Soil hydrological properties response to
Environ Earth Sci (2014) 71:745–752
DOI 10.1007/s12665-013-2476-y
ORIGINAL ARTICLE
Soil-hydrological properties response to grazing exclusion
in a steppe grassland of the Loess Plateau
Tie-Niu Wu • Gao-Lin Wu • Dong Wang
Zhi-Hua Shi
•
Received: 31 July 2012 / Accepted: 5 April 2013 / Published online: 23 April 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Soil–water characteristics are necessary for
water quality monitoring, solute migration and plant
growth. Soil–water characteristic curve (SWCC) is a relationship between suction and water content or degree of
saturation. However, little information is available concerning the impacts of grazing exclusion management on
soil–water characteristics. Here, the soil–water characteristics of grasslands, which were excluded grazing for 5
(GE5) and 15 years (GE15), were studied. The saturated
hydraulic conductivity (Ks), SWCC, particle composition,
field capacity and some other indexes were determined.
Results showed that the clay content and Ks of grassland
soil were higher for GE15 than GE5. For both treatments,
in low suction condition (B100 kPa), the water holding
capacity of 0–10 cm soil was the best. Water holding
capacity of topsoil decreased gradually with the increasing
of suction, and it reached the strongest when the suction
reached 600 kPa. In all soil water suction, the water
holding capacity of subsoil was the weakest. The van
Genuchten expression was applicable for most of the
samples, except 20–30 cm of GE5 and 10–20 cm of GE15.
T.-N. Wu G.-L. Wu D. Wang Z.-H. Shi (&)
State Key Laboratory of Soil Erosion and Dryland Farming on
the Loess Plateau, Northwest A&F University, Yangling 712100,
Shaanxi, People’s Republic of China
e-mail: shizhihua70@gmail.com
T.-N. Wu
Urban Construction Departments of Shaoyang University,
Shaoyang 422000, Hunan, People’s Republic of China
G.-L. Wu (&) D. Wang
Institute of Soil and Water Conservation of Chinese Academy
of Science & Ministry of Water Resource, Yangling 712100,
Shaanxi, People’s Republic of China
e-mail: gaolinwu@gmail.com
Dual porosity equation was applicable for all the samples.
The soil–water capability and soil structure of which was
fenced for 15 years is superior to that of 5 years. This study
suggests that the enclosure management improved the soil
structure and soil–water capability.
Keywords Grazing exclusion Arid-grassland
Soil–water characteristic curve The Loess Plateau
Introduction
Most of grasslands in the earth are over-used and poorly
managed (Oldeman 1994). The overuse of grassland is
driven by the demand for forage production since significant portions of world milk (27 %) and beef (23 %) production occurs on grasslands managed solely for those
purposes (Sere et al. 1995). A well-known management
tool to restore the degraded grassland is the establishment
of enclosures, denoting areas closed off for agriculture and
grazing for a specific period of time. Hopkins and Wainwright (1989) established the extent of changes in botanical
composition since the early 1970s in enclosed grassland in
upland areas to relate the extent of changes to the present
agricultural management. Su et al. (2005) studied the soil
physical properties in Inner Mongolia and found that the
particle size distribution showed more silt and clay in the
top soil under the non-grazed sites compared with soils
under the continuously grazed site. Li et al. (2011) examined soil physical properties under continuous grazing and
exclusion of livestock for 8 years in Horqin sandy grassland, and they found that exclosure increased the fine sand
and silt ? clay contents, which were similar with the
results of Su et al. (2005). Verdoodt et al. (2009) assessed
the vegetation and soil rehabilitation in a 23-year
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chronosequence of two different enclosure management
types. The result showed that the communal enclosure
strategy proved to be successful in improving both rangeland vegetation and soil health. Shi et al. (2010) examined
the characteristics of vegetation and soil properties in the
livestock-excluded pastures and the adjacent grazed pastures under two topographic habitats and found that livestock exclusion increased soil bulk density, soil water
content, the water retention ability of the soil, and also
effectively protect soil from loss by water erosion.
The soil–water characteristic curve (SWCC) is a relationship between suction and water content or degree of
saturation. Recently, laboratory measurements of soil–
water characteristics have been used to study the effects of
bulk density changes on soil hydraulic properties (Lv et al.
2004), and the effect of soil compaction on hydraulic
properties of the loess (Zhang et al. 2006). As the measurement of soil–water characteristics is costly and difficult, Saxton et al. (1986) derived equations to estimate
continuous relationships of soil–water moisture content to
potentials and hydraulic conductivity from soil textures.
More recently, Fredlund and Xing (1994) proposed a new
equation that can be used to fit laboratory data over the
entire soil suction range.
SWCC vary widely and nonlinearly with water content
for different soil textures (Saxton et al. 1986). Ng and Pang
(2000) studied the soil water characteristics of a volcanic
soil in Hong Kong and found that the SWCC of a recompacted specimen was very different from that of a
natural sample having the same initial soil density and
initial water content. Agus et al. (2001) studied the SWCC
of Singapore residual soils. They found that the equations
proposed by Fredlund and Xing (1994) are a reasonably
good estimate of the SWCC for Singapore residual soils.
However, in the semiarid China, the widely distributed soil
is loess and black loam rather than residual soils or volcanic soils. The studies on black loam of enclosed grassland focus on soil carbon, soil microbiology, etc. (Contant
et al. 2001; Zaller 2006; Verdoodt et al. 2009). Nevertheless, very limited research has been conducted on the
SWCC of fenced grassland soil. Soil–water characteristics
are necessary for many soil–water related investigations
such as water conservation, irrigation scheduling, solute
migration and plant growth. These investigations commonly require soil–water characteristics for the computations of soil–water storage and soil–water flow. Thus,
further studies are needed to obtain a database for model
parameters and to extend the knowledge in this respect.
In this paper, the evaluation of changes in soil–water
characteristics (soil porosity, water content, field capacity,
saturated hydraulic conductivity, mechanical components,
and bulk density) of two grazing exclusion treatments (5and 15-year) was made to determine whether long-term
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Environ Earth Sci (2014) 71:745–752
exclusion of livestock grazing can significantly affect soil
water properties. The purpose of this study is to understand
the effects of long-term grazing exclusion on soil structure
and water storage function of typical steppe ecosystem in
the Loess Plateau.
