Coupled Dynamics of Iron and Iron-Bound Organic Carbon in Forest Soils during Anaerobic Reduction.

Process for iron (Fe)-bound organic carbon (OC) in natural soils upon Fe reduction is 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 reacting with dissimilatory Fe-reducing bacteria, Shewanella oneidensis MR-1. During the reaction with S. MR-1 for 8 days, 12.6-37.7% of Fe was reduced with 0.02-4.03% released as dissolved Fe(II), and 3.8-9.9% of OC was released. The fraction of released DOC compared to Fe-bound OC was correlated with the fraction of Fe reduced, indicating the reductive release was the controlling factor for the mobilization of OC upon the microbial anaerobic reaction. Bulk and surface Fe mineral phase was changed from poorly crystalline Fe oxides-dominated to crystalline Fe oxides-dominated. Lability of OC increased after the reduction, indicating the stability of OC was decreased after the reduction because of change in mineral-OC interactions and conformation of mineral-organic matter complexes. The reduction of Fe was closely related to the electron accepting capacity of OC, ranging 0.28-0.48 mmol e^-/mol C. The role of OC in redox reaction of Fe was further supported by the change in semi-quinone functional groups in soil. Our findings demonstrate that the redox reactions of Fe, controlled by the redox reactivity of OC, play an important role in regulating the stability and transformation of OC.