Moderate grazing optimizes soil nutrient pool turnover in alpine grasslands via enhanced bacterial-driven processes

Alpine grasslands provide critical grazing resources for herbivores, and grazing activities can profoundly alter soil physicochemical properties and microbial community structure, ultimately affecting soil nutrient pools and related ecosystem functions. Nevertheless, how different grazing intensities reshape soil microbial communities and regulate nutrient pool turnover remains relatively less explored. To address this knowledge gap, we implemented a large-scale grazing intensity experiment on yak-grazed alpine pastures on the Qinghai-Tibet Plateau, examining how grazing intensity influences bacterial, archaeal, and fungal communities and their roles in regulating soil nutrient pool turnover. The results demonstrated that soil nutrient pool turnover peaked under moderate grazing but declined significantly under heavy grazing conditions. Alpha biodiversity (Shannon index and OTU richness) and community size (gene copy numbers) of bacterial communities increased from non-grazed enclosures to moderate grazing, then decreased under heavy grazing. In contrast, archaeal and fungal diversity and community size increased continuously with escalating grazing intensity. Concurrently, grazing intensity also shifted the community compositions of bacteria, archaea, and fungi, with bacterial community exhibiting the highest sensitivity ( R 2 = 0.61 for bacteria; R 2 = 0.24 for archaea; and R 2 = 0.17 for fungi). Partial least-square path modeling revealed that the effects of grazing intensities on soil nutrient pool turnover were predominantly mediated by bacterial community dynamics. Specifically, moderate grazing enhanced bacterial-driven nutrient pool processes, whereas heavy grazing amplified the contributions of archaeal and fungal communities to nutrient pool dynamics. Notably, several r-strategist bacterial lineages, including Proteobacteria (classes Alphaproteobacteria and Betaproteobacteria) and Firmicutes (class Bacilli), reached their highest abundances under moderate grazing and exhibited strong positive correlations with soil nutrient pool turnover. Collectively, these findings demonstrate that moderate grazing optimizes grassland soil nutrient pool dynamics by enhancing bacterial performance. These results provide potentially valuable guidance for grassland management in assessing the impacts of varying grazing pressures on ecosystem functions, particularly in large herbivore-dominated alpine grasslands.