Light grazing enhances microbial-mediated nitrogen transformation in desert steppe soils.

AIMS: Soil microorganisms play a central role in nitrogen transformation and availability, yet their regulatory functions in mediating nitrogen transfer between soil and plants under long-term grazing remain poorly understood. This study aims to elucidate how grazing intensity influences soil microbial communities and nitrogen-cycling functional genes, and how these microbial shifts affect nitrogen transformation processes and plant nitrogen uptake in desert steppe ecosystems. METHODS: Here, we present a comprehensive study on soil microbes and nitrogen cycling functions in a desert grassland subjected to four grazing intensities - no grazing, light (0.91 sheep/ha), moderate (1.82 sheep/ha), and heavy (2.71 sheep/ha) - over 17 years. RESULTS: We found that both the structural composition and the function of soil microbial communities varied across grazing intensities, with more pronounced changes in rhizosphere soil than in bulk soil. The relative abundance of nitrogen-cycling functional genes was generally higher under no grazing or light grazing conditions. Structural equation modeling (SEM) revealed that increases in soil pH under long-term grazing were significantly associated with shifts in microbial communities and with reduced net nitrogen mineralization rates. These changes were further linked to decreased soil nitrogen availability and lower nitrogen content in plant tissues. CONCLUSIONS: Light grazing in desert steppe ecosystems partially maintains microbial nitrogen cycling potential by supporting the abundance of nitrogen functional genes and preserving microbial community structure, despite an overall decline in nitrogen mineralization under grazing. These results suggest that low-intensity grazing may help mitigate grazing-induced suppression of nitrogen availability and plant uptake.