Coupled dynamics of iron and iron-bound organic carbon in forest soils during anaerobic reduction

The behavior of iron (Fe)-bound organic carbon (OC) under anoxic conditions in natural soils and sediments represents a critical knowledge gap for understanding the biogeochemical cycles of OC and Fe. In this study, we investigated the dynamics of Fe and OC in four forest soils in the presence of the dissimilatory Fe-reducing bacterium, Shewanella oneidensis MR-1. Over an 8-day reduction period, 3.8–9.9% of total OC was released to solution in conjunction with the reduction of 12.5–37.7% of reactive Fe. The fraction of OC released was correlated with the fraction of Fe reduced, indicating that the reductive release was the controlling factor for the mobilization of OC upon the anaerobic microbial reaction. During the reduction, the fractions of poorly crystalline Fe oxides decreased, coupling with an increase in the relative abundance of crystalline Fe oxides. Lability of OC (as reflected by water-extractable OC content) increased after microbial reduction, indicating the decreased stability of OC because of changes in mineral-OC interactions and the conformation of mineral-OC complexes. The reduction of Fe was closely related to bulk soil electron accepting capacity (0.15–0.34 mmol e⁻/mol C). Our findings demonstrate that the redox reactions of Fe, modified by the redox reactivity of OC, play an important role in regulating the stability and transformation of OC.