The Features Of Grassland Soil Degradation And Sandification In The Northwest Of Sichuan Province And The Analysis To Its Cause Of Formation

The degradation and sandification of grassland have become a worldwide problem. There is also grassland degradation occurring to a different extent in China. In recent years, the majority of experts and scholars did a lot of research work towards the rules and causes of formation of the degradation and sandification of grassland in northern Chinese droughty and semi-droughty area, but few in southern Chinese wet and semi-wet area. The plateau grassland area located in the northwestern Sichuan Province is one of the five main pasturing areas in our country, and the headstream of both Yellow River and Changjiang River at the same time, which is very important in zoology system. Recently, there are researches showing that the degradation and sandification situation in this area is fast and serious, which have impacted the development of local stockbreeding and deteriorated ecology environment, and also threatened the water resource environment of the two largest water system in our country. Therefore, a full investigation, research and mastery of the causes, features and developing trend of the grassland degradation and sandification occurred in Sichuan Province's southwestern area is of great means to Chinese zoology protection.This paper took Hongyuan County for example, adopted the method of field researches combined with analysis in lab , by region researches to the different types and degradation level soils and sampling as well as correlation analysis of sampled soils, studied soil features and relation between the cause and tenor of the soil degradation and sandification of Hongyuan County, then combined with inspections and analysis of the local plant community, weather, depasturing and economical situation, mainly discussed the reasons from both nature and human, and attempt to offer basis for the defending and recovering work for the grassland degradation and sandification of that area. The main results see below,(1) Study area belongs to continental altiplano cold temperate zone monsoon climate region. By statistic analysis of the 23 year climate data, the results showed that annual average precipitation of the area can reach 754mm ,but the distribution is odd and May to September occupied 90% of total precipitation of the whole year. Annual average temperature was 1.5℃, the highest monthly average temperature is 11.0℃, and the lowest monthly average temperature is -9.92℃. From statistic results, the tendence of precipitation declined year after year while annual average temperature raised. This kind of climate condition is highly different from that of north drought and semi-drought area.(2)Different conditions and parent materials formed various type of soils. This paper distinguishes the soil of Hongyuan County to fluvial terrace and old river sedimentary parentmaterials soil and plantation surface and diluvium and residual parent materials soil. Soils formed from the former had material basis for sandification because of separation effect of rivers, but the base of the soils formed from the later was weak because of complexity of particles structure, so the probability of sandification was relatively less than former.(3)There are degradation and sandification processes existed separately in the 2 types of soil as sandification process of bog soil—meadow soil—grassland—Aeolian sandy soil and degradation process of bog soil—meadow soil—grassland. The chemical movements in the both processes are accordant. In the degradation process, soil organic matter and humic acid showed downtrend, FA/HA showed uptrend. In the soil nutrient changes, the contents of total nitrogen, available nitrogen, total phosphorus, available phosphorus and available potassium are all showed downtrend. They are surface layer—subsurface layer—bottom layer on profile. The content of total potassium showed slight uptrend, but no changes generally. The soil pH value is also showed slight uptrend; the pH value of surface layer is less than the value of subsurface layer.(4)The terrain of Hongyuan County is high in the south and low in the north. A river called Baihe River crosses the area, flowing from the south to north. The chemical test report of the riverbank soil is showing that the river's filtration work done to the soil grains is obvious. From south to north, the soil grain shrinks from coarse to fine. However, at the very lower reaches of this river, the soil grain is coarse again, and the area is sandified.(5)The vegetation investigion report shows that the flora in the majority of Hongyuan County is degrading and succeeding. Good pasturage is gradually decreasing in variety, weeds and poisonous plants are increasing on the contrary, animals live on land and carrying capacity in unit acreage range are decreasing at the same time.(6)Via the analysis of climate, parent materials, soils and vegetation of study area, we concluded that the degradation and sandification of Hongyuan Grassland resulted from the effects of natural factors and man-made factors.Causes of formation were concluded concretely below:(1)The sand sourceg of grassland sandification was supplied with old river sedimentary sand and powder sand from rocks weathering, and the conclusion is that the characteristics of parent materials and hydrology geology is basic factors composing sandification.(2)There some aspects have also accelerated degradation and sandification of grassland .such as air temperature raising, precipitation decrease, evaporation raising, climate drying and warming, water table descendment, rivers changed their courses, lake dwindling, swamp dryingand wetland shrinking.