Materials and methods
Study site
This experiment was conducted in typical steppe grassland
at 2,000 m a.s.l. in the Loess Plateau at the Guyuan Ecological Experimental Station of Chinese Academy of Sciences (36°130 –36°190 N, 106°240 –106°280 E) in Ningxia
Province, P. R. China, which was shown in Fig. 1. The
Chinese Loess Plateau lies in the Northwest of China, the
loess-paleosol sequences range between * 100 and 300 m
in thickness and record aridity in desertified source regions
in northwestern China and Mongolia during glacial periods, while intercalated paleosols reflect intensified summer
monsoon conditions that supported soil formation during
interglacial periods. The Chinese Loess Plateau includes
three geomorphological units: the Eastern Loess Plateau,
the Central Loess Plateau and the Western Loess Plateau.
The Western Loess Plateau lies to the west of Liupan
Mountains; the mean annual precipitation decreases from
ca. 600 mm/a in the southeast to ca. 200 mm/a in the
northwest, with modern vegetation ranging from forestgrassland, grassland and desert grassland, along the same
gradient. Loess deposits in this region typically are coarsegrained, and the paleosols are relatively weakly developed
(Chen et al. 1997).
The geological loess profile was shown in Fig. 2, which
was taken at Caijiapo, Shaanxi Province. On the Loess
Plateau, loess deposits consist of two major stratigraphic
units, namely loess and paleosol. Loess horizons are
Fig. 1 The location of the study site on the Loess Plateau
Environ Earth Sci (2014) 71:745–752
747
two community blocks were established. Samples were
taken in mid-July of 2010, when biomass had reached its
seasonal peak biomass in both years. Undisturbed soil
samples were taken from the center of each sampling
quadrat of grassland. The soil in each quadrat was removed
by layers (0–10, 10–20, 20–30, 30–40 and 40–50 cm) and
taken to the laboratory. Fifteen soil samples were repeated
in each block, yielding a total of 30 samples. These samples were carried to the laboratory for measuring soil
physical properties.
Measurement methods
Fig. 2 The typical geological loess profile of the Loess Plateau
labeled Li and paleosols Si. Paleosols are defined as those
units having the appearance and a degree of pedogenic
development similar to or greater than the Holocene soil in
the same area (Ding et al. 1993). In the Caijiapo section,
nine major paleosols can be distinguished from the loess
units; they were S1–S9. However, the top four paleosols
were not shown in Fig. 2. Loess can be defined as terrestrial clastic sediment, composed predominantly of silt-size
particles, which is formed essentially by the accumulation
of wind-blown dust (Pye 1995). The predominant component of the clay fraction of the Chinese loess was illite,
which may serve as an indicator for tracing the potential
source of loess materials. The clay content of the loess and
paleosols was largely controlled by the strength of the
winter monsoon, rather than by effects of in situ pedogenetic processes (Ji et al. 1999). For the study site, the mean
annual air temperature is 5.0 °C. Mean annual precipitation
is 400–450 mm, and 65–75 % of annual precipitation
falls from July to September. Annual evaporation is
1,330–1,640 mm and accumulated temperature (C0 °C) is
2,370–2,882 °C. The vegetation is typical steppe grassland
dominated by Stipa bungeana Trin., Thymus mongolicus
Ronn., Artemisia sacrorum Ledeb., Stipa grandis P.
Smirn., Artemisia frigida Willd. The soil type in the study
area was brown humus soil which was developed from the
loess parent material (Cheng et al. 2011).
Experimental details
Two community blocks (50 9 50 m) with different grazing exclusion times were selected. One was fenced completely excluding grazing from 2005 (GE5), and the other
fenced completely excluding grazing from 1995 (GE15).
They were dominated by Stipa bungeana, Artemisia sacrorum, Thymus mongolicus and other forbs. Fifteen randomly located sampling quadrats (1 9 1 m) in each of the
Grain size was measured with Mastersizer 2000 (Malvern
Instruments Ltd. UK), in the State Key Laboratory of Soil
Erosion and Dryland Farming on the Loess Plateau, the
measurement range is 0.02–2,000 lm. No treatment procedure was adopted, for the aim of this test was to obtain
the particle composition of undisturbed soil.
Soil samples were saturated to measure saturated
hydraulic conductivity (Ks) by the constant head method
(Klute and Dirksen 1986). The soil water retention properties for soil samples were determined using the centrifugation method (Silva and Azevedo 2002). Every sample
was first saturated for 24 h and then weighted to determine
the soil water content at saturation before submitting them
to water extractions by applying centrifuge methods.
A HITACHI CR21G centrifuge was used here, with the
temperature of 20 °C. Although during water desorption
measurement, changing bulk density has an obvious
influence on the soil water characteristic curve (Lv et al.
2004), the centrifuge method was considered as an appropriate method for determining soil water retention properties (Khanzode et al. 2002; Reatto et al. 2008).The bulk
density at every degree of suction was measured, and the
volumetric water content with the changed bulk density
was calculated in the experiment. All data were expressed
as mean of quadrat values.
Equations for SWCC and software
A centrifuge is used to measure the soil–water characteristics under various suction states. These curves are fitted
by the software named Retention Curve Program (RETC),
and obtained the parameters of SWCC. Numerous mathematical models have been suggested to describe the SWCC
(Brooks and Corey 1964; Haverkamp et al. 1977; Simmons
et al. 1979; van Genuchten 1980). In this study, no model
was determined before simulation, but to select the optical
model according to RETC. Generally, the closer the value
of the coefficient of determination, R2, was equal to 1, the
better the model fit the data.
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Environ Earth Sci (2014) 71:745–752
Most popular among these functions is the equation of
van Genuchten (1980):
m
h hr
1
¼
Se ¼
;
ð1Þ
1 þ ðahÞn
hs hr
where Se is the effective saturation (dimensionless) defined
by Se ¼ ðh hr Þ=ðhs hr Þ; h is volumetric water content
(cm3 cm-3); hs is the saturated volumetric water content
(cm3 cm-3); hr is the residual volumetric water content
(cm3 cm-3); h is the metric soil water potential (104 Pa); a
is a scaling parameter that determines the position of pore
size maximum; mand n are dimensionless curve-shape
parameters.
Besides van Genuchten equation, another popular
function for describing hðhÞ has been the equation of
Brooks and Corey (1964):
Se ¼
k
h hr
¼ ðahÞ
hs hr
1
ðah [ 1Þ
ðah 1Þ
ð2Þ
where a is an empirical parameter (L-1) whose inverse is
often referred to as the air-entry value or bubbling pressure,
and k is a pore size distribution parameter affecting the
slope of the retention function.
