adawi, S.S., Reeder, R.C., 1996. Compaction and subsoiling effects on corn and soybean yields and soil physical properties. Trans. ASAE 39, 1641–1649. Alakukku, L., 1996. Persistence of soil compaction due to high axle load traffic. II. Long-term effects on the properties of fine-textured organic soils. Soil Till. Res. 37, 223–238. Arya, L.M., Farrell, D.A., Blake, G.R., 1975. A field study of soil water depletion pattern in presence of growing soybean roots. I. Determination of hydraulic properties of the soil. Soil Sci. Soc. Am. J. 39, 424–430. Assouline, S., Tavares-Filho, J., Tessier, D., 1997. Effect of compaction on soil physical and hydraulic properties: experimental results and modelling. Soil Sci. Soc. Am. J. 61, 390–398. Blake, G.R., Hartge, K.H., 1986. Particle density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I. Physical and Mineralogical Methods. Madison, WI, USA, pp. 377–378.
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