Sciences in Cold and Arid Regions  2015, 7 (6): 675-686   PDF    

Article Information

ChaoFeng Fu, JingBo Zhao, FanMin Mei, TianJie Shao, Jun Zuo. 2015.
Vertical distribution of soil moisture and surface sandy soil wind erosion for different types of sand dune on the southeastern margin of the Mu Us Sandy Land, China
Sciences in Cold and Arid Regions, 7(6): 675-686
http://dx.doi.org/10.3724/SP.J.1226.2015.00675

Article History

Received: April 17, 2015
Accepted: July 9, 2015
Citation
ChaoFeng Fu, JingBo Zhao, FanMin Mei, TianJie Shao, Jun Zuo. Vertical distribution of soil moisture and surface sandy soil wind erosion for different types of sand dune on the southeastern margin of the Mu Us Sandy Land, China[J]. Sciences in Cold and Arid Regions , 2015, 7(6): 675-686.
Vertical distribution of soil moisture and surface sandy soil wind erosion for different types of sand dune on the southeastern margin of the Mu Us Sandy Land, China
ChaoFeng Fu1,2 , JingBo Zhao2, FanMin Mei3, TianJie Shao2, Jun Zuo1     
1. Key Laboratory of Western Mineral Resources and Geological Engineering, Ministry of Education of China & Chang'an University, Xi'an, Shaanxi 710054, China;
2. College of Tourism and Environment Science, Shaanxi Normal University, Xi'an, Shaanxi 710062, China;
3. Environmental and Chemical Engineering College, Xi'an Polytechnic University, Xi'an, Shaanxi 710048, China
Received: April 17, 2015; Accepted: July 9, 2015
Fund program: The research was funded by the National Natural Science Foundation of China (41140028, 41340043), and the Central University Research Foundation, Chang'an University (310827152014) and the State Key Laboratory of Loess and Quaternary Geology (SKLLQG).
Corresponding author: ChaoFeng Fu, Associate professor of School of the Earth Science and Resources, Chang'an University. No. 126, Yanta Road, Xi'an, Shaanxi 710054, China. E-mail: fucf@chd.edu.cn
Abstract: Soil moisture is a critical state affecting a variety of land surface and subsurface processes. We report investigation results of the factors controlling vertical variation of soil moisture and sand transport rate of three types of dunes on the southeastern margin of the Mu Us Sandy Land. Samples were taken from holes drilled to a depth of 4 m at different topographic sites on the dunes, and were analyzed for soil moisture, grain-size distribution and surface sediment discharge. The results show that: (1) The average soil moisture varies in different types of dunes, with the following sequences ordered from highest to lowest: in the shrubs-covered dunes and the trees-covered dunes the sequence is from inter-dunes lowland to windward slope to leeward slope. The average moisture in the bare-migratory sand dunes is sequenced from inter-dunes lowland to leeward slope to windward slope. (2) Vegetation form and surface coverage affect the range of soil moisture of different types of dunes in the same topographic position. The coefficient of variation of soil moisture for shrubs-covered dunes is higher than that of other types of dune. (3) The effect of shrubs on dune soil moisture is explained in terms of the greater ability of shrubs to trap fine-grained atmospheric dust and hold moisture. (4) The estimated sand transport rates over sand dunes with sparse shrubs are less than those over bare-migratory dunes or sand dunes with sparse trees, indicating that shrubs are more effective in inhibiting wind erosion in the sandy land area.
Key words: soil moisture     surface sandy soil wind erosion     vegetation form     micro-geomorphology     sand dunes     desertification control    