The equation of dual porosity (Durner 1994) is given as:
k
X
h hr
m
wi ½1 þ ðai hÞni i
¼
hs hr
i¼1
ð3Þ
where h is the metric soil water potential (104 Pa); k is the
number of ‘‘subsystems’’ from the total pore size distribution; a is a scaling parameter that determines the position
of pore size maximum; mand n are dimensionless curveshape parameters; wi are weighing factors for the subP
curves, subject to 0\wi\1, and
wi =1.
RETC is a computer program for analyzing the soil water
retention and hydraulic conductivity functions of unsaturated soils. It is developed by US Salinity Laboratory. The
program uses the parametric models of Brooks–Corey and
Table 1 Soil physical
properties of two grazing
exclusion treatments (5-year,
GE5; 15-years, GE15) and five
soil depths
Grassland
type
GE5
GE15
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Soil depth
(cm)
van Genuchten to represent the soil water retention curve,
and the theoretical pore size distribution models of Mualem
and Burdine to predict the unsaturated hydraulic conductivity function from observed soil water retention data (van
Genuchten et al. 1991).
Results and discussion
Soil physical properties
From Table 1, it could be seen that there is a great difference in saturated hydraulic conductivity among the ten
soil samples. The maximum value is 1.52 mm min-1 in
20–30 cm soil of GE15 grassland, and the minimum value
appeared in 40–50 cm soil of GE5 grassland, of only
0.45 mm min-1. In general, the saturated hydraulic conductivity of the soil samples of GE15 is greater than that of
samples of GE5. For GE5, the saturated hydraulic conductivity was smaller than that of GE15 at every layer;
especially at the depth of 20–40 cm, the Ks of GE15 was
much larger than that of GE5. These results indicated that
grazing exclusion management could improve the Ks of
grassland soil. As for field capacity, the maximum value is
32.54 % in 10–20 cm soil of GE15; 0–10 cm soil of GE15
takes the second place with 31.58 %. The lowest value is
24.85 % in 20–30 cm soil of GE5. Similarly, the field
capacity of GE15 was also larger than that of GE5, especially at the depth of 0–20 cm, which showed that for the
topsoil, excluded grazing for 15 years could improve the
soil water capability more than that of 5 years.
The soil bulk density is relatively low. The maximum is
1.30 g cm-3, appeared in 40–50 cm soil of GE15 grassland; the minimum value is 1.02 g cm-3, appeared in
10–20 soil of GE15 grassland. The bulk density of these
samples is lower than the mean value of tilth soil in the
Loess Plateau (*1.30 g cm-3). It is mainly due to the
Saturated hydraulic
conductivity
(mm min-1)
Field
capacity (%)
Porosity
(%)
Bulk density
(g cm-3)
Water
content (%)
0–10
0.72
26.80
54.97
1.19
18.93
10–20
0.47
27.02
54.57
1.20
18.49
20–30
0.57
24.85
53.03
1.24
15.64
30–40
0.52
25.65
52.83
1.25
17.92
40–50
0.45
25.03
52.42
1.26
16.93
0–10
0.76
31.58
58.69
1.09
23.34
10–20
0.96
32.54
61.59
1.02
11.04
20–30
1.52
26.66
54.45
1.21
16.45
30–40
1.07
26.47
55.43
1.18
14.69
40–50
0.57
26.29
51.04
1.30
17.00
Environ Earth Sci (2014) 71:745–752
Table 2 Characteristics of
particle composition of soil
samples of two grazing
exclusion treatments (5-year,
GE5; 15-years, GE15) and five
soil depths
749
Grassland type
Soil depth
(cm)
GE5
GE15
Md (lm)
Clay
(\2 lm)
Silt
(2–50 lm)
Sand
([50 lm)
24.67
0–10
28.502
6.42
68.91
10–20
25.898
11.54
66.94
21.52
20–30
28.977
6.86
71.79
21.35
30–40
21.816
14.20
68.88
16.92
40–50
21.774
14.32
68.62
17.06
0–10
24.969
10.82
66.24
22.94
10–20
23.959
13.54
66.72
19.72
20–30
23.841
13.74
66.86
19.40
30–40
21.772
14.12
69.46
16.42
40–50
20.348
14.71
69.92
15.37
Table 3 The relationship between models in RETC and SWCC of soil samples of two grazing exclusion treatments (5-year, GE5; 15-years,
GE15) and five soil depths
The models in RETC and their applicability
Grassland
type
GE5
GE15
Soil
depth
(cm)
0–10
V G, variable
m, n Mualem
V G, variable
m, n Burdine
V G,
m = 1-1/
n Mualem
V G,
m = 1-2/
n Burdine
Brooks
–Corey,
Mualem
Brooks
–Corey,
Burdine
Log-normal
distribution,
Mualem
Dual
porosity,
Mualem
9
9
9
H
9
9
9
H
10–20
9
9
9
H
9
9
9
H
20–30
9
9
9
9
9
9
9
H
30–40
9
9
9
H
H
H
9
H
40–50
9
9
9
H
9
9
9
H
0–10
9
9
9
H
9
9
9
H
10–20
9
9
9
9
9
9
9
H
20–30
9
9
9
H
9
9
9
H
30–40
H
9
9
H
9
9
9
H
40–50
9
9
9
H
9
9
9
H
loose soil structure of the grassland at the study site. For
GE15, the bulk density was smaller than that of GE5 at the
depth of 0–40 cm, which could be attributed to more
humus and less trample of livestocks. A similar result was
obtained for the burnt and unburnt savanna in West Africa,
where the highest bulk density (1.5 g cm-3) was found in
unburnt subplots with high grazing pressure (Savadogo
et al. 2007), which implied that enclosure lessened the bulk
density of the soil. Table 2 shows the particle composition
of the soil samples. The clay content at GE15 was clearly
higher than that of GE5, while the sand content of GE15 is
relatively low. It might also be due to the added humus. For
a kindred study in Horqin, the enclosure had higher
silt ? clay (\0.05 mm) and fine sand (0.1–0.05 mm)
contents and lower coarse sand contents at all depths than
at the continuous grazing site (Li et al. 2011). Enclosure
management could obviously improve the silt content in
grassland soil.
From Tables 1 and 2, it could be seen that the physical
properties of soil were improving after grazing exclosure.
The improvements can be mainly ascribed to vegetation
recovery and litter accumulation compared with a site
subjected to continuous grazing (Su et al. 2005).
Soil–water characteristics curves
By fitting the eight SWCCs with RETC 6.2, for every curve,
no less than one out of the eight models in RETC could fit it.
The best fitted one was selected as the SWCC model for
every sample. Table 3 shows this result, and Table 4 shows
the models and parameters fitted to the ten samples.
