Elevated salinity decreases soil multifunctionality by driving bacterial community structure and network complexity

Soil salinization has emerged as a critical environmental challenge threatening the sustainable development of terrestrial ecosystems globally. While the detrimental effects of soil salinization on plant growth, soil nutrient dynamics, and microbial communities are well-documented, how salinity-driven shifts in microbial nutrient limitation and co-occurrence network complexity collectively regulate soil multifunctionality (SMF) remains poorly resolved, particularly in agroecosystems. We conducted a salinity gradient mesocosm experiment (1.23-4.30 g/kg) with sweet sorghum in northwest China. The results showed bacterial diversity (taxonomic/phylogenetic) and richness declined significantly with salinity, while fungal Shannon diversity remained stable, indicating greater bacterial sensitivity; salinity reduced positive cohesion in both microbial groups but increased fungal negative cohesion, altering community assembly (enhanced stochasticity in bacteria vs. determinism in fungi); SMF (integrating plant performance, nutrients, and enzyme activities) decreased along the gradient, strongly mediated by declines in bacterial network complexity and community structure shifts (e.g., Actinobacteriota, Proteobacteria). Critically, Partial Least Squares Path Modeling (PLS-PM) analysis revealed that salinity indirectly reduced SMF primarily via bacterial pathways (68 % total effect), while fungal contributions were marginal. Our study highlights bacterial network interactions as pivotal for maintaining SMF under salinization, providing mechanistic insights for saline soil management.