1 Introduction

In arid regions soil moisture is the main factor limiting vegetation growth and Sandy wind erosion(Han et al.,1996; Jia et al.,2002; Li et al.,2006; Robinson et al.,2008). In recent decades, there has been a considerable number of papers investigating the main factors governing the spatiaLand temporal variability of soil moisture content. These factors include:(a)soil properties(Beckett and Webster, 1971; Zhang and Berndtsson, 1988; Canton et al.,2004);(b)topography(Burt and Butcher, 1985; Bergkamp, 1998; Western et al., 1999; Gómez-Plaza et al.,2001; Sveditchnyi et al.,2003; Zhang et al.,2010; Gao et al.,2011);(c)vegetation(Yair and Danin, 1980; Hawley et al.,1983; Bromley et al.,1997; Seghieri et al.,1997; Rodriguez-Iturbe et al.,1999; Pan et al.,2008); and (d)the conditions of soil surface cover or Land use(Fu et al.,2000; Canton et al.,2004; Huang et al.,2009). The importance of topography, vegetation and soil properties has received particular attention in studies of water management and ecological-environmental protection in semi-arid areas(Loik et al.,2004; Reynolds et al.,2004; Ludwig et al.,2005). Although soil moisture exhibits a high degree of variability in both space and time, and is influenced by many factors such as weather, soil texture, vegetation and topography, an underst and ing of soil moisture balance and its variability is instrumental in quantifying the linkage between regional hydrology, ecology and geology(Petrone et al.,2004). There has been a considerable amount of research on the spatial heterogeneity of soil moisture and its determining factors. However, it is difficult to identify the relative importance of these factors because of their mutual interdependence and contradictory findings have been reported(Famiglietti et al.,1998; Western et al.,1999).

Improved underst and ing of moisture distribution and wind erosion process of Sandy soils in arid areas is of considerable practical value in the context of efficient management of water resourceSand for promoting the growth of vegetation to prevent desertification. However, very little is known about soil moisture distribution characteristicSand their controlling factorSand surface sediment discharge in desertSand especially in semi-arid climates, where long oreven medium-term meteorologicaLand hydrologicalrecords are lacking.

There has been a considerable amount of work reporting soil moisture in desert areas(Berndtsson et al.,1996; Yamanaka et al.,1998; BreshearSand Barnes, 1999; Feng and Cheng, 1999; Yamanaka and Yonetani, 1999; Guo et al.,2000; Wang et al.,2002; Lv et al.,2006; Wang et al.,2006; Wang et al.,2007) and in the Mu Us Sandy Land in particular(Guo et al.,2000; Wang et al.,2002; LV et al.,2006; Wang et al.,2006; Wang et al.,2007); however, much of this work is focused on shallow soil depths(less than about 1 m below the surface). Past studies suggested that soil moisture content in fixed sanddunes orsanddunes with dense vegetation coverwas low, but high in mobile or sparsely-vegetated dunes. However, work on the moisture content of Sandy soils at depths of 2 m or more below the surface is lacking. The relationships between soil moisture and the factors mentioned above need to be studied in a variety of localitieSand over a wide range of spatial scales(Qiu et al.,2001a, b). Meanwhile, we suggest that Sand -loess transition zone, specially, the margin of Sandy Land , is a key area for soil moisture study, because vertical variation and its influencing factors of soil moisture is closely associated with environmental improvement and desertification control in the region. Consequently, the present paper reports the results of a study on vertical distribution characteristics of soil moisture content at depths up to 4m below the surface and Sandy soil wind erosion in different types of dune on the southern margin of the Mu Us Sandy Land , China.

The specific aims of the research are:(1)to describe the soil moisture characteristics of selected dune systems in the Mu Us Sandy Land ;(2)to make comparisons of the soil moisture characteristics under different types of soil surface and vegetation coverage;(3)to improve underst and ing of the main factors controlling the moisture content of Sandy soils under the condition of non-vegetation period and non-rainy season in semi-arid environments, and (4)to simulate wind erosion sediment discharge of different types of dunes. Consequently, the Sandy soil moisture regimeSand amount of wind erosion of the most representative and different surface cover types of dunes in the Mu Us Sandy Land have been studied during a specific time period(when the dust storm frequently occurs every year) and theirrelationships with soil properties, topography and surface cover have been analyzed.

2 Study siteSand methods 2.1 Study area

The study area is located onthe southeastern border of the Mu Us Sandy Land , to the south lies the Loess Plateau, within the western part of Hengshan County in Shaanxi Province, China, and also is the Sandy loess transition zone and a typical semi-arid region(Figure 1). The Land scape is dominated by rolling sanddunes. Annual precipitation is 394.7 mm, concentrated in July to September which accounts for 80% of the annual precipitation. Annual average temperature is 7.8 °C, with a significant diurnal temperature range. Annual evaporation capacity is high: 2, 361.6 mm; and the annual sunshine amount is 2, 768.8 h. The prevailing wind direction is from the northwest and annual average wind speed is3.2 m/s, with a maximum of 24 m/s; the average wind index is 19.2 d. Winter and spring are the windiest seasons. A large number of desert plants are distributed on the fixed and semi-fixed sanddunes. The dominant species include Artemisia blepharolepis, Agriophyllumsquarrosum, Corispermumpuberulum, Psammochloavillosa, and Salix psammophylla. Certain grassLand species occur characteristically on fixed sanddunes, including Stipabreviflora, Cleistogenessquarrosa, Astragalusmelilotoides and Aster tataricus(Zhou and Zhang, 1999). There are a small number of planted trees, mainly Populus sp.. On the basis of vegetation form, vegetation coverage and Land use on the surface soil of the sanddunes, they can be divided into bare-migratory, shrubs-covered, and trees-covered dunes.

