Combined livestock grazing-exclusion and global warming decreases nitrogen mineralization by changing soil microbial community in a Tibetan alpine meadow

Alpine meadows play a key role in maintaining biodiversity, soil nutrient retention, and herders' livelihoods in mountain regions. Livestock grazing-exclusion is common in grassland management to protect and promote the sustainable use of alpine meadows. However, there are few studies evaluating the effects of traditional light grazing practices in alpine regions and grazing-exclusion impacts on soil nutrient, especially under conditions of climate warming. In this study, we conducted a five-year grazing-exclusion and warming manipulation experiment to examine the effects of grazing-exclusion on soil nitrogen fractions, microbial communities, and enzyme activities under climate warming. Treatments consisted of light-intensity grazing, grazing-exclusion, and combined grazing-exclusion and warming. Our results showed that grazing-exclusion significantly decreased potential nitrogen mineralization at a 0-5 cm depth and at altitudes of 3850 m, 4000 m, 4150 m, and 4250 m. Combined grazing-exclusion and warming also significantly decreased potential nitrogen mineralization at a 0-5 cm depth and at relatively higher altitudes of 4000 m, 4150 m, and 4250 m. Grazing-exclusion, combined grazing-exclusion and warming all shifted microbial communities from bacteria toward fungi. Grazing-exclusion significantly decreased the relative abundances of phylum Chloroflexi (bacteria) and phylum Ascomycota (fungi), while combined grazing-exclusion and warming significantly increased the relative abundances of phylum Basidiomycota (fungi). The relative abundances of phylum Ascomycota (fungi) demonstrated a positive relationship with potential nitrogen mineralization, but the relative abundances of phylum Basidiomycota (fungi) had a negative relationship with potential nitrogen mineralization. The activities of dehydrogenase and protease in grazing-exclusion and combined grazing-exclusion and warming meadows were lower than those in grazed meadows. Our results demonstrated that in the context of climate change and global warming, grazingexclusion is unsuitable as a tool for the sustainable management of alpine meadows from soil nitrogen mineralization and fractions point of view. Of concern, climate warming could potentially amplify the adverse effects of grazing-exclusion and causes depletion of soil nutrients in the alpine meadow. This research suggests that a light-intensity grazing (0.5 yak ha(-1) year(-1)) is beneficial to increase soil enzyme activity and N mineralization in alpine meadows. Our results provide substantial evidence and a useful baseline and guidance for the sustainable grazing management of alpine meadows as environments change due to climate warming in the future.