It is worth noting that although the van Genuchten
expression (m = 1-1/n Mualem) is widely used due to its
flexibility and simplicity, it is not widely used in the SWCC
of these samples. Meanwhile, dual porosity equation is
applicable for all the samples. In addition, log-normal
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Environ Earth Sci (2014) 71:745–752
Table 4 Models and parameters for soil samples of two grazing exclusion treatments (5-year, GE5; 15-years, GE15) and five soil depths
Grassland type
Soil depth (cm)
Model
hr
GE5
GE15
0–10
hs
a
N
van Genuchten, m = 1-2/n Burdine
0.1696
0.5511
0.0055
2.44
0.9981
10–20
Dual porosity, Mualem
0.1956
0.5574
0.0012
1.89
0.9973
20–30
Dual porosity, Mualem
0.1557
0.5439
0.0050
1.31
0.9963
30–40
van Genuchten, m = 1-2/n Burdine
0.1893
0.5227
0.0048
2.52
0.9966
40–50
Dual porosity, Mualem
0.2016
0.5550
0.0013
1.94
0.9971
0–10
van Genuchten, m = 1-2/n Burdine
0.2026
0.5408
0.0057
2.43
0.9944
10–20
20–30
Dual porosity, Mualem
van Genuchten, m = 1-2/n Burdine
0.1709
0.2036
0.5032
0.5207
0.0025
0.0087
1.32
2.43
0.9978
0.9984
30–40
Dual porosity, Mualem
0.2162
0.4904
0.0016
1.49
0.9968
40–50
Dual porosity, Mualem
0.2238
0.5513
0.0009
1.95
0.9939
distribution equation and van Genuchten (variable m, n Burdine; m = 1-1/n Mualem) equations are applicable for none
of the samples. The Brooks–Corey equation only fits one
sample, for it has been shown to produce fairly accurate
results for many coarse-textured soils characterized by relatively narrow pore- or particle size distributions (large
k-values). Results have generally been less accurate for finetextured and undisturbed, structured field soils, because of
the absence of a well-defined air-entry value for these soils.
Figure 3 shows the SWCCs to the experimental data,
obtained for the ten soil samples, using RETC. For both
treatments, in low-suction condition (B100 kPa), the water
holding capacity of topsoil (0–10 cm) was the best, bottom
soil (30–50 cm) took the second place, and that of subsoil
(10–30 cm) was the weakest. With the suction increasing,
the water holding capacity of topsoil decreased gradually;
when the suction reaches 600 kPa, the water holding
capacity of bottom soil was the strongest. In all soil water
suction, the water holding capacity of subsoil was the
weakest. From the fitting curves, the changing trend of F
(0–10 cm of GE15) and G (10–20 cm of GE15) are relatively stable; suggesting that in the range of effective
suction, the water capacity of grassland soil of GE15 is
better than that of the grassland soil of GE5, which is the
same as the result of field capacity.
That may be due to the porosity. As Table 2 shows, the
porosity of F and G are relatively large, are 58.69 and
61.59 %, respectively. However, the porosity of 15–5 is
only 51.04 %, which is the lowest value among the ten
samples. For undisturbed soil, if the porosity were large,
the water in big gap has been emptied in the low-suction
condition. And the fine pores have strong suction force, so
the larger soil suctions the less dehydrated water when the
same suction is increased. The slope of the curve in the desaturation zone tends to become flatter as the soil porosity
becomes smaller.
123
R2
Parameter
Therefore, at low suction values (B100 kPa), the water
capacity of soil with large porosity is stronger than that of
soil with small porosity; while in high-suction condition
(C100 kPa), the water capacity of soil with large porosity
is weaker than that of soil with small porosity. For the soil
water content of the same depth, at every suction degree,
GE15 grassland is higher than that of GE5 grassland, which
showed that grazing exclusion management significantly
increased soil water retention in studied grasslands. However, a study in the middle reaches of Kherlen River
revealed that there were no significant differences in total
porosity, storage pores for available water, and mesopores
between the grazing exclusion and grazed grasslands soils
(Hoshino et al. 2009). This difference might be due to the
particularity of the loess and the enclosed time.
Conclusions
In general, fencing improved the soil physical properties
for semiarid grassland. The saturated hydraulic conductivity and field capacity of the soil samples of GE15 were
greater than that of samples of GE5, while for GE15, the
bulk density was smaller than that of GE5 at the depth of
0–40 cm, which could be due to the adding humus.
For both treatments, in low-suction condition (B100 kPa),
the water holding capacity of topsoil (0–10 cm) was the best,
bottom soil (30–50 cm) took the second place, and that of
subsoil (10–30 cm) was the weakest. With the suction
increasing, the water holding capacity of topsoil decreased
gradually; when the suction reaches 600 kPa, the water
holding capacity of bottom soil was the strongest. In all soil
water suction, the water holding capacity of subsoil was the
weakest. That may be due to the porosity. Soils have large
particles and pores mean that although they hold large
amounts of water, they do not retain the water for long, as the
Environ Earth Sci (2014) 71:745–752
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Fig. 3 Soil–water characteristic curves of two grazing exclusion treatments (5-year, GE5; 15-years, GE15) and five soil depths by RETC
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water leaks through the pores. Clay soils are on the opposite
end of the spectrum. They have the smallest particles and
pores, which cause low porosity and high retention. Silt soils
also have high water retention, and loam soil has medium
retention. The van Genuchten expression was applicable for
most of the samples, except 20–30 cm soil of GE5 and
10–20 cm soil of GE15. Dual porosity equation was applicable for all the samples. However, log-normal distribution
equation and van Genuchten (Variable m, n Burdine;
m = 1-1/n Mualem) equation were applicable for none of
the ten soil samples.
Results suggest that the enclosure management
improved the soil structure and soil–water capability.
Further study is needed to determine the value of Ks and
parameters of SWCC of other enclosure managements,
such as the grassland which was enclosed for 10, 20 even
30 years to determine that if it is longer enclosed, the soil
physical properties would be better.
Acknowledgments We thank senior technician Shi ZhuYe for her
contribution to the laboratory analysis. This work was supported by
‘‘Strategic Priority Research Program—Climate Change: Carbon Budget
and Relevant Issues’’ of the Chinese Academy of Sciences (Grant No.
XDA05050403), ‘‘100-Talents Program’’ to Shi Zhi-Hua, and Northwest
A&F University (QN2011039), Funds from State Key Laboratory of Soil
Erosion and Dryland Farming on the Loess Plateau (10501-1215).
