Abstract: Soil microorganisms play an important role in grassland ecosystems. Understanding the changes of soil microbial activity under different grazing conditions is of great significance for evaluating the variations of soil quality. We examined the changes of soil microbial activity under different grazing intensities (no grazing, light grazing, moderate grazing and heavy grazing) in alpine grassland of Qinghai-Tibet Plateau. We measured the soil physical and chemical properties (organic carbon, pH, ammonium nitrogen, nitrate nitrogen, and available phosphorus) and microbial activities (microbial biomass carbon, microbial biomass nitrogen, microbial entropy, soil basal respiration, and microbial quotient) under different grazing intensities by combining laboratory analysis with field sampling. The results showed that: (1) Grazing decreased soil organic carbon and soil ammonium nitrogen contents, but increased soil pH in the 0-10 cm soil layer. (2) Soil microbial biomass carbon, biomass nitrogen and microbial entropy showed a decreasing trend with increasing grazing intensity. The contents of microbial biomass carbon and nitrogen were the highest in the 0-10 cm layer, which ranged from 404.44-748.49 and 101.73-173.78 mg·kg^-1, respectively. (3) Soil basal respiration gradually decreased with the intensification of grazing intensity, while microbial quotient gradually increased. (4) Soil organic carbon, pH, and ammonium were important factors affecting soil microbial activity. In summary, soil microbial biomass carbon, microbial biomass nitrogen, microbial entropy, and soil basal respiration tended to decrease with increasing grazing intensity and soil depth, while microbial quotient tended to increase.

Key words: alpine grassland, grazing intensity, soil microbial activity