Straw and nitrogen fertilization driving iron oxide transformation and soil carbon sequestration in saline paddy soils

The “enzyme latch” and “iron gate” mechanisms play essential roles in regulating soil organic carbon (SOC) cycling in coastal saline paddy soils; however, their contributions to SOC sequestration under straw incorporation and nitrogen (N) fertilization remain poorly understood. A seven-year field experiment was conducted in a coastal saline paddy soil with six treatments, including two N levels (N1: 255 kg N ha -1 and N2: 400 kg N ha -1 ) and three straw incorporation levels (S0: 0, S1: 4,500 kg straw dry mass ha −1 , and S2: 9,000 kg straw dry mass ha -1 ), with three field replicates per treatment. Moderate straw incorporation generally increased the proportion of large macroaggregates, improved aggregate stability, and promoted the accumulation of amorphous and complexed iron, as well as Fe-bound organic carbon (Fe-OC). Reduced N fertilization decreased phenol oxidase activity and cumulative CO 2 emissions compared with the conventional N level. Under the reduced N level, moderate straw incorporation increased SOC content by 8.5% compared with no straw incorporation, which was associated with lower CO 2 emissions and greater Fe-OC formation. Overall, moderate straw incorporation combined with reduced N fertilization was most favorable for SOC sequestration in coastal saline paddy soils. Therefore, in coastal saline paddy soils, the “iron gate” and “enzyme latch” mechanisms likely acted together to promote Fe-OC formation, thereby enhancing SOC sequestration.