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DOI 10.1007/s12665-013-2476-y
ORIGINAL ARTICLE
Soil-hydrological properties response to grazing exclusion
in a steppe grassland of the Loess Plateau
Tie-Niu Wu • Gao-Lin Wu • Dong Wang
Zhi-Hua Shi
•
Received: 31 July 2012 / Accepted: 5 April 2013 / Published online: 23 April 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract Soil–water characteristics are necessary for
water quality monitoring, solute migration and plant
growth. Soil–water characteristic curve (SWCC) is a relationship between suction and water content or degree of
saturation. However, little information is available concerning the impacts of grazing exclusion management on
soil–water characteristics. Here, the soil–water characteristics of grasslands, which were excluded grazing for 5
(GE5) and 15 years (GE15), were studied. The saturated
hydraulic conductivity (Ks), SWCC, particle composition,
field capacity and some other indexes were determined.
Results showed that the clay content and Ks of grassland
soil were higher for GE15 than GE5. For both treatments,
in low suction condition (B100 kPa), the water holding
capacity of 0–10 cm soil was the best. Water holding
capacity of topsoil decreased gradually with the increasing
of suction, and it reached the strongest when the suction
reached 600 kPa. In all soil water suction, the water
holding capacity of subsoil was the weakest. The van
Genuchten expression was applicable for most of the
samples, except 20–30 cm of GE5 and 10–20 cm of GE15.
T.-N. Wu G.-L. Wu D. Wang Z.-H. Shi (&)
State Key Laboratory of Soil Erosion and Dryland Farming on
the Loess Plateau, Northwest A&F University, Yangling 712100,
Shaanxi, People’s Republic of China
e-mail: shizhihua70@gmail.com
T.-N. Wu
Urban Construction Departments of Shaoyang University,
Shaoyang 422000, Hunan, People’s Republic of China
G.-L. Wu (&) D. Wang
Institute of Soil and Water Conservation of Chinese Academy
of Science & Ministry of Water Resource, Yangling 712100,
Shaanxi, People’s Republic of China
e-mail: gaolinwu@gmail.com
Dual porosity equation was applicable for all the samples.
The soil–water capability and soil structure of which was
fenced for 15 years is superior to that of 5 years. This study
suggests that the enclosure management improved the soil
structure and soil–water capability.
Keywords Grazing exclusion Arid-grassland
Soil–water characteristic curve The Loess Plateau
Introduction
Most of grasslands in the earth are over-used and poorly
managed (Oldeman 1994). The overuse of grassland is
driven by the demand for forage production since significant portions of world milk (27 %) and beef (23 %) production occurs on grasslands managed solely for those
purposes (Sere et al. 1995). A well-known management
tool to restore the degraded grassland is the establishment
of enclosures, denoting areas closed off for agriculture and
grazing for a specific period of time. Hopkins and Wainwright (1989) established the extent of changes in botanical
composition since the early 1970s in enclosed grassland in
upland areas to relate the extent of changes to the present
agricultural management. Su et al. (2005) studied the soil
physical properties in Inner Mongolia and found that the
particle size distribution showed more silt and clay in the
top soil under the non-grazed sites compared with soils
under the continuously grazed site. Li et al. (2011) examined soil physical properties under continuous grazing and
exclusion of livestock for 8 years in Horqin sandy grassland, and they found that exclosure increased the fine sand
and silt ? clay contents, which were similar with the
results of Su et al. (2005). Verdoodt et al. (2009) assessed
the vegetation and soil rehabilitation in a 23-year
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746
chronosequence of two different enclosure management
types. The result showed that the communal enclosure
strategy proved to be successful in improving both rangeland vegetation and soil health. Shi et al. (2010) examined
the characteristics of vegetation and soil properties in the
livestock-excluded pastures and the adjacent grazed pastures under two topographic habitats and found that livestock exclusion increased soil bulk density, soil water
content, the water retention ability of the soil, and also
effectively protect soil from loss by water erosion.
The soil–water characteristic curve (SWCC) is a relationship between suction and water content or degree of
saturation. Recently, laboratory measurements of soil–
water characteristics have been used to study the effects of
bulk density changes on soil hydraulic properties (Lv et al.
2004), and the effect of soil compaction on hydraulic
properties of the loess (Zhang et al. 2006). As the measurement of soil–water characteristics is costly and difficult, Saxton et al. (1986) derived equations to estimate
continuous relationships of soil–water moisture content to
potentials and hydraulic conductivity from soil textures.
More recently, Fredlund and Xing (1994) proposed a new
equation that can be used to fit laboratory data over the
entire soil suction range.
SWCC vary widely and nonlinearly with water content
for different soil textures (Saxton et al. 1986). Ng and Pang
(2000) studied the soil water characteristics of a volcanic
soil in Hong Kong and found that the SWCC of a recompacted specimen was very different from that of a
natural sample having the same initial soil density and
initial water content. Agus et al. (2001) studied the SWCC
of Singapore residual soils. They found that the equations
proposed by Fredlund and Xing (1994) are a reasonably
good estimate of the SWCC for Singapore residual soils.
However, in the semiarid China, the widely distributed soil
is loess and black loam rather than residual soils or volcanic soils. The studies on black loam of enclosed grassland focus on soil carbon, soil microbiology, etc. (Contant
et al. 2001; Zaller 2006; Verdoodt et al. 2009). Nevertheless, very limited research has been conducted on the
SWCC of fenced grassland soil. Soil–water characteristics
are necessary for many soil–water related investigations
such as water conservation, irrigation scheduling, solute
migration and plant growth. These investigations commonly require soil–water characteristics for the computations of soil–water storage and soil–water flow. Thus,
further studies are needed to obtain a database for model
parameters and to extend the knowledge in this respect.
In this paper, the evaluation of changes in soil–water
characteristics (soil porosity, water content, field capacity,
saturated hydraulic conductivity, mechanical components,
and bulk density) of two grazing exclusion treatments (5and 15-year) was made to determine whether long-term
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Environ Earth Sci (2014) 71:745–752
exclusion of livestock grazing can significantly affect soil
water properties. The purpose of this study is to understand
the effects of long-term grazing exclusion on soil structure
and water storage function of typical steppe ecosystem in
the Loess Plateau.
