Biodegradation of Ferrihydrite-Organic Carbon Coprecipitates.

Stabilization of organic matter (OM) in soil is important to preserve carbon (C) and mitigate greenhouse gas emission. Due to the high specific surface area and variably charged surface hydroxyl groups, the association of OM with Fe (oxyhydr)oxides is an important mechanism for soil C stabilization. Among the different Fe (oxyhydr)oxides, the short-range ordered Fe minerals, (e.g., ferrihydrite (Fh)) are particularly important sorbents for OM and often co-precipitate with OM when formed in OM-rich solutions. But this stabilization mechanism can be affected by the change in redox conditions, which drive Fe oxidization and reduction. In the presence of O₂, biodegradation of the coprecipitated OM on Fh could be inhibited leading to long-term soil C stabilization. However, the anoxic condition drives dissimilatory Fe reduction, which could subsequently control the release and anaerobic mineralization of Fh-associated OM. The aim of this study is to perform microbial incubation experiments under both oxic and anoxic conditions to determine the fate of coprecipitated OM associated with Fh. In this study, Fh-OM coprecipitates (C/Fe ratio 1.6) will be synthesized using humic acid as OC source and both aerobic and anaerobic incubations will be performed with two microbial systems: a microbial consortium extracted from soil and a pure culture of the Fe reducing bacteria Shewanella oneidensis MR-1. During the incubation period, we will track Fe (II), dissolved organic carbon and CO₂ production to quantify Fe reduction, OC mobilization, and mineralization rates. The results of this study will help to elucidate the importance of Fh-OM coprecipitation on soil C stabilization.