| Abstract:Aim at serious phenomenon of dried soil layer and concern about vegetation construction in the Loess Plateau, a series of questions, such as assessing standards, forming reasons, regional distribution, formation mechanism, renew condition of dried soil layer, had been discussed, by measuring soil moisture content, field investigation and experimental analysis in northern Shaanxi, which located in the middle part of the Loess Plateau, expecting to serve decision basis of vegetation construction in the Loess Plateau,especially for northern Shaanxi.The main achievements and cognitions concluded as follows:1.Dried soil layers in Yulin are stronger than in Yan’an, and spatial differentiation show sudden changes when precipitation are about550mm/year or about400mm/year. The details are as follow:in wild land, there are dried soil layers in Dingbian county(precipitation is329mm/year), there are no dried soil layer in Jingbian county(precipitation reaches400mm/year).in apple orchard about20year, there are strongest dried soil layers in Yanchuan county(precipitation is500mm/year) wherever there are weakest dried soil layers in Ganquan county(precipitation reaches550mm/year).2. The dynamic process of soil moisture in Luochuan county can be separated into notable decreasing stage of soil moisture in spring, notable increasing stage of soil moisture in summer, stable decreasing stage of soil moisture in autumn and stable increasing stage of soil moisture in winter. Mainly caused by higher temperature, higher evaporation and lower precipitation in spring, it was notable decreasing in spring. Soil moisture decreased notably not only in the depth of2meters but also under the depth of2meters. The results show spring is the main season, in which man-made plantation leads to dried soil layer.3. It was shown that the soil moisture content of artificial forest increased greatly after2003, the rainy year. The soil moisture content of dried layer in17-aged arboreal apple trees,17-aged dwarf apple trees and8-aged arboreal apples recovered respectively reached4meter,5meter and6meter, which was decided by the depth and strength of the dried soil layer. The precipitation in uttermost rainy year can enhance the soil moisture content in dried layer to field moisture capacity, which last for2or3years. The shortdated equalize can’t uproot even lighten the harm of dried layer.4.On measuring soil water unsaturated flow rate and pellicular water move rate of soil near Xi’an, it shows that soil moisture under2meters can be recharged if soil is most or more wet in gravity water zone in the depth of2meters, there would be no dried soil layer, so dried soil layers of the Loess Plateau in the semi-humid and semi-arid zone are mainly caused by shallow dried soil layer, because of lower precipitation and higher evaporation. Shallow dried soil layer leads to smaller infiltration depth, less unsaturated flow and slower pellicular water movement than in humid areas, which to be the mechanism of dried soil layer, can described as the surface-driven model of dried soil layer.5. The integrative pedological and sedimentary study was carried out in a Holocene loess profiles on the Zhouyuan loess tableland in Fufeng County, Shaanxi Province.The distinct interface between the Holocene Soil SO and loess LO indicate a monsoon climatic change. The Holocene climatic optimum was ended at3100B.P. The low magnetic susceptibility and high pH in loess mean that it was opposite dry and soil moisture was lacking during the loess deposited. The climate change indicated the climate changed from the opposite warm and humid into the opposite cold and dry, which caused the soil moisture lacking in Loess Plateau from then on. Joined with warm-dry climate, it was unavoidable that the plant degradation and dried soil layer developed.6. Drought degree of dried soil layer can be measured by Id which means index of dried soil layer and can be defined as steady moisture to soil moisture content. Soil layers can be classified as non dried soil layer, weakest dried soil layer, weaker dried soil layer, medium dried soil layer, stronger dried soil layer and strongest dried soil layer by1.0,1.3,1.6,1.9,2.2according to the score of Id. Characters of each type of dried soil layer are described by situations of growth and reproduction rate of vegetations. The construction of Id and its assessing standards provides a unified and convenient quantitative evaluation means when research dried soil layers of different area in the Loess Plateau.7. To research soil moisture dynamics, we define water-consuming index as follows: Ic=(W1-W2)/W1 As above, Ic means soil moisture consuming degree by vegetation.W1means initial soil moisture content or soil moisture content of reference profile,W2means terminal soil moisture content or soil moisture content of researching profile. When score of Ic in each depth are calculated, differences between two profiles will be showed clearly. The water-consuming index will be a useful index when we research soil moisture dynamic process or analyze soil moisture character of different plantations.8. There is a close spatial relationship between forest distribution and river location in Yan’an and Yuling of northern Shaanxi by our surveys, it shows forest have more non zonal character than zonal character in northern Shaanxi. The phenomenon is mainly caused by water redistribution, which result much more soil water can be got in lower space. Enrichment mechanism of soil moisture in lower space to be a general situation, it were testified in one slope at Chang’an district in Xi’an, where terrace were constructed. Enrichment mechanism of soil moisture in lower space leads to forest distributed along valleys in most part of the Loess Plateau.9. The stress from environment, water resources and population caused ecological deterioration in the Loess Plateau. Solely vegetation construction separated from economic development leads to lower speed and lower efficiency. The construction in the Loess Plateau should take the opportunity of reforestation, build the eco-industrial model, that according to the vegetation carrying capacity of soil water resource, focused on special agriculture, fruit industry and grass-stock industry. Thus, the sustainable eco-economic development would come true. |
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