Figure 1 Location of the study area
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2.2 Methods

Field methods-- A sanddune area in northwestern Hengshan County was selected as the study area. Sampling was done in late April 2009. The sampling sites are located along the direction of sanddune distribution(from NW to SE), and samples were taken on the windward slope, leeward slope and from the lowLand areas between dunes(see Table 1). Bare-migratory dune height is about 20-30 m and lacks vegetation. Shrubs-covered dune height is about 20-30 m, with vegetation coverage area of 40%-50%. Trees-covered dune height is about 10-15 m, with vegetationcoverage area of about 20%-30%. The soil is eolian Sand , with a mean particle size of 0.25 mm or less. The location of the sampling sites is presented in Table 1. A manually-operated corer was used to take samples to a maximum depth of about 4m. The sampling interval is10cm and the number of samples per core is 40. The sample weight was typically 50-60 g.

Table 1 Location of the sampling sites of different sectionsof sanddun
Types of sanddunes Distribution direction Sampling points(Number) Elevation(m) Latitude Longitude
Bare-migratory dunes 325° Windward(HL1-a) 1, 122 38°01′07.5″ 109°11′56.0″
Leeward slope(HL1-b) 1, 123 38°01′07.1″ 109°11′56.8″
Inter-dune depression(HL1-w) 1, 114 38°01′06.2″ 109°11′58.5″
Shrubs-covered dunes 310° Windward(HL2-a) 1, 136 38°00′42.8″ 109°11′48.1″
Leeward slope(HL2-b) 1, 156 38°00′40.6″ 109°11′56.0″
Inter-dune depression(HL2-w) 1, 122 38°00′45.3″ 109°11′44.8″
Trees-covered dunes 315° Windward(HL3-a) 1, 100 38°02′33.8″ 109°14′26.3″
Leeward slope(HL3-b) 1, 113 38°02′32.1″ 109°14′26.8″
Inter-dune depression(HL3-w) 1, 103 38°02′34.5″ 109°14′29.4″
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Moisture content-- This was determined by weighing samples before and after drying. Each sample was weighed three times to a precision of 0.003 g and the results were averaged. In order to prevent water loss, sample wet weight was obtained in the field. Sample dry weight was determined after drying to a constant weight at 105 °C. Soil moisturewas calculated as:

W =(W1 -W2)/ W2×100%(1) (1)

where, W is soil moisture content, W1 is wet weight and W2 is the weight after drying.

Particle size analysis-- In order to determine the grain size distribution characteristics of Sand layers at different types of dunes from 0 to 4 m depth, 20 samples were selected from the windward side of bare-migratory, shrubs-covered, and trees-covered dunes, respectively, sum to 60 samples. About 1-2 g of soil was used in the analysis. Prior to measurement, carbonates were removed with dilute hydrochloric acid and hydrogen peroxide was used to remove organic matter. The samples were dispersed with sodium hexametaphosphate. The particle size of samples was measured using a laser particle size analyzer(Mastersizer, 2000). The statistic parameters of soil grain sizes were fitted by log-normal model(Gomes et al.,1990).

