Shrub Encroachment and Accompanied Changes of Biogeochemistry Cycles in Semiar id and Ar id Gr asslands
干旱半干旱地区草原灌丛荒漠化及其 生物地球化学循环

As one of the most widespread vegetation types worldwide, Grasslands cover nearly one - fifth of the world' s land surface area and play a significant role in the global climate change and biogeochemistry cycle. However, a large area of grasslands in the world has been faced a serious problem- degradation and desertification. One form of desertification is the conversion of homogeneous grasslands into shrub- dominated ecosystems and shrubs encroachment has become a widespread phenomenon in arid grasslands and savannas over the past century. The transition has greatly affected ecosystem structure and function in semiarid and arid ecosystems. Overgrazing, regional climate drought and natural fires are the critical factors controlling shrubs encroachment and development. During the process of shrubs encroachment, the wind erosion and water erosion of soil will be strengthened and the soil nutrients and other noessential elements distributed uniformly in grasslands will gradually develop horizontal and vertical patterns in soil dimensions and finally lead to the formation of "islands of fertility", which makes the spatial heterogeneity of soil resource more evident. In addition, the enhancement of wind erosion with the encroachment of shrubs also increased the loss of soil resource, such as C, N, P, S and further influenced the redistribution of these elements among soil, atmosphere and biosphere. The shrubs encroachment and accompanied changes of biogeochemistry cycles will be markedly contributed to the global climate change through increasing the content of aerosol, carbon dioxide and nitrous oxide in the atmosphere and there exists a sensitive feedback mechanism between them. China has an area of grasslands of about 4 ×106 km2 or about 40% of the nation's total land surface and most of these areas have been affected by desertification, but reports on the biogeochemical effects of grass- shrub transition are limited. Therefore, Reinforcing research on shrub encroachment of grassland ecosystem along with the accompanied transformation of plant and soil resource distribution pattern is crucial for accurately understanding the desertification process and predicting the potential changes of elements biogeochemical cycle the in semi- arid and arid grasslands.