The Soil Moisture and Grain Size Characteristics of Micro-area：A Case Study of Minqin Oasis－Desert Ecotone

Ting GUO，Xianying XU，*，Peng ZHAO，Yu QIAO，Guiquan FU

1.Forestry College of Gansu Agricultural University，Lanzhou 730070，China；2.Gansu Desert Control Research Institute，Lanzhou 730070，China；3.Chinese Academy of Forestry，Beijing 100875，China

1 Introduction

Moisture is amajor restrictive factor influencingmany ecological processes in the desertecosystem［1］.Especially，in arid and semiarid areas，soilmoisture is an extremely essential factor restricting restoration， reconstruction and distribution of vegetation［2，3］.Through adjusting dynamic situation of climate，soiland vegetation system，soil moisture controls distribution pattern of vegetation［4－6］and exerts important influence on physical properties of soil and growth of vegetation［7］.However，in desert areas，it is difficult for groundwater to effectively supply soilmoisture in rhizosphere，and thus infiltrated natural rainfall becomes major source of effective soilmoisture in this area［8］.Low precipitation，cracked land in sand dunes，clay flat land，and crust ofmicroorganism in topsoilofartificial vegetation sand fixing area reduce the moisture infiltration and lead to water interception［9］.Numerous worm caves，cracksand rootholesbecome seepage passage of runoff water and some runoff water directly enters deep layer soil without entering the crust layer［10］.Runoff water resulted from high precipitation flows throughmicro-areas and infiltrates to supply soilmoisture，so the distribution and growth of vegetation in accumulated area（low-lying land）are significantly better than the runoff generation area.Besides，grain size as a quantitative indicator for classification of deposits and sedimentary rocks can reflecthydrodynamic feature of sedimentation，so it can be used asa basis for analyzing and comparing environment［11］.Themechanical composition of soil grain size is its natural property，coarse and fine feature of soil grain size is closely related to its physiochemical propertiesand biological properties，and grain size characteristics and mechanical composition can influence corrosion resistance，water retention ability and nutrients of soil，and further influence vegetation types and spatial pattern［12］.In this paper，we made comparative analysis on vegetation，soilmoisture and grain size characteristicsof different typesof sandmicro-areas in Minqin oasis-desertecotone，and elaboratedmoisture distribution laws and grain size characteristics of topsoil in Minqin oasis-desert ecotone，to provide scientific basis for vegetation restoration in Minqin sand micro-areas and provide theoretical support for ecological control and degraded vegetation restoration in Shiyang River valley.

2 Materials and methods

2.1 Overview of the study areaThe study area is situated near Minqin National Station for Desert Steppe Ecosystem Studies（Minqin NSDSES）（102°58′E，38°34′N）［1］.It belongs to desert-oasis ecotone and there iswide typicalmicro-areas such as inter-dune lowland，clay sand barrier sand fixing area，clay flat land and cracked land，etc.The climate in thisarea is temperate desert climate.Itsannualmean temperature is7.7℃，the lowest temperature is－30.8℃ （on Dec.27，1991），the highest temperature is 41℃ （on July 22，1999）；the frost-free period is172.6 days；the average annual precipitation is 115.9 mm（mainly in June to September）；the annual potential evaporation is 2452.7 mm；the averagewind velocity is2.4m／swith days of strongwind for 26.3 days，days of floating dust for 29.7 days，days of dusts and sands for 37.5 days and days of sandstorm up to 25.0 days；the groundwater level declines continuously from 2.21 m in 1961 to 18.4 m in 2001 at the rate of 0.71 m／a；the regional soil in this area is gray-brown desert soil and intrazonal soil includes three subtypes，namely，saline soil，meadow soil，and aeolian sandy soil.Natural vegetation ismanifested in the form of Tamarix ramosissima shrubs and Nitraria tangutorum sand dunes，so there is grand landscape of alternate distribution of shrubs and sand dunes.In inter-dune lowland，there aremany typesof vegetation，such as Kalidium foliatum，Limoniumau-reum，Eragrostis pilosa，Echinopilon divaricatum，Agriopyllum squarrosum，Salsola collina，Reaumuria soongorice，and Artemisia arenaria.