Estimated Sand transport rate --In order to underst and the effect of such surface features as soil moisture, grain size distribution, and vegetation on saltation, DPM(Dust Production Model)model was used to estimate saltation flux by inputing the stated parameters(Marticorena and Bergametti, 1995; Alfaro and Gomes, 2001). The equations are as follows:

$\frac{{{U_{*t}}}}{{{U_{*tr}}(z)}} = 1$,if$\eta (z) < \eta '(z)$ (2)

$\frac{{{U_{*t}}}}{{{U_{*tr}}(z)}} = {\left[ {1 + 2.21 \cdot (W(z) - W'(z) \cdot 0.68} \right]^{0.5}}$ if $\eta (z) > \eta '(z)$ (3)

$\eta '(z) = 0.0014 \cdot {C^2}(z) + 0.17 \cdot C(z)$ (4)

$\overline {{F_h}(z)} = K \cdot {\rho _a}/g \cdot U_*^3 \cdot {[1 + \frac{{{U_{*tr}}(z)}}{{{U_*}}}]^2} \cdot [1 - \frac{{{U_{*tr}}(z)}}{{{U_*}}}]$ (5)

$\overline {R(z)} = \frac{{{{[1 + \frac{{{U_{*t}}(z)}}{{{U_*}}}]}^2} \cdot [1 - \frac{{{U_{*tr}}(z)}}{{{U_*}}}]}}{{{{[1 + \frac{{{U_{*tr}}(10)}}{{{U_*}}}]}^2} \cdot [1 - \frac{{{U_{*tr}}(10)}}{{{U_*}}}]}}$ (8)

where, U* is wind friction velocity; U*tis wind friction threshold velocity of dry Sand ; U*tr(z)is wind friction threshold velocity of wet Sand layer z; η(z)is actual moisture content of wet Sand layer z; η'(z)is critical moisture content of wet Sand layer z;C(z)is clay content of wet Sand layer z; $\overline {{{\rm{F}}_h}(Z)} $is Sand transport rate of wet Sand layer z; K is proportionalitycoefficient; ρa is air mass density; g is gravitational acceleration; R(z)is the ratio of Sand transport rate of wet Sand layer z to that of surface Sand layer within 10 cm. Here, the difference of wet Sand and dry Sand is defined as if moisture content of Sand layer is greater than critical moisture content of Sand layer.

3 ResultSand discussion

The soil moisture content of Sandy ecosystems varies systematically with differences in the micro-Land scape of the dunes(Shapotou Desert Science Station, 1991; Lv et al.,2006; Wang et al.,2007). Specifically, the distribution of soil moisture is controlled by Sand movement on the windward sides of dunes; by the burial of Sand on the leeward side; by the pressure generated by fresh Sand accumulation; and by the depth of the water table in the inter-dune areas(Wang et al.,2002). Figure 2 shows the soil moisture profiles for 0-4 m for the various types of dune. Meanwhile, surface sediment discharge of different types of dune with the starting friction wind speeds on the windward sides of dunes were simulated in wind erosion procesSand results are discussed in section 3.6.

Figure 2 Soil moisture content of different types of sanddune in the Mu Us Sandy Land
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3.1 Vertical distribution of soil moisture content in relation to micro-geomorphology of the bare-migratory dunes

Figure 2a shows the soil moisture profiles(0-4 m depth)for the bare-migratory dunes. The soil moisture content values for the windward slope range from1.33%to 3.79%, with a meanof 2.37%; the values for the leeward sloperange from 0.39% to 5.04%, with a meanof 2.68%; and the values for the inter-dune lowLand range from 2.12% to 6.36%, with a mean of 3.22%. Overall, the average soil moisture content values are ordered from high to low as follows: inter-dune lowLand > leeward> windward. According to the changes ofmoisture content, soil moisture can be divided into three sections. The first section is from 0-100cm; here thesoil moisture content values of the windward, leeward slopeSand inter-dune lowLand are 2.32%, 1.67% and 2.67%, respectively. The second section is from 100-160cm; here the soil moisture content values of windward, leeward slopeSand inter-dune lowLand are 2.34%, 2.62% and 4.67%, respectively. The third section is from 160-400cm; the soil moisture content values of windward, leeward slopeSand inter-dune lowLand are 2.40%, 3.11% and 3.09% respectively. The average valu es ofmoisture content vary little on the windward slope. However, on the leeward slopethey vary significantly, from 1.67%-3.11%; the moisture content of the first section is the lowest, followed by the second section, and the third section is the highest. In the inter-dune lowLand sites, the moisture content also varies significantly, from 2.67% to 4.67%; the first section has the lowest values, the second section has the highest values, and the values in the third section are intermediate between the first two.

