Study on the Coupling and Turnover Process of Organic Carbon and Iron Minerals in Paddy Soils

Paddy soils undergo long-term redox alternations that profoundly affect the biogeochemical cycles of iron (Fe) and carbon (C).In this study,we combined physical fractionation,mineral characterisation and organic matter analysis to investigate how redox fluctuations regulate Fe mineral transformation,particle aggregation and soil organic matter (SOM) stability.By comparing the surface (leachate layer) and bottom (aggregate layer) layers of long-term paddy soils and non-paddy soils,we analysed the Fe-C distribution characteristics of particles and aggregates with different grain sizes and resolved Fe-phase variations using mineralogical means (X-ray absorption spectroscopy).Our results show that although the hydrated iron oxides in the surface paddy soils are low,their binding to minerals (especially in fine particles rich in low-crystallinity iron oxides) significantly enhances the stability of organic carbon;redox cycling enhances the structural stability of the microaggregates and alters the pattern of Fe-C distribution inside and outside the agglomerates;and,in the subsoil,the paddy soils accumulation layer is rich in short-range ordered hydrated Fe oxides (ferrihydrous sodalite),which promote microagglomerate formation and enhance carbon sequestration,an effect that is not evident in non-paddy soils.This study reveals the key mechanism of redox alternation regulating the stability of SOM through Fe-mineral-organic matter interactions,which provides a theoretical basis for the management of soil carbon sinks in rice paddies.