Materials and methods
Study site
This experiment was conducted in typical steppe grassland
at 2,000 m a.s.l. in the Loess Plateau at the Guyuan Ecological Experimental Station of Chinese Academy of Sciences (36°130 –36°190 N, 106°240 –106°280 E) in Ningxia
Province, P. R. China, which was shown in Fig. 1. The
Chinese Loess Plateau lies in the Northwest of China, the
loess-paleosol sequences range between * 100 and 300 m
in thickness and record aridity in desertified source regions
in northwestern China and Mongolia during glacial periods, while intercalated paleosols reflect intensified summer
monsoon conditions that supported soil formation during
interglacial periods. The Chinese Loess Plateau includes
three geomorphological units: the Eastern Loess Plateau,
the Central Loess Plateau and the Western Loess Plateau.
The Western Loess Plateau lies to the west of Liupan
Mountains; the mean annual precipitation decreases from
ca. 600 mm/a in the southeast to ca. 200 mm/a in the
northwest, with modern vegetation ranging from forestgrassland, grassland and desert grassland, along the same
gradient. Loess deposits in this region typically are coarsegrained, and the paleosols are relatively weakly developed
(Chen et al. 1997).
The geological loess profile was shown in Fig. 2, which
was taken at Caijiapo, Shaanxi Province. On the Loess
Plateau, loess deposits consist of two major stratigraphic
units, namely loess and paleosol. Loess horizons are
Fig. 1 The location of the study site on the Loess Plateau
Environ Earth Sci (2014) 71:745–752
747
two community blocks were established. Samples were
taken in mid-July of 2010, when biomass had reached its
seasonal peak biomass in both years. Undisturbed soil
samples were taken from the center of each sampling
quadrat of grassland. The soil in each quadrat was removed
by layers (0–10, 10–20, 20–30, 30–40 and 40–50 cm) and
taken to the laboratory. Fifteen soil samples were repeated
in each block, yielding a total of 30 samples. These samples were carried to the laboratory for measuring soil
physical properties.
Measurement methods
Fig. 2 The typical geological loess profile of the Loess Plateau
labeled Li and paleosols Si. Paleosols are defined as those
units having the appearance and a degree of pedogenic
development similar to or greater than the Holocene soil in
the same area (Ding et al. 1993). In the Caijiapo section,
nine major paleosols can be distinguished from the loess
units; they were S1–S9. However, the top four paleosols
were not shown in Fig. 2. Loess can be defined as terrestrial clastic sediment, composed predominantly of silt-size
particles, which is formed essentially by the accumulation
of wind-blown dust (Pye 1995). The predominant component of the clay fraction of the Chinese loess was illite,
which may serve as an indicator for tracing the potential
source of loess materials. The clay content of the loess and
paleosols was largely controlled by the strength of the
winter monsoon, rather than by effects of in situ pedogenetic processes (Ji et al. 1999). For the study site, the mean
annual air temperature is 5.0 °C. Mean annual precipitation
is 400–450 mm, and 65–75 % of annual precipitation
falls from July to September. Annual evaporation is
1,330–1,640 mm and accumulated temperature (C0 °C) is
2,370–2,882 °C. The vegetation is typical steppe grassland
dominated by Stipa bungeana Trin., Thymus mongolicus
Ronn., Artemisia sacrorum Ledeb., Stipa grandis P.
Smirn., Artemisia frigida Willd. The soil type in the study
area was brown humus soil which was developed from the
loess parent material (Cheng et al. 2011).
Experimental details
Two community blocks (50 9 50 m) with different grazing exclusion times were selected. One was fenced completely excluding grazing from 2005 (GE5), and the other
fenced completely excluding grazing from 1995 (GE15).
They were dominated by Stipa bungeana, Artemisia sacrorum, Thymus mongolicus and other forbs. Fifteen randomly located sampling quadrats (1 9 1 m) in each of the
Grain size was measured with Mastersizer 2000 (Malvern
Instruments Ltd. UK), in the State Key Laboratory of Soil
Erosion and Dryland Farming on the Loess Plateau, the
measurement range is 0.02–2,000 lm. No treatment procedure was adopted, for the aim of this test was to obtain
the particle composition of undisturbed soil.
Soil samples were saturated to measure saturated
hydraulic conductivity (Ks) by the constant head method
(Klute and Dirksen 1986). The soil water retention properties for soil samples were determined using the centrifugation method (Silva and Azevedo 2002). Every sample
was first saturated for 24 h and then weighted to determine
the soil water content at saturation before submitting them
to water extractions by applying centrifuge methods.
A HITACHI CR21G centrifuge was used here, with the
temperature of 20 °C. Although during water desorption
measurement, changing bulk density has an obvious
influence on the soil water characteristic curve (Lv et al.
2004), the centrifuge method was considered as an appropriate method for determining soil water retention properties (Khanzode et al. 2002; Reatto et al. 2008).The bulk
density at every degree of suction was measured, and the
volumetric water content with the changed bulk density
was calculated in the experiment. All data were expressed
as mean of quadrat values.
Equations for SWCC and software
A centrifuge is used to measure the soil–water characteristics under various suction states. These curves are fitted
by the software named Retention Curve Program (RETC),
and obtained the parameters of SWCC. Numerous mathematical models have been suggested to describe the SWCC
(Brooks and Corey 1964; Haverkamp et al. 1977; Simmons
et al. 1979; van Genuchten 1980). In this study, no model
was determined before simulation, but to select the optical
model according to RETC. Generally, the closer the value
of the coefficient of determination, R2, was equal to 1, the
better the model fit the data.
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748
Environ Earth Sci (2014) 71:745–752
Most popular among these functions is the equation of
van Genuchten (1980):
m
h hr
1
¼
Se ¼
;
ð1Þ
1 þ ðahÞn
hs hr
where Se is the effective saturation (dimensionless) defined
by Se ¼ ðh hr Þ=ðhs hr Þ; h is volumetric water content
(cm3 cm-3); hs is the saturated volumetric water content
(cm3 cm-3); hr is the residual volumetric water content
(cm3 cm-3); h is the metric soil water potential (104 Pa); a
is a scaling parameter that determines the position of pore
size maximum; mand n are dimensionless curve-shape
parameters.
Besides van Genuchten equation, another popular
function for describing hðhÞ has been the equation of
Brooks and Corey (1964):
Se ¼
k
h hr
¼ ðahÞ
hs hr
1
ðah [ 1Þ
ðah 1Þ
ð2Þ
where a is an empirical parameter (L-1) whose inverse is
often referred to as the air-entry value or bubbling pressure,
and k is a pore size distribution parameter affecting the
slope of the retention function.
