Redox-driven iron mechanisms regulating soil organic carbon stabilization in paddy soils under warming and altered flooding

Paddy soils represent a globally significant agroecosystem, functioning both as the primary environment for irrigated rice (Oryza sativa L.) production and as extensive anthropogenic wetlands with a major role in soil organic carbon (SOC) sequestration. Under climate change, increasing temperatures and reduced water availability threaten soil health, carbon (C) stability, and the long-term resilience of these systems.

This study evaluates how anaerobic digestate application influences soil organic matter (SOM) dynamics in paddy soils under simulated climate stress. A factorial field experiment tested amendment application (digestate, DS; unamended control, UN), temperature (ambient, AM; warming, +2 °C, WR), and water regime (normal flooding, NF; reduced flooding, - 30%, RF). SOM was fractionated into particulate (POM) and mineral-associated organic matter (MAOM) pools, which were characterized using Fe K-edge XANES and k²-weighted EXAFS to resolve Fe speciation and coordination environments controlling C stabilization.

Across treatments, SOC declined under warming, with WR plots losing up to 15% more SOC than AM, while digestate under NF partially mitigated SOC losses and reduced C/N ratios, indicating enhanced microbial processing.

XANES revealed strong fraction- and management-dependent shifts in Fe speciation, showing that POM was enriched in redox-sensitive Fe phases and Fe-organic complexes that responded markedly to reduced flooding and warming, whereas MAOM was dominated by illite-associated Fe and Fe (III) oxyhydroxides. Complementarily, k²-weighted EXAFS resolved the short-range Fe coordination environment, indicating that POM contained mixed crystalline Fe (III) phases (hematite and lepidocrocite) embedded within a variable mineral matrix, while MAOM was systematically enriched in poorly ordered Fe (III) phases, including ferrihydrite-like and Fe (III)-organic associations, indicative of persistent mineral-protected C pools. Together, XANES and EXAFS demonstrate that climate stress primarily destabilizes SOC by disrupting redox-controlled Fe-organic associations in the labile POM fraction, whereas long-term carbon persistence under future warming scenarios depends on the maintenance of Fe (III) oxyhydroxide-mediated protection within MAOM, only weakly modulated by organic amendment and water regime.