| Desertification is a serious problem of environment and society to the grassland, which is also the main factor of restriction for grassland resource utilizing and sustainable development of economy in the drought and semi-drought region in north of China. For better understanding the causes, the mechanisms and the ecological processes of grassland desertification and the measures of restoration, under the direction of theory of restoration ecology, method of field investigation and laboratory analysis were used in this research to study the frangible mechanism of environment, the disturbance factors, species composition, community structure, species diversity, the physicochemical properties of soil and ecological process of vegetation restoration of grassland desertification of Hulunbeir. Results are as following:(l)The causes of grassland desertification of Hulunbeir are the outcome of interaction of nature and the human beings. Among these reasons, the loose soil texture, which was formed in the fluviolacustrine sediment during Quaternary, establish the physical ground, and the drought and wind is the natural force of grassland desertification. And then over-cutting forest, over-grazing grassland, imbalance of resource utilization and estrepement reduce the coverage of vegetation, therefore provide necessary conditions for the grassland desertification.(2) Vegetation degradation of sandy grassland is a process of decrease of species quantity, simplification of community structure, lose of diversity, degeneration of ecological environment and reduction of system stability. Vegetation ecotype tend to be xerophyte and even hyper-xerophyte and finally replaced by annual plant. Community structure and species diversity also decrease as the ecological environment worsen, In this process, the height, coverage and biomass decreases by 56.6%, 80.8% and 74.2% respectively compared to steppe, and the indices of species richness, diversity and evenness decrease by 76.4%, 56.7% and 32.0%, but β diversity indices increase by 14.6 times, especially the decease of species gregariousness and the breadth and overlay of species Niche shows that grassland tend to be more sandy and vegetation retrogression succession take place.(3) Grassland desertification is also a process of comprehensive degradation of vegetation, soil and environment. Firstly, vegetation degradation enlarges bareness of ground surface, which enhance wind erosion and dry blowing, make the soiltexture coursing, increase the soil bulk volume and low down the retentiveness of water. Some results show that physical clay in 0~30cm soil layer of serious desertification grassland decreases by 97.8% and 94.6% respectively compared to steppe and potential desertification grassland, but bulk volume increases by 0.228g/cm and 0.202 g/cm ; saturation moisture retention decreases by 37.9% and 28.2%. Secondly, the content of organic, total N and available P in soil also decrease with the intensity of desertification degree and are significantly correlated to plant species diversity, which indicate that vegetation degradation is the main cause of soil nutrition reduction and the precondition of soil degradation.(4) The activity of soil biology decreased gradually in the process of grassland desertification. Regression analysis shows that microbe and fauna of soil are significantly correlated to plant biodiversity, the amount of litters, the content organic and moisture, which shows that soil biology depend heavily upon environment. The extinction of Soil biology is a synergistic reaction of vegetation retrogression and soil degradation, but soil enzyme content decrease for the reason of reduction of soil biochemical activity and the block of material recycle and energy flow.(5) Fencing is one of the most economic and effective measures. After fencing, disturbance will be removed and plant community transform from annual plants to secondly stable shrub or relatively stable steppe, and biodiversity also increase, ecosystem tend to be more stable. Soil develop from mobi...
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