Water-saving irrigation mitigates methane emissions from paddy fields: The role of iron

Iron (Fe) and methane (CH4) emissions play crucial roles in the carbon cycling of paddy field ecosystems. However, little is known about the effect and mechanism of Fe transformation on the CH4 emission from paddy fields, especially with different water management methods. In this study, the dynamic changes in CH4 emissions and different forms of Fe and soil organic carbon content were observed in paddy fields under flooded irrigation (FI) and water-saving irrigation (WSI) (including intermittent irrigation (II) and controlled irrigation (CI)). In addition, the structural equation model was used to clarify the response relationship between CH4 emissions and Fe ions in paddy fields. The results indicate that the cumulative CH4 emissions under WSI were reduced by 29.03–61.29 %. The reasons are as follows (i) WSI increased the oxidation ability of Fe to CH4 by increasing the soil ferric iron (Fe3+) content in paddy fields; (ii) WSI could reduce the substrate utilization by methanogens by transforming the Fe oxide forms to increase the binding capacity of Fe to dissolved organic carbon, which reduced the CH4 production. In addition, compared with II, CI had higher soil Fe3+ content, Fe-bound organic carbon content, and lower acetate content, so it had lower CH4 production capacity. In summary, WSI promotes the direct oxidation of CH4 by Fe3+ in paddy fields and indirectly reduces CH4 production by facilitating the combination of Fe oxides with the substrates. These findings deepen our understanding of the mechanisms of CH4 emission in paddy fields and provide new insights for research on carbon sequestration and emission reduction in paddy field ecosystems.