Interactive Effects of Salinity and Land Use Changes on Depth-Dependent Soil Organic Carbon Fractions and Biological Activity

Land-use change (LUC) and salinization interact synergistically to regulate depth-dependent fractionation and biological mediation of soil organic carbon (SOC) in vulnerable agroecosystems. Unlike previous syntheses addressing these drivers separately, the present review integrates them within a depth-resolved biological framework to reveal their combined effects on fraction-specific distribution under contrasting anthropogenic and ionic regimes. In the topsoil (0–30 cm), LUC and salinity synergistically collapse fungal networks, suppress carbon use efficiency, and restructure microbial communities to accelerate particulate organic matter (POM) turnover and impair mineral-associated organic matter (MAOM) formation. In the subsoil (>30 cm), salinity-driven clay dispersion and pore occlusion restrict oxygen diffusion and carbon accessibility, while LUC-induced loss of deep-rooting vegetation reduces carbon supply to mineral-associated pools. These depth-decoupled mechanisms render subsoil MAOM relatively resilient to direct ionic stress but highly vulnerable to land-use legacy, a distinction rarely represented in existing conceptual models. The evidence highlights key management implications, including restoring biological complexity in topsoil through reduced tillage, mycorrhizal re-establishment, and osmotic stress alleviation; conserving subsoil carbon by restoring deep-rooting vegetation and maintaining favorable ionic conditions for organo-mineral stabilization; and using depth-specific biomarkers, including enzymatic stoichiometry, fungal-to-bacterial ratios to detect SOC vulnerability before measurable losses occur. Future research should prioritize depth-explicit monitoring and integrated biological–physicochemical approaches to improve predictions of SOC dynamics. The resulting framework provides a mechanistic basis for depth-differentiated carbon management in salinizing landscapes.