Vegetation Mosaic Effects on Soil Microbial Community Structure and Enzyme Functioning in Relation to Nutrient Heterogeneity in a Mountainous Ecotone

Vegetation mosaics characterize mountainous agroforestry ecosystems, yet how their spatial configuration shapes soil microbial assembly and functions remains unresolved. This study investigated how mosaic elements (monocultures, shrublands, and ecotones) drive microbial communities and enzyme activities across a forest–shrubland–farmland mosaic in western Hunan, China. Nutrient stoichiometry, microbial biomass (PLFA), and six enzyme activities were analyzed via variance partitioning, partial least squares regression, and ordination analysis. Fungal biomass dominated, peaking in ecotones and showing the lowest values in monocultures and shrublands. Microbial assembly was regulated by soil nutrients (31%) rather than soil texture (15%). Fungi (variable importance in projection, VIP = 1.287) and bacteria (VIP = 1.003) were key drivers, indicating distinct functional compartmentalization: fungi drove oxidative enzymes, whereas bacteria mediated nutrient cycling. Actinomycetes and total PLFA acted as secondary drivers, with VIP values of 0.932 and 0.939, respectively. Soil organic matter, dissolved organic carbon, silt content, and available nitrogen were key abiotic predictors. Collectively, vegetation configuration regulates soil functioning via nutrient-mediated microbial assembly and functional differentiation across mosaic elements. These findings underscore the role of landscape heterogeneity in sustaining soil fertility, suggesting that protecting ecotones and maintaining mosaic complexity should be prioritized in mountainous agroforestry management to enhance soil ecological functioning under global land-use change.