Alpine grassland degradation affects soil properties and the abundance of N cycling functional genes

Alpine grassland on the Tibetan Plateau plays a crucial role in carbon and nitrogen fixation and the maintenance of biodiversity. Numerous studies have investigated the effects of grassland degradation on vegetation, soil, and microorganisms at different spatial scales; however, the response of N-cycling functional genes to grassland degradation and the relationship with microbial communities are still not clear. In this study, we used soil macrogenome sequencing technology to study the effects of alpine grassland degradation on soil properties, microbial communities, and functional genes of N cycling. The results showed that grassland degradation decreased soil organic carbon, total nitrogen, total phosphorus, available nitrogen, available phosphorus, cellulose, sucrase, and urease, but increased pH. Grassland degradation has shifted the soil microbial community from eutrophic groups (Actinobacteria and Bacteroidota) to oligotrophic groups (Acidobacteria and Chloroflexota) could help communities adapt to environmental disturbance. In addition, grassland degradation increased the abundance of pmoC-amoC genes related to the soil nitrification process and decreased the abundance of soil assimilatory nitrate reduction and dissimilatory nitrate reduction genes (nxrA, nasD, gdh, nirD). Finally, structural equation modeling was constructed to show that soil TN and AN were the main soil factors driving the changes of N cycle functional genes. This study links soil microbial communities with functional genes of the N cycle to further deepen our understanding of degraded alpine grassland, and then to propose grassland conservation measures.