Characteristics of soil water repellency after sand dune stabilization in the Tengger Desert

HaoTian Yang,LiChao Liu,XinRong Li,YongPing Wei,YanHong Gao,XiaoJun Li,RongLiang Jia,Lei Huang

Shapotou Desert Research and Experiment Station,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

Characteristics of soil water repellency after sand dune stabilization in the Tengger Desert

HaoTian Yang*,LiChao Liu,XinRong Li,YongPing Wei,YanHong Gao,XiaoJun Li,RongLiang Jia,Lei Huang

Shapotou Desert Research and Experiment Station,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

Soil water repellency (SWR) is one of the most important physical properties of soils found all over the world,and it may have significant effects on the eco-hydrological processes of land ecosystems.In this study,the Capillary Rise Method was used to measure the SWR in the artificial vegetation area in Shapotou,located in the southeast area of the Tengger Desert,Ningxia Province of western China.The variation of the soil water repellency among different minor topographies,different depths and different particle sizes was analyzed.The results of the study indicate that the SWR shows distinct changes with vegetation restoration,and it increases with an increase in the period of dune stabilization.In the same vegetation area,the SWR of soils in inter-dune depressions or windward slopes is slightly greater than that in crest or leeward slopes.The SWR of 0–3 cm topsoil is significantly greater than that in the 3–6 cm soil layer.The SWR decreases with an increase in grain size and the differences among the SWRs of different sieved soil fractions are found to be significant.There is also a significantly positive correlation between the SWR and the proportion of soils with grain sizes of 0–0.05,0.05–0.01 and 0.01–0.15 mm,and a significantly negative correlation between the SWR and the propotion of soils with grain sizes exceeding 0.15 mm.The increase of SWR in revegetation areas may depend on the continuous depositing of atmospheric dust on the stabilized dune surface as well as the formation of biological soil crusts,especially on the formation of algal and lichen crusts.Enhanced SWR influences the effectiveness of water use of sand plants inhabiting the sand dunes.

soil water repellency;soil contact angle;Capillary Rise Method;vegetation restoration;southeast area of the Tengger Desert

1.Introduction

The term of soil water repellency (SWR) refers to the physical phenomenon (Carter and Hetherington,1994) that it is impossible or very difficult for water to moisten the surface of soil grains,i.e.,water droplets are not capable of quickly infiltrating into or spreading at the surface of soil with water repellency,and it is generally represented by soil contact angle (Carilloet al.,1999;Bachmannet al.,2000).SWR is the key factor influencing most surface and underground hydrological processes,and it has important implications for soil water movement.On the one hand,SWR is able to significantly prevent water infiltration (van Damet al.,1990;Hendrickxet al.,1993;Ritsemaet al.,1993),promote surface runoff and soil erosion (Witteret al.,1991;Terry and Shakesby,1993;Jungerius and Ten, 1994;Doerr and Thomas,2000) and carry chemical substances to pollute the groundwater (Hendrickxet al.,1993;Ritsemaet al.,1993) through preferential flow.On the other hand,water repellency causes the surface to become dry which may result in wind and water erosion and as well as land degradation,thus having a major impact on ecosystem patternsand processes (House,1991;York,1993;Biemeltet al.,2000).In addition,SWR may also affect the soil water use and growth of plants.Therefore,SWR has received much attention from scientists throughout the world.

The initial reports regarding soil water repellency mostly originated from coarsely textured sandy soils with small surface areas (Dekker,1990).Later studies have proven that medium texture soil (containing 20%–30% clay) and even clay itself show strong water repellency (McGhie and Posner,1980;Chan,1992;Dekker and Ritsema,1996).In recent years,observational evidence has shown that SWR is a global phenomenon,and it appears in regions with various climatic backgrounds,soil types and vegetation covers(DeBano,1981;Doerr and Thomas,2000;Jaramilloet al.,2000).At present,studies concerning SWR mainly involve habitat conditions such as forest vegetation,shrub vegetation,grassland,agricultural land,golf courses,sand dunes and pastures (DeBano,1981;Wessel,1988;Dekker and Ritsema,1994).Studies have shown that SWR is more likely to cause water repellency in arid and semi-arid areas,and persistent droughts in these areas often exacerbate this phenomenon.However,as of yet,very few studies concerning SWR in temperate desert soils have been performed.

