Abstract: Soil and root-associated microorganisms are fundamental drivers of plant nutrition and environmental adaptation. However, along aridity gradients, the response patterns and regulatory mechanisms of microbial assemblages across bulk soil, rhizosphere soil, and root endosphere remain largely unclear. In this study, we focused on the leguminous plant Lespedeza davurica. We synchronously collected bulk soil, rhizosphere soil, and root samples from 27 sites along a natural aridity gradient of approximately 1200 km across the Inner Mongolian grasslands. By integrating high-throughput sequencing with structural equation modeling, we examined how aridity affects bacterial and fungal diversity and community composition across different ecological niches. The results indicated that aridity directly and significantly reduced bulk soil and rhizosphere fungal diversity. Aridity indirectly suppressed root endosphere bacterial diversity through changes in soil properties. Total nitrogen, soil organic carbon, and soil water content were identified as significant predictors of root endosphere bacterial diversity, whereas total nitrogen was a significant predictor of fungal diversity in both bulk soil and rhizosphere soil. Aridity altered microbial community composition across different ecological niches. The relative abundance of Proteobacteria increased significantly, whereas Acidobacteria showed a significant decline in the root endosphere. In contrast, the relative abundance of Zygomycota decreased significantly in bulk soil and rhizosphere soil. Under intensifying aridity, plants might selectively recruit specific microorganisms via a “cry-for-help” strategy. This study reveals niche-specific microbial responses to aridity across bulk soil, rhizosphere soil, and root endosphere in the grassland ecosystem, providing new insights into microbial adaptive strategies under aridity conditions.

Key words: Aridity, Lespedeza davurica, Root-associated microbiota, Microbial diversity, Community composition