Soil diazotrophs introduce significant amounts of new nitrogen (N) into forest ecosystems through the process of free-living N fixation. However, the factors influencing their community structure and function remain elusive, particularly in subalpine forests that are often characterized by N limitation. Here, we explored the patterns and drivers of the diazotrophic community structure (abundance, diversity, composition, and inter-specific interactions) and N fixation potential (nifH mRNA copy number) along a 400-m elevational gradient in subalpine forests on the eastern edge of Tibetan Plateau, using droplet digital PCR, nifH gene sequencing, and metagenomic sequencing. The results showed that the diazotrophic community varied significantly across different elevations, with Bradyrhizobium consistently identified as the key taxa. Our findings also demonstrated the predominant role of environmental filtering in shaping the diazotrophic community. Specifically, the elevational pattern of diazotrophic abundance was primarily driven by soil resource availability, while those of α-diversity, community composition, and co-occurrence network were jointly modulated by temperature and soil properties. In comparison to total bacteria, the α-diversity of diazotrophs was more sensitive to changes in temperature. The positive relationship between temperature and diazotrophic diversity indicates that future subalpine forests may support more diverse soil diazotrophic communities due to projected increases in temperature. Moreover, we found a weak linkage between diazotrophic community structure and N fixation potential, indicating the complexity of the diazotrophic structure-function relationship in subalpine forests. Overall, our work highlights the sensitivity of diazotrophic community structure to future environmental changes in subalpine forests. Further studies are needed to simultaneously monitor the diazotrophic community structure and N fixation rates across a broader range of sites.