Influence On Environment From Physicochemical Characteristics Of Soil Crust And The Reflection Spectrun Analysis In Sandy Desert Of Yanchi, Ningxia

Based on lots of the outdoor investigations, the main types of soil crust were determined. The characteristics of physical and biological soil crust, including color, thickness had been analyzed. The distribution features of soil crust in different sand dune types, in different location, and under different communities were discussed. The spectral and physicochemical characteristics of soil crust had been analyzed. The influences on soil moisture, vascular plant species, coverage and diversity of soil crust were studied. The research through the remote sensing image analysis will provide the theory evidence for restoration and management of degraded ecosystem in YanChi sandy land. The major conclusions are as follows:Soil crust in sandy desert of Yanchi is divided into physical crust and biological crust. Physical crust is widely distributed in the sand cultivating more than 3 years, algal crust and lichen crust are widely distributed in the sand cultivating more than 5 years, moss crust is widely distributed in the sand cultivating about 9 years.Degree of dune stability has a certain influence on the distribution of soil crust.There is no formation of soil crust in mobile sand dunes. The soil crust in semi-mobile sand dunes was mainly physical crust. The soil crust in fixed sand dunes was mainly biological crust. Physical crust occurs mainly on the top of the fixed sand dune. The weak development of algal crust occurs between the top to the side slope of the fixed sand dune. The better development of algal crust and a small amount of lichen crust occurs on the middle of the fixed sand dune. The well developed moss crust occurs under low shrubs at the bottom of the dune. The algal crust occurs mainly on windward slope. The lichen crust occurs on the bottom of leeward slope. Artemisia sphaerocepha, Nitraria tangutorum, Salix cheilophyla and Caragana korshinskii are the existing common shrub plants in Ningxia. Due to the differences of plant characteristics and structure, the distribution types of soil crust are different.As the development of soil biological crust and the fixation of shifting sand dune, the reflectance features of the sand surface changing with its thickness.The reflectance features of the biological soil crusts are different under the condition of dry and hydration. The reflectivity of biological soil crusts under the hydration was significantly lower than the dry. The reflectance features of old time soil biological crust are similar to the green vegetation. Therefore while using images of remote sensing to carry out desertification land assessment, one must emphasize on the option of image, if funds adoptedly, as far as possible to reduce other influence for evaluating results.With the increase of Cultivating age, the evolution of the soil crust habitat becomes from mobile sand dunes to the recovery of fixed sand dunes, physical soil crusts grows into algae crusts, lichen crusts and moss crusts, the crusts will turn thicker, the bulk density will decrease, the amount of coarse sand will decrease and the amount of fine, silt and clay will increase. The thickness of soil crust in different landscape of sand dune has close relationship with the biological components of the crust. The soil crust thickness on the top of the sand dune was smaller. The thickness of leeward slope was higher than the windward slope. The thickness was the highest at the bottom of the dune. The soil crust bulk densities on the top and windward side were higher than the bottom and leeward slope. The particle composition of biological crust was mainly fine, coarse sand took second place on the top, windward slope and leeward slope of the sand dune. The particle composition of biological crust was mainly fine, silt and clay took second place at the bottom of the sand dune. Meanwhile, soil crusts will decrease the pH, increase the EC, and accumulate nutrients of the soil. Moss crust has obvious accumulation effect of soil nutrients. The nutrient contents of soil crust under the four plants were significantly higher than flowing sand. The crusts under the plant have enrichment effect for soil nutrient.Soil crust covered directly affects the soil natural moisture, and different types of biological crusts have different impact on soil natural moisture conditions. Soil crust has effect on rainfall interception. The increase of clay content of the crust can help increasing water-holding capacity of the soil. It makes the soil having significantly higher water retention compared to flowing sands. Different types of soil crusts have different impact on water-holding capacity. In the initial stage, soil water infiltration rate is faster, as the infiltration time increasing, infiltration rate fell sharply; gradually achieve stability infiltration.The steady infiltration time decreases with the increase of crust development degree and the thickness of the crust. The existence of biological crust hindered the infiltration of moisture and inhibited the growth of vegetation. Biological crust caused rainfall loss and consumption in the process of infiltration, had significant effect on rainfall interception, made the most of rainfall become ineffective rainfall, reduced the amount of water infiltration, and restrained the growth of the plants in the sand cultivating more than 9 years. Therefore, optimum tillage or grazing in the closure area can reduce the negative effects of soil crust to water infiltration.The land desertification was toward moderate desertification situation as a whole, and the development trend is easing. Climate and human factors are becoming such a driving force of the dynamic change of desertification. Desertification changing trend obvious slow down since the county-wide implements the grazing prohibition. The vegetation coverage showed a trend from increase to decrease along with the increasing of cultivating age. The vegetation coverage has decreased in the sand cultivating more than 9 years. In the early of cultivating period, there is a mutual promotion stage between biological crust and plants under bad environmental condition and unstable soil. As the increase of cultivating age, biological crust and plants have a mutual inhibition stage. Biological crust reduced the diversity and stability of the plant in the the sand cultivating more than 9 years.The development of soil crusts need more than 5 years. Biological crust in the sand cultivating more than 9 years limited the growth and recovery of vegetation. The key is to grasp the development "degree" of biological crust in the pasture management and ecological restoration process of dry region and semi-arid region. We should make different measures according to the different situations. We will through the protection and control of biological crust, to promote the growth of vegetation.