| Loess plateau is a very important part of west development of China, and to restore its fragile ecological environment is the fundamental factor for keeping its sustainable development. As for eco-environment construction of Loess Plateau, the key point is to return farmland to forest and grassland. Since it is located at a semi-arid and arid climate region, water is main factor that refrains the vegetation recovery and eco-reconstruction. Therefore, it is very significant to study soil water eco-environment, which will be essential for eco-environment restoration.Based on the serious shortage of soil moisture, soil aridity and vegetation recession in this region, the following issues are studied in this dissertation: (1) the meaning of soil water and its role in the water circulation; (2) soil water deficit and its influence to vegetation growth; (3) the definition of soil dried layer and the causes of its formation; (4) soil dried layer distribution and characteristics in north of Shaanxi loess plateau; (5) the primary study of measurable index of soil dried layer.The main results are as follows based on field survey and analysis:1 Soil water is the water stored in the soil that can be absorbed by plants in some depth of soil, and the efficiency of soil water plays the critical role in plant growth and development. So soil water is the resource that is very crucial to vegetation. In addition, Rainfall is the only source of soil water in loess plateau that has the key function in the water circulation.2 The comparisons between soil water deficiency in natural and artificial forest land show that: (1) the concept of "forestland steady moisture capacity" is presented and defined as the lowest soil water content that can not affect the forest regular growth primarily and cause inverse succession in community. (2) There are significant differences of soil water contents between natural and artificial forestland. In general, soil moisture condition is better for the former. According to "forestland steady moisture capacity", water deficiency in the former is only in surface and recovers timely after rain season. But in artificial forestland, there is an apparent shortage especially in deep layer, so it affectsgrowth and development of vegetation. (3) In artificial forestland, soil dried layer has been formed because of water deficit. In 0-200cm light dried layer was formed and can be recovered partly after rainfall, but beneath 200cm the serious perpetual dried layer has lower water content and larger thickness and is very difficult to be recovered in short period because of the special hydrological characteristics in the Loess Plateau where infiltration depth is about 200cm and could not percolate into deeper layer. (4) The landscape ecology in Fuxian County shows that the two kinds of forest community have not declined. It points out that the rational and stable inner structure cannot lead to serious soil moisture shortage; its natural climate condition can meet the needs of forest community. It is the irrational management that causes the serious soil moisture shortage in artificial forestry land. If we intervene properly and create some growth condition similar to natural vegetation, the recessed artificial Robinniapseudoscacia can restore benignly.3 The influence of water on vegetation growth: (1) soil water shortage refrains the height, basal diameter and stock of trees. The artificial Robinnia pseudoscacia forest growth is becoming worse and worse as the soil moisture content decreases from south to north from Fuxian County. (2) Little precipitation and water shortage is the main reason of low efficiency and productivity forest in hill and gully area of Loess Plateau. (3) Soil dried layer is the dual result of forest growth and arid climate, that is high density and extensive irrational cultivation accelerate the soil aridity. The more the water shortage, the thicker the dried layer. The dried layer baffles the deeper soil water deficiency, and the vegetation can only live on ye...
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