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Soil & Tillage Research 90 (2006) 117–125

Effect of soil compaction on hydraulic

properties of two loess soils in China

Shulan Zhang a,b,*, Harald Grip b, Lars Lovdahl b

a College of Resources and Environmental Sciences, Northwest Agriculture & Forestry University,

Yangling 712100, Shaanxi, Chinab 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: zhangshulan@nwsuaf.edu.cn (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 environ-

mental quality.

The effect of soil compaction depends on the

compaction effort, soil type, water status, landscape

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125118

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. Compaction-

induced 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 con-

ductivity 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 south-

eastern 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 (Perkin-

Elmer 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 para-

meters. 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.

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125 119

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

# ¼�

r

rw

�w (1)

where r is the soil particle density, rw is the water

density (assumed to be 1 g cm�3), and w is the gravi-

metric 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 ¼1

½1þ ðacÞn�m (2)

where

Se ¼u � ur

us � ur¼ #� #r

#s � #r;

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).

2.4. Determination of soil hydraulic conductivity

Soil cores from different treatments (five replicates)

were saturated to measure saturated hydraulic con-

ductivity (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

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:

Dð#Þ ¼ 1

2t

dz

d#

Z #i

#

zd# (3)

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 pro-

cedure 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 con-

sidered, i.e.

Kð#Þ ¼ Dð#Þ d#

dc(4)

2.5. Statistical analysis

Mean values, standard deviations and standard errors

are reported for each of the measurements. ANOVAwas

used to assess the effects of compaction on the

measured variables. When ANOVA indicated a sig-

nificant 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.

3. Results

3.1. Soil characteristics

According to the USDA soil texture classification

system, both soils were silty loams (Table 1). Never-

theless, 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

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125120

Table 1

Selected physical properties of the soils

Site Depth (cm) Compaction

level

Bulk density

(g cm�3)

Particle density

(g cm�3)

Sand 0.05–2

mm (%)

Silt 0.002–0.05

mm (%)

Clay <0.002

mm (%)

Organic carbon

(g kg�1)

Heyang 0–5 C0 1.27 (0.09)a 2.65 2.8 76.5 20.6 13.5b

C1 1.37 (0.10)

C2 1.60 (0.12)

10–15 C0 1.29 (0.11) 2.65 1.6 73.6 24.8

C1 1.45 (0.16)

C2 1.65 (0.14)

Mizhi 0–5 C0 1.30 (0.02) 2.65 27.7 60.6 11.7 5.5

C1 1.45 (0.03)

C2 1.61 (0.03)

10–15 C0 1.34 (0.03) 2.65 29.5 60.3 10.2

C1 1.47 (0.01)

C2 1.69 (0.04)

a Numbers in parentheses are standard deviations.b 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)

Fig. 1. Soil water retention curves at different levels of compaction (C0, 0%;

from the Heyang (left panel) and Mihzi (right panel) sites at 0–5 cm (upper)

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.

C1, 10%; and C2, 20%; VG, fitted by the van Genuchten model) of soil

and 10–15 cm (lower) soil depths. The same symbols mean replicates.

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125 121

Table 2

Parameters from the van Genuchten model fit (m = 1 � 1/n)

Site Soil depth (cm) Compaction level Wsa (cm3 cm�3) Wr (cm3 cm�3) a (kPa�1) n R2

Heyang 0–5 C0 1.165 0.125 3.221 1.203 0.926

0–5 C1 0.967 0.125 0.405 1.221 0.926

0–5 C2 0.757 0.125 0.125 1.264 0.926

10–15 C0 1.154 0.125 2.552 1.180 0.924

10–15 C1 1.054 0.125 0.992 1.195 0.924

10–15 C2 0.708 0.125 0.134 1.176 0.924

Mizhi 0–5 C0 1.005 0.080 0.089 2.173 0.986

0–5 C1 0.919 0.080 0.080 2.190 0.986

0–5 C2 0.832 0.080 0.074 2.061 0.986

10–15 C0 1.001 0.080 0.095 2.204 0.982

10–15 C1 0.897 0.080 0.082 2.214 0.982

10–15 C2 0.764 0.080 0.071 2.072 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

Fig. 2. Saturated hydraulic conductivity at different levels of compaction (C0

15 cm) from Heyang (left panel) and Mihzi (right panel). Error bars indica

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, nvalues 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

, 0%; C1, 10%; and C2, 20%) of soil from two layers (0–5 cm and 10–

te standard errors.

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125122

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). Further-

more, 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 con-

ductivity 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

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125 123

(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 develop-

ment 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 valuesarepredominantlygovernedbytheabundanceof

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

andMizhi soil (Fig.2),correlatingwellwith thechanges 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 fromHeyangwas sufficiently strong, in relation

to field variations, to significantly reduce Ks values.

Because the soil at Mizhi was very homogenous, even the

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 conduc-

tivity 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 cm�3) 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 (Guerif

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

S. Zhang et al. / Soil & Tillage Research 90 (2006) 117–125124

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 non-

compacted 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 compac-

tion 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).

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