Abstract: The effects of grazing intensities on soil enzyme activities (urease, dehydrogenase, sucrase, catalase) and soil microbial biomass were studied in a Stipa krylovii steppe in Hulunbuir of Inner Mongolia during different phenological periods. The results showed that soil dehydrogenase activity in moderately grazed plots was significantly higher than that in lightly grazed plots in periods of seedling establishment, heading, flowering and seed maturation (P<0.05). Catalase activity in lightly and moderately grazed plots was significantly higher than in heavy grazed plots in periods of seedling establishment, heading, flowering and seed maturation (P<0.05). Under the light grazing, soil microbial biomass carbon was significantly lower in heading period than in seed maturation period (P<0.05), and soil microbial biomass nitrogen was significantly higher in heading and seed maturation periods than in seedling establishment and flowering periods (P<0.05). Under moderate grazing, soil microbial biomass carbon was significantly higher in seedling establishment than in heading, flowering and seed maturation periods (P<0.05), and soil microbial biomass nitrogen was significantly higher in seedling establishment period than in heading, flowering and seed maturation periods (P<0.05). Under heavy grazing, soil microbial biomass carbon was significantly higher in seed maturation period than in seedling establishment, heading and flowering periods (P<0.05), and soil microbial biomass nitrogen was significantly higher in seed maturation period than in seedling establishment, heading and flowering periods (P<0.05). Correlation analysis showed that soil catalase activity was positively correlated with soil microbial biomass nitrogen, and soil microbial biomass carbon was positively correlated with soil microbial biomass nitrogen.

Key words: ecological water demand, hydrological variation, the southern three outlets of Jingjiang River.