Abstract: Nitrogen is the main limiting nutrient for the productivity of grassland ecosystems, and can indirectly affect the structure of microbial communities, especially bacterial communities, by altering soil properties. In this study, we conducted a field nitrogen addition experiment in the alpine meadow of Haibei in the northeastern Tibetan Plateau. Urea was used as the nitrogen source. Five nitrogen addition levels were set up: N₀ (control, no nitrogen addition), N₅₀ (50 kg N·hm^-2·a^-1), N₁₀₀ (100 kg N·hm^-2·a^-1), N₁₅₀ (150 kg N·hm^-2·a^-1), and N₂₀₀ (200 kg N·hm^-2·a^-1). In the third year of the experiment, we collected soil samples of the surface layer to measure soil physical and chemical properties, stable isotope δ¹⁵N, and microbial biomass. The microbial stoichiometric imbalance was calculated, and bacterial community characteristics (composition, diversity, and community assembly) along the nitrogen addition levels were analyzed by 16S rRNA high-throughput sequencing. Through correlation analysis, non-metric multidimensional scaling analyses, and phylogenetic-bin-based null model analyses, we investigated the driving mechanisms of the changes in soil bacterial community composition and community assembly. The results showed that: 1) Nitrogen addition significantly altered soil bacterial community composition, but did not change the α diversity of soil bacteria. 2) Nitrogen addition significantly increased soil inorganic nitrogen content by 85.7% and reduced the stoichiometric imbalance of carbon:nitrogen by 40.6%. Soil inorganic nitrogen content and carbon:nitrogen stoichiometric imbalance were significantly correlated with bacterial community composition and the relative abundance of the dominant phylum (i.e., Bacteroidetes), indicating that bacterial taxa were significantly influenced by soil available nitrogen and stoichiometric imbalance. 3) The stochastic process (54.7%-56.8%) dominated the community assembly of soil bacteria across all treatments. Nitrogen addition had no significant effect on the community assembly of soil bacteria. In conclusion, soil available nitrogen and the resulting stoichiometric imbalance were the primary factors regulating the relative abundance of bacterial taxa under nitrogen addition. Our findings provide a scientific basis for predicting the changes of soil microbial communities in alpine meadows in the context of future environmental changes.

Key words: available nitrogen, ecological stoichiometry, high-throughput sequencing, community assembly, meadow