Distinct stabilization characteristics of organic carbon in coastal salt‐affected soils with different salinity under straw return management

Abstract Straw returning effectively promotes the soil organic carbon (SOC) pool, but the straw‐induced change in SOC quality (C fraction) remains unclear, particularly in saline soil. Here, we investigated how straw returning and soil salinity (low, medium, and high) affect SOC fractions, including easily oxidized and particulate organic C (EOC and POC) and mineral‐associated organic C (MOC) at aggregate scales, including macroaggregates (MA), microaggregates (MI), and silt + clay particles. The results revealed that SOC fraction contents in the whole soil decreased with increasing soil salinity, and straw returning increased the total organic C (TOC) in both low‐ and medium‐salinity soils. In low‐salinity soil, straw returning induced decreases of 12.1% and 30.6% in MI‐associated EOC and POC, respectively, and promoted significant increases of MI‐associated MOC, indicating that 'hierarchical aggregation' theory (i.e., single soil particles gradually form clusters, MI and MA) dominated during soil aggregate formation. In medium‐salinity soil, straw application increased all SOC fraction contents in MA, and raised the POC/TOC of the whole soil by 56.6%, suggesting that the 'macroaggregate turnover' theory (MI is formed within MA) played a more important role in forming aggregates. Our results demonstrated that low‐ and medium‐salinity soils had different C‐increasing strategies under straw returning: MA contributed more to the increased SOC than MI in low‐salinity soil, whereas the reverse was true in medium‐salinity soil. These findings suggest that higher soil salinity would weaken the C sequestration potential of straw returning, and that targeted measures should be applied to enhance reclamation efficiency.