The equation of dual porosity (Durner 1994) is given as:
k
X
h hr
m
wi ½1 þ ðai hÞni i
¼
hs hr
i¼1
ð3Þ
where h is the metric soil water potential (104 Pa); k is the
number of ‘‘subsystems’’ from the total pore size distribution; a is a scaling parameter that determines the position
of pore size maximum; mand n are dimensionless curveshape parameters; wi are weighing factors for the subP
curves, subject to 0\wi\1, and
wi =1.
RETC is a computer program for analyzing the soil water
retention and hydraulic conductivity functions of unsaturated soils. It is developed by US Salinity Laboratory. The
program uses the parametric models of Brooks–Corey and
Table 1 Soil physical
properties of two grazing
exclusion treatments (5-year,
GE5; 15-years, GE15) and five
soil depths
Grassland
type
GE5
GE15
123
Soil depth
(cm)
van Genuchten to represent the soil water retention curve,
and the theoretical pore size distribution models of Mualem
and Burdine to predict the unsaturated hydraulic conductivity function from observed soil water retention data (van
Genuchten et al. 1991).
Results and discussion
Soil physical properties
From Table 1, it could be seen that there is a great difference in saturated hydraulic conductivity among the ten
soil samples. The maximum value is 1.52 mm min-1 in
20–30 cm soil of GE15 grassland, and the minimum value
appeared in 40–50 cm soil of GE5 grassland, of only
0.45 mm min-1. In general, the saturated hydraulic conductivity of the soil samples of GE15 is greater than that of
samples of GE5. For GE5, the saturated hydraulic conductivity was smaller than that of GE15 at every layer;
especially at the depth of 20–40 cm, the Ks of GE15 was
much larger than that of GE5. These results indicated that
grazing exclusion management could improve the Ks of
grassland soil. As for field capacity, the maximum value is
32.54 % in 10–20 cm soil of GE15; 0–10 cm soil of GE15
takes the second place with 31.58 %. The lowest value is
24.85 % in 20–30 cm soil of GE5. Similarly, the field
capacity of GE15 was also larger than that of GE5, especially at the depth of 0–20 cm, which showed that for the
topsoil, excluded grazing for 15 years could improve the
soil water capability more than that of 5 years.
The soil bulk density is relatively low. The maximum is
1.30 g cm-3, appeared in 40–50 cm soil of GE15 grassland; the minimum value is 1.02 g cm-3, appeared in
10–20 soil of GE15 grassland. The bulk density of these
samples is lower than the mean value of tilth soil in the
Loess Plateau (*1.30 g cm-3). It is mainly due to the
Saturated hydraulic
conductivity
(mm min-1)
Field
capacity (%)
Porosity
(%)
Bulk density
(g cm-3)
Water
content (%)
0–10
0.72
26.80
54.97
1.19
18.93
10–20
0.47
27.02
54.57
1.20
18.49
20–30
0.57
24.85
53.03
1.24
15.64
30–40
0.52
25.65
52.83
1.25
17.92
40–50
0.45
25.03
52.42
1.26
16.93
0–10
0.76
31.58
58.69
1.09
23.34
10–20
0.96
32.54
61.59
1.02
11.04
20–30
1.52
26.66
54.45
1.21
16.45
30–40
1.07
26.47
55.43
1.18
14.69
40–50
0.57
26.29
51.04
1.30
17.00
Environ Earth Sci (2014) 71:745–752
Table 2 Characteristics of
particle composition of soil
samples of two grazing
exclusion treatments (5-year,
GE5; 15-years, GE15) and five
soil depths
749
Grassland type
Soil depth
(cm)
GE5
GE15
Md (lm)
Clay
(\2 lm)
Silt
(2–50 lm)
Sand
([50 lm)
24.67
0–10
28.502
6.42
68.91
10–20
25.898
11.54
66.94
21.52
20–30
28.977
6.86
71.79
21.35
30–40
21.816
14.20
68.88
16.92
40–50
21.774
14.32
68.62
17.06
0–10
24.969
10.82
66.24
22.94
10–20
23.959
13.54
66.72
19.72
20–30
23.841
13.74
66.86
19.40
30–40
21.772
14.12
69.46
16.42
40–50
20.348
14.71
69.92
15.37
Table 3 The relationship between models in RETC and SWCC of soil samples of two grazing exclusion treatments (5-year, GE5; 15-years,
GE15) and five soil depths
The models in RETC and their applicability
Grassland
type
GE5
GE15
Soil
depth
(cm)
0–10
V G, variable
m, n Mualem
V G, variable
m, n Burdine
V G,
m = 1-1/
n Mualem
V G,
m = 1-2/
n Burdine
Brooks
–Corey,
Mualem
Brooks
–Corey,
Burdine
Log-normal
distribution,
Mualem
Dual
porosity,
Mualem
9
9
9
H
9
9
9
H
10–20
9
9
9
H
9
9
9
H
20–30
9
9
9
9
9
9
9
H
30–40
9
9
9
H
H
H
9
H
40–50
9
9
9
H
9
9
9
H
0–10
9
9
9
H
9
9
9
H
10–20
9
9
9
9
9
9
9
H
20–30
9
9
9
H
9
9
9
H
30–40
H
9
9
H
9
9
9
H
40–50
9
9
9
H
9
9
9
H
loose soil structure of the grassland at the study site. For
GE15, the bulk density was smaller than that of GE5 at the
depth of 0–40 cm, which could be attributed to more
humus and less trample of livestocks. A similar result was
obtained for the burnt and unburnt savanna in West Africa,
where the highest bulk density (1.5 g cm-3) was found in
unburnt subplots with high grazing pressure (Savadogo
et al. 2007), which implied that enclosure lessened the bulk
density of the soil. Table 2 shows the particle composition
of the soil samples. The clay content at GE15 was clearly
higher than that of GE5, while the sand content of GE15 is
relatively low. It might also be due to the added humus. For
a kindred study in Horqin, the enclosure had higher
silt ? clay (\0.05 mm) and fine sand (0.1–0.05 mm)
contents and lower coarse sand contents at all depths than
at the continuous grazing site (Li et al. 2011). Enclosure
management could obviously improve the silt content in
grassland soil.
From Tables 1 and 2, it could be seen that the physical
properties of soil were improving after grazing exclosure.
The improvements can be mainly ascribed to vegetation
recovery and litter accumulation compared with a site
subjected to continuous grazing (Su et al. 2005).
Soil–water characteristics curves
By fitting the eight SWCCs with RETC 6.2, for every curve,
no less than one out of the eight models in RETC could fit it.
The best fitted one was selected as the SWCC model for
every sample. Table 3 shows this result, and Table 4 shows
the models and parameters fitted to the ten samples.
