Abstract: Aims: As an important part of the global ecosystem, alpine grasslands are especially sensitive to climate change and human activities. The carbon and nitrogen pools of plants and microorganisms are significant components of the carbon and nitrogen pools in grassland ecosystems. Grazing, as one of the main utilization ways of alpine grasslands, directly determines the distribution, storage and utilization of carbon and nitrogen resources by plants and soil microorganisms. Methods By setting up experiments with different grazing methods in Xihai Town, Haibei Prefecture, Qinghai Province, i.e., Yak grazing alone (YG), Tibetan sheep grazing alone (SG), Yak-Tibetan sheep 1:2 mixed grazing (MG1:2), Yak-Tibetan sheep 1:4 mixed grazing (MG1:4), Yak-Tibetan sheep 1:6 mixed grazing (MG1:6), and no-grazing treatment (CK). Important findings In terms of carbon pools, separate grazing by yaks and Tibetan sheep significantly reduced the carbon pool of the plant community, but did not affect the microbial biomass carbon pool. Mixed grazing also reduced the carbon pool of the plant community, yet significantly increased the microbial biomass carbon pool. Regarding nitrogen pools, when grazing separately, the nitrogen pool of the plant community remained unchanged, while the microbial biomass nitrogen pool increased significantly. Under mixed grazing conditions, the nitrogen pool of the plant community was significantly reduced, and the microbial biomass nitrogen pool was not affected. Studies on the trade-offs between plant and microbial carbon and nitrogen pools found that in the situations of no grazing and separate grazing by yaks and Tibetan sheep, the trade-off of carbon and nitrogen pools favored the plant community. Under mixed grazing conditions, the trade-offs of MG1:2 and MG1:4 were inclined to the microbial biomass carbon pool, and that of MG1:6 was inclined to the microbial nitrogen pool. The above results indicate that grazing may regulate the carbon content in plants by consuming the above-ground parts of plants. This change further affects the trade-off state between the carbon pool of the plant community and the microbial biomass carbon pool. Meanwhile, grazing promotes plant regeneration through consumption, increases the nitrogen content of the plant community, and changes the trade-off relationship between the nitrogen pool of the plant community and the microbial biomass nitrogen pool.

Key words: grazing, alpine grassland, carbon pool, nitrogen pool, trade-off