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Coupled Biogeochemical Cycles of Iron and Organic Carbon in Soils
Civil and Environmental Engineering
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Soil organic carbon (OC) is one of the largest carbon (C) reservoirs on the Earth’s surface. Because of the high sorption affinity of iron (Fe) minerals for OC, the redox reactions of Fe potentially play an important role in regulating the stability and transformation of OC in soils. Fate of Fe-bound OC in natural soils upon Fe redox reactions is a critical knowledge gap for understanding the coupled biogeochemical cycles of C and Fe.This study comprehensively investigated the amount and characteristics of Fe-bound OC in forest soils as well as the coupled biogeochemical reactions of Fe and OC during redox processes. Iron-bound OC contributed substantially to total organic carbon (TOC) in forest soils, representing an important component of C cycles in terrestrial ecosystems. The ecogeographical parameters, such as latitude and annual mean temperature, are governing factors for the fraction of Fe-bound OC in TOC (fFe-OC). Iron-bound OC was less aliphatic, more carboxylic, and more enriched in 13C, compared to non-Fe-bound OC. Our studies also demonstrated the closely coupled biogeochemical reactions of Fe and OC during redox processes. We found that microbial reduction of Fe can lead to substantial mobilization of OC in natural soils under anaerobic incubation. OC electron accepting capacity (EAC) strongly regulated Fe reduction, demonstrating that the biogeochemical cycles of Fe and OC are coupled together through two-way interactions. After transferring to the aerobic condition, Fe(II) in pre-reduced soils was oxidized in conjunction with oxidation of OC. OC oxidation was much lower for soils exposing to the anaerobic-aerobic transition, compared to soils only aerobically incubated, potentially because of secondary Fe minerals formed during the transition sequestrating OC. These results provide novel insights into the impact of anaerobic-aerobic transitions on the dynamics of OC in ecosystems undergoing the anaerobic-aerobic transitions frequently. Therefore, we argue that it is critical to include the redox reactions in biogeochemistry models for evaluating and predicting C stability and cycles.