Redox‐Induced Transformations of Iron Oxides Enhance Soil Organic Carbon Stabilization in an Estuarine Soil

Abstract We investigated temporal changes in operational Fe pools and their association with soil organic carbon (SOC) fractions in an estuarine soil affected by the deposition of iron‐rich mine tailings over 6 years in Southeast Brazil. Sequential Fe extractions revealed a 56% decrease in Fe extracted with citrate–bicarbonate–dithionite alongside a 50% increase in Fe extracted with citrate–ascorbate, consistent with a shift from more crystalline Fe oxide pools toward poorly crystalline Fe phases. Over the same period, Fe‐bound organic carbon increased by 61% and mineral‐associated organic carbon (MAOC) increased by 25%, whereas particulate organic carbon remained relatively stable. These patterns suggest that SOC accrual was preferentially associated with the mineral‐associated fraction rather than with particulate organic matter accumulation. After six years, Fe‐bound organic carbon represented 44 ± 9% of MAOC and 36 ± 9% of total SOC, highlighting the importance of Fe‐mediated organo‐mineral associations for SOC retention under dynamic estuarine redox conditions. Overall, our results provide field‐based evidence that Fe geochemical changes and substantial SOC accrual can be closely coupled in redox‐active coastal wetland soils over relatively short timescales. These findings provide insights into how Fe‐rich anthropogenic substrates may interact with SOC under redox‐active estuarine conditions and may inform the design of constructed wetlands as potential carbon retention systems, provided that ecological and geochemical risks are carefully constrained.