Aggregate structure and microbial shifts shape carbon stability in saline–alkaline soils

Soil salinization and alkalization are widely associated with reduced agricultural productivity and soil carbon sequestration, and are accompanied by changes in soil physicochemical properties and microbial processes. The differential responses of labile and recalcitrant soil organic carbon (SOC) pools to different salinization types remain poorly understood. Here, we investigated saline, sodic, and secondary salinized soils across salinity gradients in the Ningxia Irrigation District, analyzing soil properties, aggregate structure, carbon fractions, enzyme activities, and microbial communities. We found that higher salinity was associated with lower macroaggregate proportions, >60% reductions in labile carbon pools (POC, DOC, and MBC), and decreased enzyme activities and microbial diversity, whereas mineral-associated organic carbon (MAOC) showed comparatively smaller variation. Partial least squares structural equation modeling (PLS-SEM) revealed distinct association patterns among soil types. In HN, labile carbon was most strongly associated with enzyme activity, whereas MAOC was associated with salinity stress and mechanical aggregates. In XDT, salinity stress showed the strongest negative association with labile carbon, while nutrient status was positively associated with enzyme activity and aggregate structure. In HSB, nutrient status was strongly associated with aggregate structure, and water-stable aggregates were positively associated with both labile and MAOC pools. These findings indicate that SOC responses under salinization vary among soil types and exhibit distinct association patterns rather than a single universal pattern.