Characteristics of soil fertility variation and their relationships were studied in Stipa purpurea steppes, the most representative alpine steppes in northern Tibet. Based on the same parent material(lake sediments）and soil texture (sandy soil）during the rapid growing period of steppe plants, we selected four sampling areas located at the altitude of 4550-4740 m with a distance of 40-50 km away from each other. In each sampling area, according to the classification standard of steppe degradation, we investigated four kinds of steppes(no degradation, slightly degraded, moderately degraded and seriously degraded） with an area of one hectare each. All the sampling sites(3 sites per area）were chosen randomly in our study. Soil samples in 0-10 and 11-20 cm layers were collected by a soil auger (4 cm inside diameter）in steppes with different degradation grades in each sampling area. Sol samples used for measuring indexes concerned with soil microorganisms were from 2-10 and 11-20 cm layers. At each sampling site, ten soil cores were mixed thoroughly to form one composite sample for chemical and biological analyses related to soil fertility.
The results were listed as follows: (1）Under the ecological conditions of the Northern Tibet Plateau, steppes degradation had consistent effects on soil physical, chemical and biological fertilities. These soil physical, chemical and biological fertilities were associated with each other and set the direction of soil fertility evolvement in the alpine steppes. (2）The degradation of soil fertility in alpine steppes was mainly depended on the changes of soil biological properties while it was also regulated by the variations of soil physical factors, such as soil structure, soil bulk density and soil moisture. In general, soil biological, physical and chemical fertilities in the slightly degraded steppe showed increasing trends at various levels because of the decline in soil moisture content and soil frozen status induced by slight soil desertification. (3）With increased steppe degradation grades, soil bulk density at both 0-10 and 11- 20 cm layers showed a decreasing trend (the difference between slightly degraded steppe and normal steppe was not significant）, while soil porosity and moisture content increased or decreased significantly although steppe degradation influenced soil moisture content more than soil porosity. In alpine steppe soils, water stable aggregates (>0.25 mm）content positively and significantly correlated with soil moisture content, indicating that organic matter played an important role in the improvement of soil structure and the increase in soil moisture content. (4）Soil organic matter content as well as total N, P and K contents were increased in slightly degraded steppe but decreased in moderately and seriously degraded steppes compared with no degradation steppe. Soil available N, P and K showed a decreasing trend with steppe degradation. The proportions of soil humus C to soil organic C, HA-C to soil humus C and the ratio of HA/FA improved with the increase in steppe degradation grades. (5）The quantities of soil microorganisms (e.g. bacteria, fungi and actinomycetes）and soil microbial biomass (BC and BN）, and the activities of soil enzymes (cellulase, urease and alkaline phosphatase）in 2-10 cm layer of different degraded steppe soils showed the same trends with soil organic matter content, in general. There was a positive relationship between the ratios of BC/BN and TC/TN (r=0.937 0**，p≤0.01）, and the ratos of BC/BN and TC/TN increased largely in the slightly and moderately degraded steppes and decreased only in seriously degraded steppe. (6）There were significantly (p≤0.05 or 0.01） positive relationships between soil microbial biomass, the activities of three mentioned soil enzymes and soil organic matter, total N, available N, available K contents in alpine steppe soils. In contrast, we found negative relationships between soil microbial biomass or the activities of soil enzymes and soil available P content.