3.2 Vertical distribution of soil moisture content in relation to micro-geomorphology of the shrubs-covered dunes

Figure 2b shows the soil moisture content profiles(0-4 m depth)for the shrubs-covered dunes. The values for the windward slope range from 0.74%to 11.92%, with a meanof 4.87%; the values for the leeward slope range from 0.89%-12.08%, with a mean of4.40%; and the values for the inter-dune lowLand range from 1.34%to 9.24%, with a meanof 6.43%. Overall, the average soil moisture content values are ordered from high to low as follows: inter-dune lowLand > windward > leeward. According to the changes in moisture content, soil moisture can be divided into three sections. The first section is from 0-60 cm, and here the average soil moisture content values for the windward, leeward slopeSand inter-dune lowLand are 3.64%, 10.80% and 3.05%, respectively. The average values of soil moisture content are ordered as follows: leeward> windward > inter-duneslowLand . The second section is from 60-250cm; here the average soil moisture content values of windward, leeward slopeSand inter-dune lowLand are 7.83%, 4.75% and 6.17%, respectively. The average values of soil moisture content are ordered as follows: windward > inter-duneslowLand > leeward. The third section is from 250-400 cm depth; here the average soil moisture content values of windward, leeward slopeSand inter-duneslowLand are 1.62%, 1.40% and 8.10%, respectively. The average values of soil moisture content are ordered: inter-duneslowLand > windward > leeward.The soil moisture content on the windward slope varies significantly, from1.62% to 7.83%. The moisture content of the third section is the smallest, followed by the first section, and the second section is the largest. For the leeward slope, the soil moisture content varies significantly, with a range from1.40% to 10.80%. The moisture content of the first section is the largest, followed by the second section, and the third section is the smallest. At the inter-duneslowLand , the soil moisture content also varies significantly, ranging from 3.64% to 8.10%; the first section is the smallest, the third section is the largest, and the second section is intermediate.

3.3 Vertical distribution of soil moisture content in relation to micro-geomorphology of the trees-covered dunes

Figure 2c shows the soil moisturecontent values(0-4 m depth)for the trees-covered dunes. The values for the windward slope range from 1.51% to4.66%, with a meanof 2.78%; values for the leeward slope range from 0.74%to7.04%, with a mean of 2.58%; and the values for the inter-dune lowLand range from 0.97% to 9.46%, with a mean of 5.03%. Overall, the average soil moisture ranges from high to low as follows: inter-duneslowLand > windward > leeward. According to the changes in moisture content, the soil moisture can be divided into three sections. The first section is from 0-60 cm; here, the average soil moisture content values of windward, leeward slopeSand inter-dune lowLand are 2.45%, 1.73% and 2.77%, respectively. The average of soil moisture content can be ordered as follows: inter-duneslowLand > windward > leeward. The second section is from 60-200cm; here, the aver age soil moisture content values of windward, leeward slopeSand inter-dune lowLand sites are 2.51%, 3.01% and 4.69%, respectively. The average moisture content can be ordered as follows: inter-dune lowLand > leeward > windward. The third section is from 200-400 cm; the average soil moisture content values of windward, leeward slopeSand inter-dune lowLand are 3.07%, 2.54% and 5.95%, respectively. The average moisture content can be ordered as: inter-dune lowLand > windward > leeward. The soil moisture content on the windward slope varies relatively little, ranging from 2.45% to 3.07%. The moisture content of the first section is the smallest, followed by the second section, and the third section is the largest. For the leeward slope, the soil moisture content also varies little, ranging from 1.73% to 3.01%. The moisture content of the first section is the smallest, increases in the second section and is lower in the third section. For the inter-dune lowLand , the soil moisture content varies significantly, rangingfrom 2.77% to 5.95%; the first section is minimum, the third section is maximum, and the second section is intermediate between the two.

(a)HL1-b, HL1-a and HL1-w are the hole numbers on the leeward slope, windward slope and inter-duneslowLand of thebare-migratory dunes, respectively;(b)HL2-b, HL2-a and HL2-w are the hole numbers on the leeward slope, windward slope and inter-duneslowLand of the shrubs-covered dunes, respectively;(c)HL3-b, HL3-a and HL3-w are the hole numbers on the leeward slope, windward slope and inter-dune lowLand of the trees-covered dunes, respectively

