Salinity-induced the decrease of soil organic carbon controlled by micro-food web networks complexity in saline soil

Soil salinization undermines the soil structure and microbial carbon cycling efficiency. However, the pathways by which salt stress reorganizes the microbial food web to decrease soil organic carbon (SOC) and degrade soil quality remain poorly understood. We analyzed natural and agricultural sites of low- and high-salinity soils in coastal China according to SOC, soil quality index (SQI), microbial carbon use efficiency (CUE), microbial necromass carbon (MNC), enzyme activities, and microbial community composition. Compared to adjacent low-salinity soils, high-salinity soils exhibited lower SQI and SOC, CUE, and MNC (by 16.0–21.1%, 16.7–22.0%, and 34.8–40%, respectively) but double the maintenance respiration, indicating a shift from growth to survival metabolism. The SQI in highly saline soils was positively correlated with SOC, CUE, and MNC, but negatively associated with microbial C and P limitation, highlighting the pivotal role of microbially mediated C turnover in soil quality under salt stress. Salinity favored halotolerant Proteobacteria, Crenarchaeota, and protists, displacing key bacterial and fungal decomposers. Unexpectedly, network complexity increased (nodes by 50–80% and edges by 3–11‑fold) with heightened positive cohesion, reflecting close cooperative interactions that nonetheless intensified resource competition and accelerated SOC mineralization. Structural equation modeling revealed a cascade of effects, whereby salinity disrupted soil aggregation and nutrient balance, which increased network connectivity and reduced microbial metabolism efficiency, driving SOC loss and SQI decline. Saline soil management should therefore combine aggregate stabilization, inoculation with osmolyte‑producing microbes, and modular, resilient, food web architectures to sustain SOC sequestration and soil health.