2.2 Study methodsIn July 2013，wemade a field survey in sandy areas near Minqin NSDSES where micro-areas are widely distributed.Wemade a record of typicalmicro-areas（fixed Nitraria tangutorum sand inter-dune lowland，clay flat land，clay sand barrier＋Haloxylon ammodendron forest）.In the runoff generation and accumulated area of each type ofmicro-areas，we set up sample plot（10m x 10m）.Using the soil profile andmultipoint samplingmethod，we collected samples of 0－10，10－30，30－50，50－70，and 70－100 cm soil for making relevant analysis of soil grain size，soilmoisture and physiochemical properties.Detailed information about sample plots is listed in Table 1.

Table 1 Basic information about sample plots of 3 types of typicalm icro-areas

2.3 Lab analysisWe adopted Malvern 2000 laser diffraction particle size analyzer tomeasure particle size of soil，used drying method（DHG-9055A Stainless Steel Air Blast Drying Oven）to measure soilmoisture，and made statistical analysis using Excel 2010 and SPSS 17.0.

3 Results and analyses

3.1 Characteristics of soilmoisture changes in accumulated area and runoff generation area ofm icro-areas

3.1.1Changes of soilmoisture in accumulated area and runoff generation area of the same typemicro-areas.From Fig.1，it can be known that soilmoisture in accumulated area and runoff generation area firstly increases，then drops，and finally becomes stable with increase of soil depth.Soilmoisture of corresponding layer is better in the former than the latter.The corresponding depth of lowestmoisture runoff generation area（20 cm）is shallower than accumulated area（35 cm）；there is less change in soilmoisture in 0－20 and 20－40 cm accumulated area compared with runoff generation area.On the whole，soilmoisture of accumulated area is better than runoff generation area in fixed Nitraria tangutorum sand inter-dune lowland.In the runoff generation area（biological crust3／4，physical crust1／4，biological crust4－6mm，physical crust2－4 mm），the soil crust development is better than accumulated area（physical crust2－3 mm）.Crust has certain interception and absorption function for natural rainfall.It stores rainfall in topsoil，so it is difficult to supply water for deep soil.In addition，high intensity rainfallwill produce surface runoff.Much accumulated water will permeate and thus supply soilmoisture in this area.Further，Nitraria tangutorum ismore in runoff generation area than accumulated area.The distribution of shrub root system in soil also influences level of soilmoisture.In all，in the action of various comprehensive factors，moisture in accumulated area is better than runoff generation area.

Fig.1 indicates that the soil moisture change is similar to that in fixed Nitraria tangutorum sand inter-dune lowland.The20 cm topsoil moisture is significantly higher in accumulated area than runoff generation area and the correspondingmoisture change is great.For soil layer below 20 cm，themoisture is better in runoff generation area than accumulated area.In the runoffgeneration area，there are herb plants such as Salsola collina，Eragrostis pilosa，and Echinopilon divaricatum（the vegetation coverage is 0.81%）and there is heavy biological crust（8－10 mm）.In accumulated area，there are Nitraria tangutorum，Eragrostis pilosa，Salsola collina，and Limoniumau-reum，and physical crust（2－4 mm）.Runoff generated from rainfall is gathered in accumulated area，permeates downward and makes 0－20 cm topsoilmoisture higher than runoff generation area.For accumulated area below 20 cm，soilmoisture is influenced from shrub（Nitraria tangutorum）root system and other factors，so themoisture of clay flat land isas shown in the figure.This is close to conclusions ofWang Fanglin et al［13］and Chang Zhaofeng et al［14］.

In clay sand barrier＋Haloxylon ammodendron forestsample plot（see Fig.1），the soilmoisture of0－25 cm topsoil in runoff generation area is better than accumulated area；for the soil below 20 cm，the accumulated area isbetter than runoffgeneration area.The lowest soilmoisture and depth in accumulated area is lower and shallower than the runoff generation area；for soil layer below 25 cm，the moisture change in runoff generation area is smaller than accumulated area.This is because there is Haloxylon ammodendron forest（canopy18.2%）in sample plot.Under the forest，there is well developed biological crust（4－6 mm），which increases resistance ofwater permeation and sand in deep layer fails to obtain water supply［15］.Root system of Haloxylon ammodendron mainly uses60－120 cm soilmoisture［14］，and itexerts high influence on moisture of deep layer soil in runoff generation area.In accumulated areawithout arbor and shrub，biological crust（5－9 mm）is well developed，root system of vegetation（coverage 3.20%）such as Halogeton glomeratus，Eragrostis pilosa，and Limoniumau-reum mainly influences 0－20 cm soilmoisture［14］but has little influence on deep layer soilmoisture.