In summary, soil moisture content is affected by Land scape position on the dunes. Past studies also found that the localtopography and vegetation cover were the main factors influencing soil moisturevariations(English et al.,2005; Hebrard et al.,2006; Pan et al.,2008). In the present work, for the range of 0-4 m depth, the moisture content of shrubs-covered and trees-covered dunes has the same trend of variation: inter-dune lowLand > windward > leeward; and for bare-migratory dunes, the average moisture content has the trend: inter-dune lowLand > leeward> windward. The differences are related to the distribution direction of the sanddunes. The sanddunes covered by vegetation are in general the fixed dunes, and the vegetation growsmore rapidly on the leeward slope than on the windward slopebecause of the greater insolation. Therefore water evaporation and plant transpiration on the leeward slope are greater than that on the windward slope in plant growing period. The slope on the windward side of the bare-migratory dunes is comparatively gentle, and Sand onthe slope was visibly eroded. In contrast, the leeward side of the bare-migratory dunes is comparatively steep, and eroded Sand was deposited here, and the effective area of sunlight on the windward side is larger than that on the leeward side. Therefore the evapotranspiration rate is much greater on the windward side than that on the leeward side. Furthermore, soil moisture content at different depths in different types of dune is different, with the range of variation of the moisture content at different depths of the shrubs-covered dunes being much greater than that of trees-covered and bare migratory sanddunes.

3.4 Characteristics of soil moisture variation in the same micro-geomorphological parts of the three types of dune

The data presented in Table 2 and Figure 2 show that the average soil moisture content on the windward slope of the dunes is ordered as follows: trees-covered dunes>shrubs-covered dunes> bare-migratorydunes; the coefficient of variation of soil moisture content of the shrubs-covered dunes is the largest, and soil moisture contentfluctuates significantly with depth. On the leeward slope, the average soil moisture content is ordered as follows: shrubs-covered dunes> bare-migratory dunes>trees-covered dunes, and the coefficient of variation of soil moisture content for the trees-covered duneSand bare-migratory dunesissmall. In the inter-dune lowLand , the average soil moisture content is ordered as follows: shrubs-covered dunes>trees-covered dunes>bare-migratory duneSand the coefficient of variation of soil moisture content for the trees-covered dunesis the largest, and soil moisture content fluctuates significantly with depth.

Table 2 Moisture content at different topographic locations on different types of sanddune
Types of sanddunes Windward slope Leeward slope Inter-duneslowLand
Mean value(%) St and ard deviation Variation coefficient(%) Mean value(%) St and ard deviation Variation coefficient(%) Mean value(%) St and ard deviation Variation coefficient(%)
Bare-migratory dunes 2.37 0.42 17.72 2.68 0.84 31.34 3.22 0.88 27.33
Shrubs-covered dunes 4.87 3.45 70.84 4.40 3.45 78.40 6.43 2.18 33.90
Trees-covered dunes 2.93 1.19 40.61 2.58 0.95 36.82 5.03 2.33 46.32
Note: The coefficient of variation is the ratio of st and ard deviation to mean value.
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The coefficient of variation of moisture content for different micro-geomorphological parts of the bare-migratorydunes is significantly lower than that of trees-covered and shrubs-covered dunes(Table 2). This demonstrates that the influence of vegetation is more significant compared with that of aspect and the micro-geomorphology of the dunes. The windward, leeward slopeSand inter-duneslowLand of shrubs-covered dunes are significantly larger than trees-covered dunes in terms of the mean, st and ard deviation and coefficient of variation of moisture content, which suggests that the influence of shrubs onthe moisture content of duneSand its vertical variationis greater than that of trees. As a result, cultivating suitable plants on sanddunes is potentially very important for conserving moisture in the Mu Us Sandy Land area.

3.5 Grain size distribution characteristicSand its influence on the soil moisture content of the sanddunes

The results of the grain size analysis are presented in Figure 3 and Table 3.

