绿洲边缘夹粘沙丘持水特性

夹粘沙丘是指剖面中分布有一层红褐色粘化层的沙丘,其夹粘层埋藏深度常在120—500cm左右,厚度在40—100cm之间,粘粉粒含量达到90%以上。夹粘沙丘人工梭梭固沙植被郁闭度在0.5以上,明显高于相同水热条件下的植被生产力,这可能与夹粘层通过改变土壤持水性及水分状况,从而提高了土壤水分承载力有关。以河西走廊绿洲边缘夹粘沙丘为研究对象,在测定0—8.0m剖面土壤物理性质和土壤水分特征曲线的基础上,计算了土壤孔隙分布和持水特性,研究了剖面中土壤持水性的变化规律及其影响因素。结果表明:(1)夹粘层土壤含水量最高,上覆沙土土壤含水量最少且随深度增加而增加,下伏沙土受地下水毛管作用影响水分含量较高;(2)土壤机械组成决定了不同土层的持水特性,土壤中粘粉粒含量越高,田间持水量和饱和含水量就越大,夹粘层的土壤持水性远高于沙土层;(3)夹粘层是沙丘土壤水分暂时贮存的重要土层,可以为植被提供深层土壤水分,这对绿洲边缘人工固沙植被的生存和繁衍有重要影响,而土壤持水性和毛管作用的强弱决定了夹粘层对沙丘水分调节能力的强弱。本研究为深入理解绿洲边缘雨养固沙植被斑块状分布的机理提供了一定的理论依据。

Water retention of the clay interlayer of dunes at the edge of an oasis

A sandwich dune''s profile consists of layers with different soil textures, the upper and lower of which are sandy soil layers, and there is a fine soil pan between the surface and the subsoil. The clay interlayer is composed of silty loam containing over 90% silt and clay. Deposits of this layer occur at depths of 120-500cm and it ranges in thickness from 40 to 100cm. In arid areas with precipitation of less than 200mm, artificial sand-fixing vegetation coverage is approximately 10% to 15%; however, the vegetation coverage of the study area with 117mm mean annual precipitation is approximately 65%, which does not meet the zonal water and heat conditions. Water retention is considered to play an important part in affecting the water conditions and soil water carrying capacity of dunes. With regards to water retention, Betti et al. have analyzed the effects of soil texture on water retention, revealing that when the sand content is increased, water retention becomes weaker, and the soil layer has a significant effect on the water-holding capacity, and their studies have focused on analysis of the water-holding capacity of homogeneous soil. In order to study the characteristics and mechanisms of the response of the clay interlayer to water retention, we measured the soil water characteristic curve based on the centrifuge test, particle size measured by Mastersizer-2000, and pore distribution based on the measured soil water characteristic curve and the calculated equivalent porosity at different depths of a sandwich dune. Our results show the following patterns. (1) The soil moisture content of the clay interlayer was the highest, soil water content increased slightly with depth in the surface soil, and the soil moisture of the subsoil was higher than in the surface, which was affected by the capillary action of groundwater. (2) Field capacity, wilting coefficient, saturated water content, total porosity, and aeration porosity were significantly correlated with soil physical properties, such as bulk density, sand content, clay content, and particle size. Sandy soil layers were at 0-4.0m and 4.6-8.0m. The soil water characteristic curves of these profiles were steep and the soil was loose (aeration porosity, 14%-27%) with poor soil water retention. The layer at 4.0-4.6m was an interbedded layer of silty loam, the soil water characteristic curves of which was smooth and the soil water content was high when the soil water suction was high.This type of soil had less aeration porosity, leading to higher soil water-holding capacity than the upper and lower sandy layers. Furthermore, correlation analysis showed that capillary porosity was not significantly correlated with soil physical properties, but was weakly related to soil water content, and increased significantly only at both the interface between the sandy layer and silty loam layer, and the water table. To a certain extent, the distribution of soil porosity affected the soil moisture content and water conveyance between the clay interlayer and the sand layer. In conclusion, soil textures lead to considerable differences in soil water retention of dunes at the edge of an oasis. The clay interlayer can hold more water compared with all profiles, which improves the water conditions of the dune. (3) The soil pan is the region where water collects, and provides water resources for plants. It plays an important role in the improvement of sand-fixing vegetation of dunes at the edge of an oasis. Further, the ability of the clay interlayer to regulate the water conditions of dunes depends on soil water retention and soil capillarity. In conclusion, our findings should provide strong support for future understanding of the mechanism of spatial patterns of artificial sand-fixing vegetation.