Shapotou,a district of Zhongwei City in Ningxia Province,is located in the southeast end of the Tengger Desert.In the area a sand-binding vegetation protection system has been established by means of artificially planting drought-tolerant plants since the 1950s.With the development of sand-binding vegetation,the original mobile dune landscape has gradually evolved into a complex ecosystem covered by artificial sand-binding vegetation and naturally settled vegetation (Liet al.,2004).The physical and chemical properties of the surface soil have changed significantly due to the evolution of artificial vegetation,and have exerted a large impact on the surface eco-hydrological processes.Many studies have reported the influence of the vegetation construction in the area on hydrological processes such as dune soil infiltration,surface evaporation and condensation water formation,and water balance (Liet al.,2004;Liuet al.,2006;Liet al.,2007;Wanget al.,2007;Zhanget al.,2008);however,little is known about the soil hydrophobic characteristics and the eco-hydrology effect of fixed sand dunes in the area.Discussion regarding this issue is of great significance to maintaining and managing the stability of the artificial sand-binding vegetation systems here.

The objective of this study was to investigate the characteristics of soil water repellency of air dried samples taken in the artificial vegetation protection system in southeast fringe of the Tengger Desert,and to clarify the effect of soil particle size on water repellency.

2.Materials and methods

2.1.The study area

The experiment was conducted in the Shapotou Desert Research and Experiment Station,Chinese Academy of Sciences.The station is located in southeast fringe of the Tengger Desert (37°30′N,105°02′E) at an altitude of 1,234 m,western China.According to the records of local climate station,annual mean temperature is 10.4 °C,and the mean monthly air temperature in July and January are 24.9 °C and−6.3 °C respectively.Mean annual precipitation amount is about 184 mm with high annual and inter-annual variability,about 80% of the rainfall occurs from May through September.The annual potential evaporation is 2,800 mm.The number of days with dust events is about 122 days.The depth of underground water is more than 80 m and is unavailable by plant.The area is a typical transitional zone from desert steppe to steppified desert,and the transitional belt between desert and oasis (Liet al.,2004).Soil at the study site is loose,infertile and mobile,belonging to orthic sierozem or an aeolian sandy soil (Liet al.,2007).

In northwestern China,vegetation restoration is one of the common methods to stabilize sand dunes.To prevent the Baotou-Lanzhou Railway from sand encroachment in Shapotou area where the railway passes through sand dunes,the artificial vegetation protection system has been established since the 1950s.Liet al.(2004) particularly described the method used for establishing the protection system.The main technique is to plant drought-enduring shrubs such asArtemisia ordosicaKrash andCaragana korshinskiiKom following the stabilization of sand dunes with straw checkerboards.Now it has been formed a green corridor across the Tengger Desert with 16 km in length and more than 700 m in width.

2.2.Research methods

Experimental samples are taken from the non-irrigation artificial sand-binding vegetation area to the north of the Shapotou Railway.we selected a sample belt,which is 50 meters wide from east to west and 600 meters long from north to south,in the summer of 2009;the belt is composed of five sample plots,namely the artificial vegetation areas respectively established in 1956,1964,1981 and 1987 as well as the quicksand areas in sequence,each with an area of 50m×100m.Representative terrains were selected in each sample plot,including (A) inter-dune lowland,(B) windward slope,(C) crest and (D) leeward slope.Eight sampling points were randomly selected in each terrain,and soil samples were extracted from two layers,respectively 0–3 cm(upper) and 3–6 cm (lower),using a sampling frame with a size of 20cm×20cm,then the eight duplicate soil samples were uniformly mixed in order to reduce soil spatial heterogeneity.After natural drying,removal of litter and passage through a 0.5-mm soil sieve,the samples were divided into two parts,one part of which was used to directly determine the SWR and the composition of its grain diameters,and the remaining part was graded by being passed through soil sieves with sieve diameter of 0.05,0.10,0.15 and 0.25 mm,then used to determine the water repellency of soil sampleswith 4 grain sizes,namely 0 to 0.05,0.05 to 0.10,0.10 to 0.15 and 0.15 to 0.25 mm.Among the samples obtained those with grain sizes of 0.25 to 0.50 mm were quite rare,therefore the hydrophobic determination was not carried out for samples within this range of grain sizes.

The grain sizes of samples were determined via a laser grain size analyzer (MS-S,Britain Malvem Instruments Company).

The Capillary Rise Method (Sieboldet al.,1997;Juanet al.,2008) was used to determine the contact angle of surface soil in the artificial vegetation area and to represent the scale of the SWR,i.e.,SWR increases with an increase in soil contact angle.In order to minimize the rate of experimental error,caution was taken to place the soil samples in the glass columns as uniformly as possible,and sample mass (g) and filling height (cm) were used to control the filling density.During the experiment,the temperature and humidity of the laboratory were respectively controlled at 20 °C and 50%(Sieboldet al.,1997;Juanet al.,2008).After the commencement of measurements,the times when the liquid wetting peaks rose to a certain height in the cylindrical soils were recorded.