Table 3 Grain size distribution characteristics of different types of sanddune
Types of sanddunes Medium Sand class(0.5-0.25 mm) Fine Sand (0.25-0.125 mm) Very fine Sand (0.125-0.063 mm) Silt(0.063-0.0039 mm) Clay(<0.0039 mm)
Mean value(%) St and ard deviation Mean value(%) St and ard deviation Mean value(%) St and ard deviation Mean value(%) St and ard deviation Mean value(%) St and ard deviation
Bare-migratory dunes 0.000 0.00 5.148 2.33 62.080 2.72 31.900 3.36 0.868 0.44
Shrubs-covered dunes 0.000 0.00 4.169 4.37 29.900 16.22 61.500 17.49 4.435 3.23
Trees-covered dunes 0.000 0.00 14.885 4.91 57.068 5.10 27.090 9.34 0.953 0.44
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Figure 3 Grain-size distribution and cumulative grain size distribution curves for different types of sanddune.HL1-a, HL2-a and HL3-a are the hole numbers on the windward slope of thebare-migratory dunes, the shrubs-coveredduneSand the trees-covered dunes, respectively
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The water holding capacity of soil depends mainly on its texture, clay content and soil pore size distribution.The size of the field soil water holding capacity affects how well plants can absorb water. The clay content of Sandy soil directlyaffects field moisture capacity and plant wilting point moisture, and also affects the height of capillary rise and permeabilityconstant of Sandy soil(Feng and Cheng, 1999). The clay content of the soil is important for improving soil texture and increasing the capacity of Sandy soil to hold water.

From Figure 3 and Table 3 it can be seen that soil of the study region is an eolian Sand soiLand its mechanical composition consistsmainly of very fine Sand and silt, with a small proportion of clay. There is hardly any Sand above the medium Sand class. Thus, the particle size composition of the study regionis characteristic of the transition zone between the Loess Plateau and the Sandy Land region(Xiao et al.,2002b). The clay content of the soils of the shrubs-covered dunes is higher than that of the other types ofdunes.In the case of the windward site in the shrubs-covered dunes, for example, it can be seen that the variation trend of soil moisture content with depth is the same as that for the clay and silt content, but this is the opposite to the depth variation trend of very fine Sand content(Figure 4).In addition, the variation trend of soil moisture content is the opposite of the variation trend of median particle diameter and mean grain size. These studies further indicate that the content of clay and silt in the sanddunes is an important factor affecting soil moisture content.

Figure 4 Comparison of the vertical distribution of moisture content of the shrubs-covered duneSand various grain-size fractions
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Based on the reported dominant plant species growing in the Mu Us Sandy Land , the main period of vegetative growth is from early May to early September(Xiao et al.,2002a). Therefore, the effect of evapotranspiration by plants on the moisture content of the samples collected in the present study can be ignored, because the samples were collected in April, prior to the main period of vegetative growth. The effect of vegetation on the moisture content of the sanddunes is most likely due to the role of vegetation in intercepting and promoting the deposition of atmospheric dust and thereby increasing the silt and clay content of the vegetated areas of dunes. As a result, the moisture-holding capacity of the sanddunes will be increased, and this explains the greater moisture content of the shrub and trees-covered dunes compared to the bare-migratory dunes. According to the grain size distribution characteristics of the windward slope of the three types of dune(Table 3), the sediment origin is the same. All of the dune sediments are composed of grains in the range from 0 to 250<µm, with 80% consisting of fine Sand and very fine Sand (4-125µm diameter). This indicates that the eolianSand particles are homogeneous. According to the sedimentary facies characteristics of samples, the eolian Sandy soil is derived from the eluvium of the Mu Us Sandy Land , and that the three types of dune are derived from the same parent material. Furthermore, the main difference between the three types of dunes is in terms of the silt and clay content(Table 3). The silt and clay content of shrubs-covered dunes is up to 61.50% and 4.44%, respectively; for bare-migratory dunesit is 31.90% and 0.87%, and for trees-covered dunes it is 27.09% and 0.95%. In the case of shrubs-covered dunes, the silt content is up to 61.50% which is probably caused by the trapping effect of the vegetation, and the higher clay content is the result of stronger pedogenesis than in the other types of dune. In Table 3, it can also be seen that the silt content of the shrubs-covered dunes is higher than that of the trees-covered dunes, further indicating that shrubs trap fine particles more effectively than do sparse trees.Comparison of the elevation of the sampling sites indicates that the trees-covered dunesare 10-20 m lower than thebare-migratory and shrubs-covered dunes, and thus fine Sand and very fine Sand grains remain in the low elevation areas as the result of saltation transport, resulting in a higher fine Sand content compared to the other types of sanddune. The clay content is slightly higher than that of the bare-migratory duneSand this may be the result of the presence of herbaceous plantSand sparsepoplar trees on the trees-covered dunes.

