【Objective】 Plant-soil microbe interactions are the cornerstone of grassland ecosystem function and stability. Elucidating the relationship between plant biomass and soil microbial diversity, along with its environmental dependencies, is essential for understanding ecosystem maintenance mechanisms and predicting future dynamics. However, most studies focus primarily on bulk soil microbes, while integrated investigations examining the effects and relative contributions of both bulk and rhizosphere soil microbes on plant biomass remain scarce. Therefore, this study aimed to elucidate the effects of soil microbes on aboveground biomass (AGB) under the combined influence of biotic and abiotic factors.【Method】 This study was conducted in the desert steppe on the eastern foothills of the Helan Mountains. The characteristics of soil microbial communities in both bulk soil and the rhizosphere soil of dominant plants across different plant communities were analyzed. The effects of soil microbes on aboveground biomass (AGB) under the combined influence of biotic and abiotic factors were elucidated. 【Result】 The results revealed that: 1) No significant difference was observed in microbial alpha diversity between bulk and rhizosphere soils (P > 0.05), whereas beta diversity showed significant differences (P < 0.05). The absolute abundance of both bacteria and fungi in the rhizosphere soil of all dominant plants was higher than that in the corresponding bulk soil, except for bacterial abundance in the rhizosphere of Stipa breviflora. Stochastic processes dominated the microbial community assembly in both bulk and rhizosphere soils. 2) AGB was significantly positively correlated with the biotic factors of plant diversity and Faith’s phylogenetic diversity of fungi in bulk soil, as well as the abiotic factors of ectorhizosphere soil pH and total nitrogen content (P < 0.05). Conversely, it was significantly negatively correlated with mean annual temperature, soil moisture content, and available phosphorus in bulk soil (P < 0.05). 3) Plant diversity was the primary factor explaining the variation in AGB, accounting for 41.5% of the explained variance, followed by soil physicochemical properties. 4) In contrast to rhizosphere microbes, bulk soil microbial diversity acted as the key mediator linking environmental factors to AGB. The effects of climatic and soil physicochemical factors on AGB were primarily indirect, transmitted through this mediating pathway. 【Conclusion】 In conclusion, the diversity of plant and bulk soil microbiomes is a key biological regulator in maintaining plant AGB of the desert steppe ecosystem, a role that is modulated by climatic factors and soil physicochemical properties.