Soil microbial communities exhibit pronounced spatial heterogeneity in diversity patterns and assembly processes along environmental gradients. This heterogeneity, governed by the complex interplay of climatic, edaphic, and biotic factors, exerts a profound influence on ecosystem functions. However, our understanding of these patterns and mechanisms remains limited, especially in ecologically fragile regions like the Qinghai-Tibet Plateau (QTP). Through a large-scale survey across four grassland types on the northern QTP, we investigated the spatial variation of microbial diversity, its drivers, and its connection to ecosystem multifunctionality (EMF). Microbial communities displayed distinct biogeographic patterns, with alpha diversity significantly lower in arid alpine desert grassland (ADG) compared to more mesic sites. Mean annual precipitation (MAP) and plant richness emerged as the strongest predictors of diversity. While significant distance-decay relationships were observed for all communities, bacteria exhibited steeper spatial turnover (slope =-0.042 to-0.044) than fungi (slope =-0.024 to-0.028). Community assembly was predominantly governed by dispersal limitation (fungi: 69.7-80.5 %; bacteria: 83.2-89.1 %) and homogeneous selection (fungi: 16.6-26.5 %; bacteria: 6.7-12.3 %). Crucially, we found that environmental factors like MAP and plant richness influence EMF primarily through indirect pathways, by regulating microbial community structure and assembly processes. Our findings highlight that climate change, by altering precipitation regimes and biotic communities, could disrupt the microbial-mediated pathways that sustain EMF in these vulnerable alpine grasslands.