Nitrogen addition has negative effects on bacterial diversity and stability, but simulated grazing mitigates these effects.

Introduction: Nitrogen addition and grazing, as common management tools in grasslands, alter the structure and function of soil microbial communities and have far-reaching effects on grassland ecosystems. However, the mechanisms by which nitrogen addition and grazing regulate the diversity and stability of soil microbial communities remain insufficiently understood. Methods: In this study, a field experiment was conducted in the temperate desert grassland of Xinjiang, combining nitrogen addition treatments with simulated grazing to investigate the response mechanisms of soil microbial communities to nitrogen addition and simulated grazing. The regulation of soil microbial community diversity and stability under the combined effects of nitrogen addition and grazing was examined by using mowing to simulate aboveground vegetation disturbance. Results: The results showed that inorganic nitrogen (nitrate and ammonium nitrogen) was a key factor driving nitrogen-induced changes in microbial community structure, increasing the availability of soil nitrogen. Moderate nitrogen addition promoted bacterial community diversity, whereas excessive nitrogen input weakened this effect and reduced bacterial community complexity and co-occurrence network stability. Simulated grazing enhanced organic nitrogen catabolism through increased leucine aminopeptidase activity, thereby stabilizing bacterial community interactions under nitrogen-enriched conditions and alleviating the negative effects of nitrogen addition. Discussion: These results indicate that grazing can buffer nitrogen-induced destabilization of soil microbial communities and highlight its role in maintaining microbial functional stability in grassland ecosystems under increasing nitrogen deposition.