Spatial distribution characteristics of soil moisture in different types of sand dune in the Mu Us Sandy Land, adjacent to north of Chinese Loess Plateau

The distribution of soil moisture in arid and semiarid regions is a major environmental factor and is regulated by regional topography, vegetation and soil texture. Here, we present the results of a study of the spatial distribution characteristics of soil moisture in the Mu Us Sandy Land, which is the transitional area between the northwestern deserts and the Chinese Loess Plateau, in North China. Samples were taken from holes drilled to a depth of 4 m in 52 different microtopographic positions on different types of dune (bare dunes, shrub-covered dunes and tree-covered dunes). The sites were located in the northwestern margin, the central region and the southeastern margin. All samples were analyzed for moisture content and grain size distribution. The results show that: (1) for the same type of dune, the soil moisture content varies in different microtopographic positions. The soil moisture content on windward slopes is greater than on leeward slopes in the shrub-covered dunes and the tree-covered dunes, while this is the case in only some of the bare migratory sand dunes. In addition, the soil moisture content on leeward slopes is greater than on the corresponding windward slope. (2) The vegetation type and density have a large influence on the moisture content of sandy soils; specifically, the presence of shrubs and trees significantly affects the soil moisture content of windward and leeward slopes and the inter-dune lowland. (3) Soil moisture content is positively correlated with the clay and silt content of sandy soils. From northwest to southeast across the Mu Us Sandy Land, the silt and clay content increases gradually; however, in the case of dunes covered with planted trees, a peak in the content of fine-grained material occurs in the central region, while for shrub-covered sand dunes, the peak occurs in the southeastern margin. In addition, the correlation between soil moisture and soil grain size distribution of the three types of dunes varies from northwest to southeast. (4) The proportion of fine-grained material and the correlation between the content of fine-grained material and soil moisture are the two main factors influencing the soil moisture distribution of the different types of dune. A soil moisture concentration index can be used as a rough indicator of the distribution characteristics of soil moisture.