Understanding the relationship between biodiversity and ecosystem multifunctionality remains a central challenge in ecology. Although biodiversity is generally associated with enhanced multifunctionality, this relationship can vary across ecosystems and environmental contexts. It remains unclear how concurrent global changes, such as drought and nitrogen deposition, modulate biodiversity–multifunctionality relationships, in particular the moso bamboo forests in subtropical China. We conducted a field manipulation experiment combining throughfall reduction and nitrogen addition to simulate drought and nitrogen deposition, and applied amplicon sequencing to examine how these global change factors independently and interactively influence soil microbial diversity, multifunctionality, and their interrelationships. We found that throughfall reduction significantly reduced soil multifunctionality, likely due to lower soil water availability, and nitrogen addition exacerbated this negative effect. However, environmental changes did not affect bacterial and fungal α-diversity, nor was soil multifunctionality correlated with microbial diversity. Instead, shifts in soil multifunctionality were closely linked to changes in microbial community composition and co-occurrence network structure. In particular, increasing relative abundances of Acidobacteriota, Myxococcota, and Chytridiomycota were positively associated with higher soil multifunctionality. Our findings demonstrate that soil multifunctionality responses to throughfall reduction and nitrogen addition are primarily mediated through microbial community reorganization rather than diversity loss, providing new insights into the mechanisms linking global change drivers, microbial communities, and ecosystem functioning.