Rice Cultivation Duration Drives Soil Organic Carbon Stabilization in Saline–Alkaline Paddy Soils via Mineral-Associated Organic Carbon Accumulation and Biological Pathways

Continuous paddy reclamation is a promising strategy for restoring sodic lands, yet the exact mechanisms driving soil organic carbon (SOC) stabilization remain insufficiently quantified. Here, we investigated SOC dynamics across a 12-year rice cultivation chronosequence (0, 2, 5, 10, and 12 years) at 0–20 cm and 20–40 cm depths in western Jilin Province, China. Successive rice cultivation progressively alleviated saline–alkaline stress, with electrical conductivity (EC) decreasing by 70.11% in topsoil after 12 years, establishing a stabilized soil environment by years 10–12. Concurrently, topsoil SOC and total nitrogen (TN) increased by 63.09% and 26.02%, respectively. This physicochemical amelioration triggered a directional carbon transformation: while particulate organic carbon (POC) accumulated in early stages, mineral-associated organic carbon (MAOC) dominated medium-term storage, expanding by 147.12% in topsoil and elevating its share of SOC. Fourier transform infrared (FTIR) spectroscopy confirmed a shift toward molecular structural persistence, marked by increased aromaticity and hydrophobicity. Partial least squares path modeling (PLS-PM) demonstrated that cultivation duration directly drove soil stability (β = 0.94, p < 0.001), operating through a hierarchical cascade where management-induced stress reduction enhanced soil enzyme activity and microbial processing, thereby accelerating the conversion of labile plant inputs into mineral-protected MAOC. Overall, this study quantifies the pivotal role of paddy management in driving organo-mineral protection and chemical persistence, providing a mechanistic framework for carbon sequestration in degraded agroecosystems.