Alpine meadow degradation reshapes diazotroph community assembly and enhances putative recruitment of diazotrophs from legume soils to sedge soils.

INTRODUCTION: Biological nitrogen fixation is a key microbial process sustaining nutrient cycling and plant productivity in nitrogen-limited alpine meadows. However, how alpine meadow degradation reshapes diazotrophic community composition and assembly across different plant species, and how these changes relate to aboveground biomass, remain poorly understood. METHODS: We used the nifH gene as a molecular marker to investigate diazotrophic communities in rhizosphere and bulk soils associated with the legume Oxytropis ochrocephala, and the sedge Carex alatauensis in undegraded and degraded alpine meadows on the Qinghai-Tibet Plateau. RESULTS: Meadow degradation markedly reduced diazotroph α-diversity and shifted community composition, with Proteobacteria, Cyanobacteria, Firmicutes, and Actinobacteriota dominating the assemblages. Diazotrophic communities in legume soils were primarily structured by deterministic processes, whereas those in sedge soils were more strongly influenced by stochastic processes. Source-tracking analysis suggested that legume soils may serve as an important reservoir of diazotrophs for sedge soils, with the potential contribution increasing under degradation. Exploratory PCA-based piecewise SEM indicated that degradation mainly affected soil nutrient status and diazotroph β-diversity, while aboveground biomass was only weakly mediated by the soil nutrient-diazotroph-vegetation diversity pathway. DISCUSSION: These findings indicate that alpine meadow degradation reorganizes diazotrophic diversity, community assembly, and potential source contributions between legume and sedge soils. Legume soils may therefore represent an important diazotroph reservoir for sedge-associated soils during alpine meadow degradation.