3.1.2Changes of dynamic characteristics of soilmoisture in accumulated area and runoff generation area of different types ofmicro-areas.Through comparative analysis of fixed Nitraria tangutorum sand inter-dune lowland，clay flat land，clay sand barrier＋Haloxylon ammodendron forest，we know that the soilmoisture of fixed Nitraria tangutorum sand inter-dune lowland and clay flat land is basically consistent and is 1.5 times the clay sand barrier＋Haloxylon ammodendron forest.Themoisture of three types has declining trend：clay flat land＞clay sand barrier＋Haloxylon ammodendron forest＞fixed Nitraria tangutorum sand inter-dune lowland.In inland arid and semi-arid areas，solar radiation is strong and sunshine time is long，topsoilmoisture of sandy land is low.In the beginning of survey，little rainfall brought bettermoisture of0－10 cm topsoil.At20－40 cm soil layer，themoisture of soil in accumulated area is the lowest for all three types ofmicroareas.The moisture of clay flat land and clay sand barrier＋Haloxylon ammodendron forest is the lowest，basically consistent，and obviously lower than the fixed Nitraria tangutorum sand interdune lowland.The lowestmoisture value appearance depth of the first two types is shallower than the latter.Besides，change range ofmoisture of fixed Nitraria tangutorum sand inter-dune lowland and clay flat land is smaller than clay sand barrier＋Haloxylon ammodendron forest，indicating that restoration of20－40 cm soil moisture in clay sand barrier＋Haloxylon ammodendron forest micro-areas is better than other two types ofmicro-areas.In 40－70 cm soil，moisture of all three types is increasing and the change range is fixed Nitraria tangutorum sand inter-dune lowland＞clay flat land＞clay sand barrier＋Haloxylon ammodendron forest.When soil layer is above 70 cm，soil moisture change of three types is gentle and relatively stable.

From Fig.2，in 0－15 cm topsoil，moisture is basically consistent in fixed Nitraria tangutorum sand inter-dune lowland and clay sand barrier＋Haloxylon ammodendron forest，and is about 2 times higher than the clay flat land.In 15－20 cm soil layer，the lowest soilmoisture appears in fixed Nitraria tangutorum sand inter-dune lowland and clay flat land and the former is greater than the latter；in about25 cm soil layer，the soilmoisture of clay sand barrier＋Haloxylon ammodendron forest is the lowest and the soil moisture is as follows：clay flat land＞clay sand barrier＋Haloxylon ammodendron forest＞fixed Nitraria tangutorum sand inter-dune lowland.In 25－40 cm soil layer，soil moisture of three types is increasing and the level sequence is as follows：fixed Nitraria tangutorum sand inter-dune lowland＞clay flat land＞clay sand barrier＋Haloxylon ammodendron forest，and themoisture is as follows：clay flat land＞fixed Nitraria tangutorum sand inter-dune lowland＞clay sand barrier＋Haloxylon ammodendron forest.In 40－100 cm soil layer，change of soilmoisture is generally gentle，and the soil moisture of clay sand barrier＋Haloxylon ammodendron forest is significantly lower than fixed Nitraria tangutorum sand inter-dune lowland and clay flat land.The clay sand barrier＋Haloxylon ammodendron forest sample plot is situated in the 1960s Haloxylon ammodendron forest，while the Haloxylon ammodendron forest root system mainly uses 60－120 cm soil moisture［14］，which well explains the fact that the soil moisture of clay sand barrier＋Haloxylon ammodendron forest in 40-100 cm soil layer of runoff generation area ismuch lower than that of other twomicro-areas.

3.2 Distribution characteristics of soil grain size in different types ofm icro-areasFrom analysis on soil grain composition of three types ofmicro-areas（as listed in Table2），we know that for fixed Nitraria tangutorum sand inter-dune lowland：fine sand in accumulated area（46.02% －68.47%）and runoff generation area（topsoil17.69%，other layers57.10% －79.12%）takesup a larger portion in grain size classification.The sum of coarse and fine sand grains takes up 53.4－96.0%in each layer of both areas（except in 0－10cm of runoff generation area，the content of coarse and fine sand grain is20.7%），which is basically consistentwith researchesofother scholars in inter-dune lowland.In 0－10 cm topsoil accumulated area，the content of clay and powder grains（1.42%and 3.83%）ismuch lower than runoffgeneration area（17.90%and 61.38%）；for 10－100 cm soil layer，the situation is opposite.The development of surface crust（biological crust 3／4，physical crust 1／4，biological crust 4－6 mm，and physical crust2－4 mm）in runoff generation area is better than accumulated area（physical crust 2－3 mm）.In the study area，plant growing in sand dunes blocks foreign dust and withered things and forms crust in the action ofmicroorganism decomposition［16］.Formation and existence of biological crust greatly increase content of clay and powder grains in the runoff generation area.In 10－50 cm soil layer accumulated area，the content of clay and powder grains is obviously better than runoff generation area，so leaching only occurs in shallow layer of soil（0－40 cm）［17］.Short and heavy rainfall in deserts can generate runoff，gather and permeate downward，thus powder and clay grains in topsoil of accumulated area will be leached to deeper layer soil.

