Fungal diversity and keystone microbial clusters drive soil multifunctionality along a land-use intensity gradient in Ultisols

Maintaining soil multifunctionality—the concurrent performance of nutrient cycling, microbial activity, and organic matter decomposition—is a central challenge under land-use intensification. We investigated how microbial properties regulate multifunctionality across an intensification gradient in red-soil Ultisols of Jiangxi, China. Twenty-seven 5 × 5 m plots were established in abandoned land, woodland, and farmland. Soil multifunctionality was assessed from physicochemical and enzymatic indicators, and its microbial determinants were analyzed through diversity metrics, co-occurrence network complexity, and ecological cluster composition. Both bacterial and fungal richness declined significantly with increasing land-use intensity, accompanied by clear shifts in community structure. Microbial interaction networks became progressively simplified from abandoned to cultivated soils, indicating weakened microbial associations. Soil multifunctionality decreased along the same gradient and was positively related to fungal richness (R² = 0.3919), bacterial richness (R² = 0.3103), and network complexity (R² = 0.2962). Among five ecological clusters identified, one keystone cluster (“Cluster 4”) exhibited the strongest links to multifunctionality through both relative abundance (R² = 0.5693) and internal diversity (R² = 0.455). These results demonstrate that land-use intensification diminishes microbial diversity and interaction complexity, thereby constraining soil multifunctionality. Moreover, fungal diversity is a more powerful predictor than bacterial diversity, and a keystone ecological cluster explains more variation than whole-community metrics. Our findings highlight the pivotal role of fungi and dominant microbial clusters in maintaining multifunctionality of highly weathered Ultisols, providing mechanistic insights for biodiversity-based soil management under intensified land use.