2.3.Data processing and analysis

The soil contact angleθ(°) is calculated on the basis of the Washburn Formula:

wherehrepresents the height (m) of the front of the rising liquid in the cylinder,rrepresents the effective radius (m) of the uniform pore,expressing the idealized pore system of granular material,γ1represents the surface tension of the liquid (J/m2),ηrepresents the viscosity of the liquid (Pa·s),andtrepresents time (s).These values are determined respectively with distilled water and octane.Octane is a type of entirely wetted liquid,i.e.,the contact angle is 0 degrees(Sieboldet al.,1997),and therefore in this experiment octane is selected as the reference liquid to obtain factorrin the formula.

SPSS16.0 software was used to conduct statistical analysis of the data,one-way ANOVA was used to determine the water repellency and significance of soil difference in vegetation areas of different ages,different terrains and different grain sizes,and the Tukey test was used for multiple comparisons.By taking the SWR as the dependent variable and the soil particle size as the independent variable,the relationship between the SWR and soil grain size was analyzed by means of linear regression.

3.Experimental results

3.1.SWR characteristics of sand-fixing vegetation and quicksand areas in different years

The SWR of the artificial vegetation area is greater than that of the quicksand area,and the SWR increases with the age of the vegetation (Figure 1).Statistical results show that the differences in SWR between earlier sand-fixing vegetation areas (vegetation areas established in 1956,1964 and 1981) and quicksand areas have all reached significant level(p＜0.01),while the differences in SWR between late vegetation areas and quicksand areas are not significant (p＞0.05);the main reason for this is the fact that the vegetation area established in 1987 is located near the quicksand area,and therefore it has suffered serious shifting sand erosion.

Figure 1 The variation of soil water repellency (Mean±SD) with different revegetation ages at depth of 0–3 and 3–6 cm

For different parts of fixed dunes,the SWR in inter-dune lowlands is greater than that in leeward slopes and crests on the whole.For the water repellency of different levels of soil,the results of the study show that the SWR in the upper level of the soil shows a downward trend following inter-dune lowlands,windward slope,leeward slope and crest;the SWR in the lower layer of soil shows a downward trend following inter-dune lowlands,windward slope,crest and leeward slope (Figure 2).

The SWR in different terrain parts of the artificial vegetation area is greater than that in the quicksand area (Figures 3a,b).The SWR in the upper level of the soil shows an upward trend with the increase of vegetation restoration years(Figure 3a).The SWR in the lower layer of the soil does not show an upward trend with the increase of vegetation restoration years,and the laws under various terrain conditions differ.The following specific results were found: in inter-dune depression,1964 ＞ 1987 ＞ 1956 ＞ 1981 ＞ quicksand area;in windward slope,1956 ＞ 1987 ＞ 1964 ＞ 1981 ＞quicksand area;in crest,1964 ＞ 1956 ＞ 1981 ＞ 1987 ＞quicksand area;in leeward slope,1981 ＞ 1956 ＞ 1964 ＞1987 ＞ quicksand area (Figure 3b).

Figure 3 The variation of soil water repellency (Mean±SD) with different revegetation ages at depth of 0–3 and 3–6 cm for four topographies:(A) inter-dune lowland,(B) windward slope,(C) crest and (D) leeward slope.

In the inter-dune depressions,the differences in SWR between the quicksand and vegetation areas respectively established in 1987 and 1981 are significant (p＜0.05);those between the quicksand and vegetation areas established in 1964 and 1956 are very significant (p＜0.01);and those among different vegetation areas are not significant.In the crest,the differences in SWR among quicksand and vegetation areas established in 1981,1964 and 1956 are significant (p＜0.01);those between quicksand and vegetation areas established in 1987 is not significant (p＞0.05);and those among vegetation areas established in 1987 and 1981,1964 and 1956 have reached a significant level (p＜0.05).In the leeward slope,the differences in SWR between quicksand and vegetation areas established in each year are very significant (p＜0.01),and the differences between the vegetation areas established in 1987 and 1956 are significant (p＜0.05).

3.2.Influence of grain size on SWR

The water repellency of soils with different grain sizes is shown in Figure 4.The water repellency of soils with different sieved soil fractions of the vast majority of the samples decreases with an increase in grain size (Figures 4a,b).The water repellency of the soil in the sieved soil fraction of 0–0.05 mm is significantly greater than that of the soils in the other sieved soil fractions.The water repellency of the soil in the sieved soil fraction of 0.05–0.10 mm is greater than those of 0.10–0.15 and 0.15–0.25 mm in 28 samples among a total of 32;the water repellency of the soil in the sieved soil fraction of 0.10–0.15 mm is greater than that of 0.15–0.25 mm in 26 samples among a total of 32.