It is worth noting that although the van Genuchten
expression (m = 1-1/n Mualem) is widely used due to its
flexibility and simplicity, it is not widely used in the SWCC
of these samples. Meanwhile, dual porosity equation is
applicable for all the samples. In addition, log-normal
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Environ Earth Sci (2014) 71:745–752
Table 4 Models and parameters for soil samples of two grazing exclusion treatments (5-year, GE5; 15-years, GE15) and five soil depths
Grassland type
Soil depth (cm)
Model
hr
GE5
GE15
0–10
hs
a
N
van Genuchten, m = 1-2/n Burdine
0.1696
0.5511
0.0055
2.44
0.9981
10–20
Dual porosity, Mualem
0.1956
0.5574
0.0012
1.89
0.9973
20–30
Dual porosity, Mualem
0.1557
0.5439
0.0050
1.31
0.9963
30–40
van Genuchten, m = 1-2/n Burdine
0.1893
0.5227
0.0048
2.52
0.9966
40–50
Dual porosity, Mualem
0.2016
0.5550
0.0013
1.94
0.9971
0–10
van Genuchten, m = 1-2/n Burdine
0.2026
0.5408
0.0057
2.43
0.9944
10–20
20–30
Dual porosity, Mualem
van Genuchten, m = 1-2/n Burdine
0.1709
0.2036
0.5032
0.5207
0.0025
0.0087
1.32
2.43
0.9978
0.9984
30–40
Dual porosity, Mualem
0.2162
0.4904
0.0016
1.49
0.9968
40–50
Dual porosity, Mualem
0.2238
0.5513
0.0009
1.95
0.9939
distribution equation and van Genuchten (variable m, n Burdine; m = 1-1/n Mualem) equations are applicable for none
of the samples. The Brooks–Corey equation only fits one
sample, for it has been shown to produce fairly accurate
results for many coarse-textured soils characterized by relatively narrow pore- or particle size distributions (large
k-values). Results have generally been less accurate for finetextured and undisturbed, structured field soils, because of
the absence of a well-defined air-entry value for these soils.
Figure 3 shows the SWCCs to the experimental data,
obtained for the ten soil samples, using RETC. For both
treatments, in low-suction condition (B100 kPa), the water
holding capacity of topsoil (0–10 cm) was the best, bottom
soil (30–50 cm) took the second place, and that of subsoil
(10–30 cm) was the weakest. With the suction increasing,
the water holding capacity of topsoil decreased gradually;
when the suction reaches 600 kPa, the water holding
capacity of bottom soil was the strongest. In all soil water
suction, the water holding capacity of subsoil was the
weakest. From the fitting curves, the changing trend of F
(0–10 cm of GE15) and G (10–20 cm of GE15) are relatively stable; suggesting that in the range of effective
suction, the water capacity of grassland soil of GE15 is
better than that of the grassland soil of GE5, which is the
same as the result of field capacity.
That may be due to the porosity. As Table 2 shows, the
porosity of F and G are relatively large, are 58.69 and
61.59 %, respectively. However, the porosity of 15–5 is
only 51.04 %, which is the lowest value among the ten
samples. For undisturbed soil, if the porosity were large,
the water in big gap has been emptied in the low-suction
condition. And the fine pores have strong suction force, so
the larger soil suctions the less dehydrated water when the
same suction is increased. The slope of the curve in the desaturation zone tends to become flatter as the soil porosity
becomes smaller.
123
R2
Parameter
Therefore, at low suction values (B100 kPa), the water
capacity of soil with large porosity is stronger than that of
soil with small porosity; while in high-suction condition
(C100 kPa), the water capacity of soil with large porosity
is weaker than that of soil with small porosity. For the soil
water content of the same depth, at every suction degree,
GE15 grassland is higher than that of GE5 grassland, which
showed that grazing exclusion management significantly
increased soil water retention in studied grasslands. However, a study in the middle reaches of Kherlen River
revealed that there were no significant differences in total
porosity, storage pores for available water, and mesopores
between the grazing exclusion and grazed grasslands soils
(Hoshino et al. 2009). This difference might be due to the
particularity of the loess and the enclosed time.
Conclusions
In general, fencing improved the soil physical properties
for semiarid grassland. The saturated hydraulic conductivity and field capacity of the soil samples of GE15 were
greater than that of samples of GE5, while for GE15, the
bulk density was smaller than that of GE5 at the depth of
0–40 cm, which could be due to the adding humus.
For both treatments, in low-suction condition (B100 kPa),
the water holding capacity of topsoil (0–10 cm) was the best,
bottom soil (30–50 cm) took the second place, and that of
subsoil (10–30 cm) was the weakest. With the suction
increasing, the water holding capacity of topsoil decreased
gradually; when the suction reaches 600 kPa, the water
holding capacity of bottom soil was the strongest. In all soil
water suction, the water holding capacity of subsoil was the
weakest. That may be due to the porosity. Soils have large
particles and pores mean that although they hold large
amounts of water, they do not retain the water for long, as the
Environ Earth Sci (2014) 71:745–752
751
Fig. 3 Soil–water characteristic curves of two grazing exclusion treatments (5-year, GE5; 15-years, GE15) and five soil depths by RETC
123
752
water leaks through the pores. Clay soils are on the opposite
end of the spectrum. They have the smallest particles and
pores, which cause low porosity and high retention. Silt soils
also have high water retention, and loam soil has medium
retention. The van Genuchten expression was applicable for
most of the samples, except 20–30 cm soil of GE5 and
10–20 cm soil of GE15. Dual porosity equation was applicable for all the samples. However, log-normal distribution
equation and van Genuchten (Variable m, n Burdine;
m = 1-1/n Mualem) equation were applicable for none of
the ten soil samples.
Results suggest that the enclosure management
improved the soil structure and soil–water capability.
Further study is needed to determine the value of Ks and
parameters of SWCC of other enclosure managements,
such as the grassland which was enclosed for 10, 20 even
30 years to determine that if it is longer enclosed, the soil
physical properties would be better.
Acknowledgments We thank senior technician Shi ZhuYe for her
contribution to the laboratory analysis. This work was supported by
‘‘Strategic Priority Research Program—Climate Change: Carbon Budget
and Relevant Issues’’ of the Chinese Academy of Sciences (Grant No.
XDA05050403), ‘‘100-Talents Program’’ to Shi Zhi-Hua, and Northwest
A&F University (QN2011039), Funds from State Key Laboratory of Soil
Erosion and Dryland Farming on the Loess Plateau (10501-1215).
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