Table 2 Characteristics of soil grain size in different samp le p lots

Soil composition in clay flat land of accumulated area and runoff generation areamainly consists of coarse sand（35.83－42.30%and 24.03－43.62%），fine sand（50.62－59.85%and 48.65－64.93%），clay（1.00－2.93%and 1.42－2.94%），and powder（1.93－8.11%and 3.74－8.10%）.In vertical direction，variation of changes in different levels of grain size is significant（P＜0.05）.In each corresponding layer，the clay grain content is better in runoff generation than accumulated area.Related researches show that if one-time rainfall exceeds 5 mm，it will generate surface runoff［18］.Since sample plots in accumulated area and runoff generation area are low（the accumulated area is 0.8 m lower than runoff generation area），the development of crust in runoff generation area is better than accumulated area（the runoff generation area is 8－10 mm thick biological crust；the accumulated area is2－4mm physical crust），biological crust absorbsand blocksupstream runoff to reach the accumulated area.Such hydrological process brings fine soil to low-lying land，and increases clay particle of soil to a great extent.More important，such soil and hydrological process greatly promotes growth and development ofmicroorganisms in crust.Thus，this is possibly the most essential factor for increase of clay particle content［16］.With wash of natural rainfall，fine soil in clay sand barrier isbrought to runoff generation area butnot reaches the accumulated area.In 10－30 cm soil layer，clay and powder particle content in accumulated area is better than runoff generation area，which is connected with permeation and leaching of accumulated water in themicroarea，and also related with Haloxylon ammodendron forest blocking dust in air.Various factors determine grain size distribution of accumulated area and runoff generation area in this type ofmicro-area.

4 Conclusions and discussions

（i）In sandmicro-area，rainfall heavier than certain intensity will generate runoff and the runoff will flow to accumulated area and supply partof soilmoisture.In three types ofmicro-areas，vegetation growth（canopy 2.88－6.20%）is better in accumulated area than in runoff generation area（canopy 0.65－1.64%）.This indirectly reflectswater catch function of soilmoisture inmicro-areas.（ii）Variation of soilmoisture in accumulated area and runoff generation area of three types ofmicro-areas reflects water household.The level of soilmoisture influences distribution of shallow root and deep root plants and crust，while moisture is also influenced by other factors，such as rainfall，vegetation，crust，and terrain，so the situation is relatively complex.（iii）Topsoilmoisture of three types ofmicro-areas rises due to little rainfall in the early period.As a result，soilmoisture in accumulated area and runoff generation area takesonan S form.In 20－40 cm soil layer，the lowest soilmoisture of fixed Nitraria tangutorum sand interdune lowland is significantly higher than that in clay flat land and clay sand barrier＋Haloxylon ammodendron forest in accumulated area of three types ofmicro-areas.In runoff generation area，the lowest soilmoisture level（0.18% －0.41%）and depth are basically consistent.The depth of lowest soil moisture in three types ofmicro-areas is closely related with vegetation growth in accumulated area and runoff generation area，and also connected with specific condition of little rainfall in sand area，and interception and absorption function of dry sand and soil crust reducessupply of rainfall for deep soil［19］.（iv）Soil grain size in three types ofmicro-areas is influenced by rainfall，vegetation，crust，original soil texture factors，the influence process is relatively complex and to be further studied.Besides，it is necessary to study how to promote sand under crust to change to soil and the lawsofwater cycle inmicro-areas.（v）At present，the understanding of ecological and hydrological process is limited［20］，and there are few foreign reports aboutmoisture in sand micro-areas.Through analysis on distribution laws of soilmoisture and characteristics of grain size in 3 types ofmicro-areas，it is hoped to fill the gap of domestic researches in this area and provide scientific basis and theoretic support for vegetation restoration and comprehensive ecological control，aswellas restoration of degraded vegetation inmicro-areasof northern desert in China.

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