Rice Cultivation Alters Soil Aggregates by Changing the Distribution of Humic Substances in Saline–Sodic Soils

Rice cultivation is widely used for the reclamation of saline–sodic soils. However, the mechanisms by which prolonged flooding alters soil chemical conditions and regulates carbon redistribution and stabilization across the soil profile remain unclear. This study compared soils reclaimed for 6 years (R6) and 17 years (R17) with unreclaimed saline–sodic soil (CK) in the Songnen Plain, Northeast China, and evaluated changes across three depths (0–20, 20–40, and 40–60 cm). Reclamation significantly improved aggregate stability, with corresponding increases in mean weight diameter and water-stable aggregates. R17 and R6 promoted greater soil organic carbon (SOC) retention within macroaggregates and increased humic substance concentrations, indicating improved structural protection of carbon. The fulvic/humic acid (FA/HA) ratio increased with depth under flooded conditions, suggesting greater fulvic acid mobility. Although HA and humin (HM) decreased with depth, their concentrations, particularly the HM/SOC ratio, remained higher and more stable in R17. Reductions in salinity acted as a key mediating pathway, regulating carbon redistribution across the soil profile, with mobile carbon fractions destabilizing surface aggregates but promoting organo-mineral bonding and aggregate formation at subsurface depths (20–40 cm). Overall, these findings indicate that rice-based reclamation stabilizes carbon via interconnected processes of salinity reduction, vertical carbon redistribution, and aggregation driven by carbon quality, highlighting subsurface layers as essential for long-term carbon stabilization in saline–sodic soils.