| One of the important water-resource in Beijing is Miyun reservoir, where water quality is influenced directly by the soil erosion of its neighborhood. This thesis applied 137Cs tracer technique to study the contents of 137Cs and the trend of soil nutriment change under different landscape ecosystem and land using models in Miyun reservoir neighborhood and the upper reaches region. It also investigated conditions of land using and social economic development of Miyun reservoir neighborhood, mensurated the content of soil nutriment and nuclear elements such as 137Cs, analyzed the rules and trends of modulus soil erosion under different landscape ecosystem and land using models, analyzed the soil nutrient losing conditions and quantitative change relations between soil erosion and soil nutrient losing, and established mathematic model between nuclear elements and soil fertility ingredients, set up forecast system.Miyun reservoir region is mainly constituted by four types of physiognomy units including mountainous country (inside mountain, low mountain), mound, plain and water areas. The characteristic of all levels physiognomy type is obvious, the structure is clear. According to field investigation, different landscape ecosystem and land using models and physiognomy shapes, the 29 sampling districts were ascertained. There were 100 sampling spots in mountainous region from summit of hill to the foot of a hill. The sampling method is to dig the soil section plane in each sampling spots, 10 cm is the space between two collect the section sample. Then collected surface layer soil on the circle inside that the section circumference 1 s, and mixed it uniformly as the surface layer sample. The contents of 137Cs,other nuclear elements and fertility factors were mensurated and analyzed.The conclusions of analysis are as follows.(1) Because of the activity of mankind that do to the transference and retroflexion of grains of soil, the intensity of soil erosion and nutrient losing has been enhanced, whether it is in the flat, farmland or woodland.(2) With the depth's increase, the content of 137Cs takes on decrease trend in the soil section. 137Cs mainly distribute in the upside of uncultivated and undisturbed soil, and get together in the interspaces of surface layer.(3) According to the conclusion which draw from the soil test of the soil in Miyun reservoir neighborhood and the upper reaches region of Baihe River, the distribution of 137Cs in the surface soils accords with the change of topography and physiognomy. The contents of l37Cs on the top of hillside were lower than it in the middle and low hillside, but the distribution of 137Cs contents was contrary on a flat slope in the mountaintop or on a sharp slope in the lowland.(4) Comparing the soil erosion modulus calculated by the 137Cs content in Miyun reservoir neighborhood with the standard of Ministry of Water Conservancy of China, it was proved that this region mainly belongs to the slight and middling degree of erosion, but some regions' erosion conditions were serious, even reached the severe degree of erosion.(5) Different land using models affected the distribution of the nutrient in soil greatly. The content of organic matter from bush, woodland, and orchard to farmland takes the trend of decrease. Just as the regulation of organic matter, the content of total N is from bush, farmland, orchard to woodland, the content of hydrolyzed N is from bush, woodland, farmland to orchard, the content of total P is from farmland, woodland, bush to orchard, and the content of available P is from farmland, woodland, bush to orchard. The contents of N and P in the reservoir neighborhood are higher than in the upper reaches region. Irrational humanity activities increased the soil erosion degree and losses of soil nutrient seriously. For the woodland and bush, the distribution of the content of organic matter and N is in accord with tracer such as 137Cs.(6) The diagram manifests that it is in direct correlation between the content of l37Cs, 210Pb and soil...
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