Differential Soil Organic Carbon Accumulation Patterns Following Cropland-to-Grassland Conversion in Non-Saline and Saline–Alkali Soils

Agricultural expansion and intensification generally lead to a depletion in soil organic carbon (SOC). While converting cropland to grassland is a recognized strategy for SOC accumulation, the patterns of SOC accumulation under different grassland types and soil conditions remain unclear. This study evaluated the long-term effects of two perennial grasses—alfalfa (a legume) and switchgrass (a non-legume)—on SOC composition, specifically lignin phenols and amino sugars, in non-saline and saline–alkali soils, using a conventional wheat–maize rotation as a control. Our results showed that both alfalfa and switchgrass significantly enhanced SOC content compared to a wheat–maize rotation, but their accumulation pathways differed between non-saline and saline–alkali soils. In non-saline soils, increases in both lignin phenols and amino sugars (muramic acid and glucosamine) were observed under both perennial grasses. In saline–alkali soils, however, the accumulation was primarily driven by glucosamine. While no significant difference was observed in amino sugars content between the two grasses, switchgrass showed significantly higher lignin phenols content than alfalfa under saline–alkali conditions. This indicated that litter quality regulated the accumulation of plant-derived C in saline–alkali environments, but has no significant impact on the accumulation of microbial-derived C. These findings elucidate the divergent mechanisms that drive SOC sequestration following cropland-to-grassland conversion in contrasting non-saline and saline–alkali soils, highlight the dominant role of microbial processes in SOC accumulation following such conversion.