Dynamics of soil microorganism s in stipa pu rpurea alp ine steppes different in degradation intensity in North Tibet Plateau were studied.Four p lots were selected,coincident in parent rock and soil texture,but different in degradation intensity,i.e.non-degraded(ND),slightly degraded(SD),moderately degraded(MD) and severely degraded(VD).Soil samp les were taken using the grid method from the four p lots in both warm and cold seasons.Results indicate that under alp ine arid conditions,both degradation intensity and seasonal variation have significant influences on soil microbial activities in alp ine steppe.A similar trend of variation of soil bacteria population,microbial biomass(carbon(C) and nitrogen(N)) and soil enzyme(cellulase,urease and alkaline phosphatase) activities as a whole was found in both warm and cold seasons,and so was high correlation between the two.In the warm season,the steppes were in the order of SD > ND >MD > VD in soil microbial activity.The numbers of fungi and actinomycetes were positively correlated with soil microbial biomass,but negatively with soil enzyme activity to a varying extent in warm season,while an opposite trend was found in cold season.The soil microbial activities in cold season declined significantly,compared with those in warm season.The ratio of soil bacteria in warm season to that in cold season in the four grades of steppes was 206 10,251.7,18.4 and 87.4,respectively,of fungi,14.7,1.132,0.6 and 0.9 and of actinomycetes,0.1,10.5,10.0 and 14.9.With respect to ratio of B_C(microbial biomass C) and B_N(microbial biomass N) in warm-season to those in cold-season,SD was the highest and followed by ND,MD,and then VD.and sharp variation of B_N was observed between sea-sons.The ratios of B_C/B_N in cold season and in warm season were more or less the same in variation,but the B_C/B_N.ratio was much higher in cold season than in warm season.However,the values of B_C/T_C and B_N/T_N were higher in cold season than in warm season for all of the four grasslands,except VD,which showed a reverse trend.Soil urease was much higher than other soil enzymes in activity in warm season.The ratio of soil urea activity in warm season to that in cold season ranged from 31.5 to 781.5.The activity of soil cellulolytic enzyme,especially alkaline phosphatase,were higher in cold season than in warm season,and the ratio of the activity in warm season to that in cold season ranged from 0.46 to 1.01 for soil cellulolytic enzyme and 0.40 to 1.37 for alkaline phosphatase.