Biogeochemical Fate and Stability of Iron Oxide-Organic Carbon Complexes.

Soil organic carbon (OC) is one of the largest carbon (C) reservoirs on the Earth’s surface, and understanding soil OC residence time is crucial for accurately modeling global C dynamics. Soils also contain a high quantities of iron (Fe) minerals, which can strongly bind with OC depending on the chemical composition of the molecules. Additionally, soils constantly experience transient anaerobic conditions that promote reductive dissolution of Fe(III) minerals and compromise the stability of Fe-bound OC. Thus, understanding which functional groups are responsible for the formation of Fe-OC complexes and evaluating the fate of Fe-bound OC under Fe reducing conditions is important for predicting OC residence time in soils. More specifically, we studied the fate and stability of Fe oxide-OC complexes during abiotic and biotic Fe(III) reduction.

During the abiotic reduction of Fe in hematite-OC complexes, the release rate for Fe bound-OC to solution was faster than the rate of Fe reduction.¹ Aromatic OC was released during the early stage of reduction, and aliphatic OC was enriched in the residual fraction.^1,2These results partially explained the widely-observed accumulation of aliphatic OC.

During the microbial reduction of ferrihydrite (Fh)-OC co-precipitates by Shewanella putrefaciens strain CN32, higher C/Fe ratios in the co-precipitates facilitated Fe reduction and subsequent reductive release of Fe-bound OC.³ However, aromatic and carboxylic OC were preferentially retained in the complex during the reduction. For Fh-OC co-precipitates synthesized using various model organic compounds (alginate, amylose, benzoquinone, glucosamine, glucose, and tyrosine), the presence of OC with high electron shuttling capacity increased the rate of microbial Fe reduction.⁴

Our results highlight that Fe mineral phase, C/Fe ratio, and OC chemical composition are all important factors regulating the fate of Fe-bound OC during Fe reduction. Such information will help develop process-based models for predicting C biogeochemical cycle in terrestrial environments.