Figure 4 The variation of soil water repellency (Mean±SD) of different particle size with different revegetation ages and topographies at depth of 0–3 and 3–6 cm: (A) inter-dune lowland,(B) windward slope,(C) crest and (D) leeward slope.

The water repellency of soils with different grain size ranges shows a decreasing trend with an increase in grain size (0–0.05,0.05–0.10,0.10–0.15 and 0.15–0.25 mm),and water repellency with various grain sizes in the lower layer of soil is lower than that in the upper layer (Figure 5).There is a very significant difference in water repellency of soilwith the various grain sizes (p＜0.01).

Figure 5 The variation of soil water repellency (Mean±SD) with different particle sizes at depth of 0–3 and 3–6 cm

The variations of water repellency of soils with different grain sizes in the vegetation areas established in different years vary quite widely.The differences in water repellency of soils with different grain sizes in the vegetation area established in 1956 are very significant (p＜0.01).In the vegetation area established in 1964,except for the differences in water repellency of the soils in the sieved soil fractions of 0.10–0.15 mm and 0.15–0.25 mm,the differences in water repellency of soils in the other sieved soil fractions all reached a significant level (p＜0.05).The differences between the water repellency of soils in the sieved soil fraction of 0–0.05 mm and the soil in the other sieved soil fractions in the vegetation area established in 1981 are all very significant (p＜0.01);the difference between the sieved soil fractions of 0.05–0.10 and 0.10–0.15 mm is significant (p＜0.05),and the difference between the sieved soil fractions of 0.05–0.10 and 0.15–0.25 mm is very significant (p＜0.01);the difference between the sieved soil fractions of 0.10–0.15 and 0.15–0.25 mm is not significant (p＞0.05).The differences between the sieved soil fractions of 0–0.05 mm and other sieved soil fractions in the vegetation area established in 1987 are all very significant (p＜0.01);the differences between other grain sizes are not significant.

The soil grain composition and distribution of soil samples are shown in Table 1,and the relationships between SWR and soil grain composition and distribution are shown in Table 2.The results show that there is a highly significantly positive liner correlation between the SWR and grain size proportions of ＜0.05,0.05–0.10 and 0.10–0.15 mm (p＜0.001),and the respective correlation coefficients are 0.818,0.911 and 0.676;there is also a highly significantly negative liner correlation between SWR and the grain sizeproportions of 0.15–0.25 and 0.25–0.50 mm (p＜0.00),and the respective correlation coefficients are −0.83 and −0.68.

Table 1 Distribution of particle size fractions in soils of different years since revegetation

Table 2 The relationship between soil water repellency and soil particle size distribution

4.Discussion

SWR is an important physical property of soil which is affected by soil organic matter,soil grain composition,vegetation distribution,soil moisture characteristics,soil microorganisms,as well as other factors (DeBano,2000;Doerret al.,2000).SWR is also one of the key factors in soil water movement,because it is capable of enhancing surface runoff and may lead to the uneven distribution of soil moisture.It is difficult to humidify hydrophobic soil,therefore hydrophobic soil affects seed germination,seedling settlement and plant growth;meanwhile,hydrophobic soil also makes the earth surface dry,which may lead to wind and water erosion and result in land degradation,these are not conducive to maintaining the sustainability of ecosystems.

