Iron–carbon synergy mitigates carbon emissions via microbial and redox regulation in paddy fields of the Ningxia Yellow River irrigation zone

Iron oxides play curial roles in stabilizing soil carbon, but their impact on carbon emissions under anaerobic conditions, particularly in organically amended paddy fields, remains unclear. A pot experiment using alluvial soil from the Ningxia irrigation region explored the synergistic effects of organic amendments and iron oxides on soil carbon dynamics. Six treatments were established: CK (control, without organic amendments or ferrihydrite), B (biochar), S (straw), F (ferrihydrite), BF (biochar combined with ferrihydrite), and SF (straw combined with ferrihydrite). Soil samples were collected at the rice-heading stage to measure the soil carbon fractions, ferric oxide forms, and functional microbial abundances. Cumulative carbon dioxide (CO 2 ) and methane (CH 4 ) emissions were evaluated using anaerobic slurry incubation. Combined application of ferric oxide and organic materials reduced cumulative CO 2 and CH 4 emissions after 45 days of incubation compared to organic materials alone. The cumulative CO 2 emissions were positively correlated with microbial biomass carbon (MBC) and negatively correlated with free iron oxide (Fed), suggesting that ferric oxides suppressed microbial carbon mineralization by stabilizing organic matter and competing with microbial respiration for electrons under anaerobic conditions. This trend was consistent with the highest amorphous ferric oxide (FeO) and Geobacter abundances being observed in the BF, which also exhibited the lowest MBC and CO 2 emissions. FeO had the strongest negative correlation with cumulative CH 4 emissions, suggesting that iron reduction under anaerobic paddy field conditions suppressed methane production stimulated by organic amendments. These results establish a comprehensive theoretical framework for optimizing soil fertility and minimizing carbon emissions in paddy field systems.