The correlationcoefficients between moisture content and the percentages of four different grain-size classes for the three types of sanddune are listed in Table 4. It is clear that there is no significant correlation between soil moisture content and any of the four grain-size classes for the bare-migratory dunes; and this implies that for these dunes the grain size distribution characteristics are not the major factor determining the soil moisture content. In fact, in the case of these dunes the grain size is relatively uniform and exhibits little change with depth, and in addition the st and ard deviation of the content of each of the four grain size classes is lower than for the vegetation-covered dunes(Table 3). This is presumably because of the effect of continuous eolian reworking of these dunes.

Table 4 Correlation coefficients of moisture content and percentage composition of different particlesize classes on the windward slope of the different types of sanddune
Types of sanddunes Fine Sand (0.25-0.125 mm) Very fine Sand (0.125-0.063 mm) Silt(0.063-0.0039 mm) Clay(<0.0039 mm)
Bare-migratory dunes 0.192 0.044 −0.176 0.058
Shrubs-covered dunes −0.658 −0.810 0.752 0.884
Trees-covered dunes −0.542 −0.381 0.475 0.375
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The vertical variation of moisture content in the shrubs-covered dunesis significantly positively correlated with clay and silt contents(correlation coefficient>0.75, Table 4), and is significantly negatively correlated with fine Sand and very fine Sand contents. This indicates that clay and silt contents are important factors influencing the vertical variation of moisture content. In the case of trees-covered dunes there is some degree of correlation between the vertical variation of moisture content and the contents of clay, silt, very fine Sand and fine Sand in trees-covered dunes; however it is not very distinct and therefore this suggests that there are other more important factors influencing the moisture content of these dunes. A significant finding of this research is the important role of different types of vegetation coverage in influencing moisture content. In particular, the presence of shrubSand trees has been shown to impact vertical distribution of moisture content on the windward and leeward slopeSand in the inter-dune lowLand areas.

3.6 Theaeolian Sand transport rates over the three types of sanddunes estimated by DPM Model

Soil grain size, soil moisture and aerodynamic roughness length that depends on roughness elements' density, height, shape and erodible particles' size, affect wind threshold friction velocity, thereby, further affect Sand transport rate of dunes(Marticorena and Bergametti, 1995; Marticorena et al.,1997; Alfaro and Gomes, 2001). As supposed aerodynamic roughness length of the stated corresponding sanddunes is 0.003, 0.1 and 0.05 cm, respectively, the wind friction threshold velocity of these dunes estimated by DPM model corresponds to 0.36, 0.61 and 0.54 m/s, respectively. The wind friction velocity over bare-migratorydunes is lower than that of sanddunes with sparse shrub or trees due to lower soil moisture and roughness length. The Sand transport rates within 10 cm surface layers of the three types of sanddunes estimated by DPM model Sand soil under different wind friction velocities are presented in Figure 5, with the following order: bare-migratory dunes>trees-covered dunes> shrubs-covered dunes in the same wind speed condition. This demonstrates that planting shrubs is more effective in preventing soil wind erosion and improving the retention of soiLand water.

Figure 5 Sand transport rate of different types of duneswith alteration of wind speeds
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4 Conclusions

Study of the vertical distribution of moisture content and surface soil wind erosion of various types of sanddunesin the Mu Us Sandy Land yields the following conclusions:

The average soil moisture content at a depth interval of 0-4 m varies in different types of dunes, and in addition the variation of soil moisture content at different depths within each dune type is different. Moreover, the variation range of moisture content at different depths of shrubs-covered dunes is much greater than that of the trees-covered and bare-migratory sanddunes.

Vegetation type and vegetation coverage significantly affect the soil moisture content in the same topographic position of different types of dune. This is probably because of the greater ability of shrubs to trap aeoli and ustSand to improve Sandy soil quality.

The estimated Sand transport rates over sanddunes with sparse shrubs is less than that of bare sanddunes or sanddunes with sparse trees, the three types of sanddunes in terms of Sand grain size distribution, soil moisture and vegetation, indicating that shrubs are more effective for conservation of water and soil in the Sandy Land area. These findingsresult fromthe complicated interactions among sanddune types, soil moisture, Sand particle size distribution and vegetation.

Acknowledgments:

We thank Dr. Jan Bloemendal for discussion, revising and modifying the manuscript; and Zhang Yun, Dong Junchao for their field and experiment work assistance. The research was funded by the National Natural Science Foundation of China(41140028, 41340043), and the Central University Research Foundation, Chang'an University(310827152014) and the State Key Laboratory of LoesSand Quaternary Geology(SKLLQG).

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