The phenomenon of SWR is mainly caused by soil organic matter covering the surface of soil grains,and there is a significantly positive correlation between SWR and soil organic matter (Bond,1964;DeBano and Letey,1969;Roberts and Carbon,1971;Chen and Schnitzer,1978;Ma’shum and Farmer,1985;Ma’shumet al.,1988;Walliset al.,1990;Bisdomet al.,1993;Hudsonet al.,1994;Francoet al.,1995;Berglund and Persson,1996;Neinhuis and Barthlott,1997;McKissocket al.,1998;Francoet al.,2000).The results of this study show that SWR increases with the extension of sand-binding vegetation age (Figure 1).On the one hand,after the establishment of artificial vegetation in the quicksand area at the southeast edge of the Tengger Desert,a relatively stable biological and physical environment has been formed.Later,herbs gradually invaded and settled,and thus the perennial shrub-based artificial vegetation was gradually replaced by herb-based vegetation.Vegetation coverage increases year by year,and plant litter is also on the rise (Liet al.,2004,2009).Meanwhile,the establishment of artificial vegetation changes the underlying surface roughness,aids the dune surface in obtaining more dust fall,and provides favorable conditions for the settlement of cryptogamic plants.Therefore,artificial vegetation enables a large number of soil crusts composed of lichens,algae,mosses and other cryptogamic plants to colonize (Liet al.,2000);it also allows organic matter formed in the decomposition of litter from herbs and cryptogamic plants to continue to enter the soil,and organic matter to continue depositing (Fearnehoughet al.,1998).On the other hand,the establishment of artificial vegetation creates favorable conditions for the movement of insects and soil animals;for example,ants and sand lizards extensively thrive in the artificial vegetation area of Shapotou,and their excreta increases the organic matter content in the surface soil of the artificial vegetation area.These biotic and abiotic processes cause the soil organic matter in the artificial vegetation areas of Shapotou to increase with an extension in the sand-fixing period;also,the physical and chemical properties of surface soil constantly improve and the thickness of surface soil increases (Liet al.,2009),so as to enhance the level of SWR.In addition,there are a large number of microorganisms inhabiting the biological soil crusts,and their secondary metabolites are important factors in the increase of SWR in the artificial vegetation area.

There is no significant difference in the water repellency between the soil layers of 0–3 and 3–6 cm in the quicksand area (p＞0.05);however,the water repellency of the soil layer of 0–3 cm is significantly greater than that of 3–6 cm(p＜0.05).This is mainly due to the following reasons: the artificial vegetation in the Shapotou area was originally established in the shifting sand dunes;the influence of the establishment of artificial vegetation on soil properties is located mostly in shallow surface soil;the organic matter and sticky silt content in the surface soil is in a downward trend with soil depth;and the content in the soil layer of 0–3 cm is much greater than that of 3–6 cm (Liet al.,2009),thereby the SWR of the two soil layers is affected.We believes that the differences in water repellency of different soil layers may gradually decrease with the extension of the establishment period of artificial vegetation,and eventually the respective water repellencies will become closer to the same figure.

The results of this study show that SWR continues to decrease with an increase in grain size (Figures 4 and 5),supporting the study results of Bachmannet al.and Juanet al.(Bachmannet al.,2000;Juanet al.,2008).In addition,the correlation between SWR in the sand-binding vegetation areas of different ages and soils in different sieved soil fractions shows the following: the soil grain in the three sieved soil fractions (%) of 0–0.05,0.05–0.10 and 0.10–0.15 mm is capable of significantly increasing SWR,while the increase of soil grain in the two sieved soil fractions (%) of 0.15–0.25 and 0.25–0.50 mm will not increase SWR.The increase of SWR may be the result of the combined action of wettability,capillary phenomenon and increase of organic matter content with the decrease of soil grains.

SWR is capable of controlling the distribution of mois-ture and vegetation by affecting the processes of surface and groundwater hydrology,therefore further research regarding infiltration,evaporation,runoff,preferential flow,soil erosion and other processes influenced by SWR may be able to aid in explaining the hydrological,ecological and water balance processes in arid vegetation areas,and provide support for current theories and methods,thus improving the use of water and the ecological environment in arid areas.

5.Conclusion

More than 50 years after the artificial sand-fixing vegetation was established in Shapotou,an artificial vegetation system has gradually evolved into a natural system,the physical and chemical properties of the surface soil in this area have continued to change,the texture has been gradually fine grained,and organic matter content has gradually increased.At the same time,the SWR in the vegetation areas is significantly greater than that in the quicksand area.SWR shows an increasing trend with the expansion of establishment period of vegetation;the SWR of fixed inter-dune lowlands and windward slope in the vegetation areas is greater than that of tune top and leeward slope,and the water repellency of the soil layer of 0–3 cm is significantly greater than that of the soil layer of 3–6 cm.With an increase in soil grain,SWR continues to decrease.There is a significantly positive correlation between SWR and the percentage content of soil grain of less than 0.15 mm,and a significantly negative correlation between SWR and the percentage content of soil with grain sizes of 0.15–0.50 mm.The continuous colonization and development in the area,as well as the constant input of sand-fixing organic matter,are the most important causes of SWR here.

Acknowledgments:

This research was supported by the National Basic Research Program of China (Grant No.2009CB421303) and the National Natural Sciences Foundation (Grant Nos.40971031,40701002).

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February 20,2012 Accepted: June 17,2012

*Correspondence to: HaoTian Yang,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences.No.320,West Donggang Road,Lanzhou,Gansu 730000,China.Tel: +86-931-4967185;Email:yanghaotian6516@163.com