Redox Fluctuations Across Capillary Fringe Drive Dynamics in Iron-Organic Matter Binding, Bioavailability of Dissolved Organic Carbon and Microbial Metabolic Potential

Abstract:

Soil minerals, particularly iron (Fe) oxides, are known to stabilize and protect soil organic carbon from microbial degradation and carbon (C) mineralization. Recent experimental studies show that certain environmental factors such as anoxia, elevated moisture, and input of root exudates can counteract such protective mechanisms. Reductive dissolution of Fe-OM complexes may lead to C loss by leaching as dissolved organic carbon (DOC) which is microbially more accessible. In terrestrial subsurface environment, capillary fringe is a highly redox dynamic zone with frequent changes in hydrology and geochemistry thus, fueling microbial activity. So far, the Fe-OM biogeochemistry at capillary fringe is poorly examined, and its impacts on microbial activity is not clear. In this study, we aim to address two knowledge gaps: (i) which sediment-derived DOM fractions get preferentially stabilized and protected via Fe-OM association, and which fractions may be released back to the DOC pool from Fe-OM decomposition due to redox fluctuation; and (ii) how does Fe-OM decomposition under redox fluctuation impact bioavailability of DOC pool and subsequent microbial metabolic potential in groundwater. Here we synthesize Fe-OM complexes via adsorption and co-precipitation, using ferrihydrite (Fh) and sediment-derived DOM, and conduct a redox incubation study under varying redox conditions to stimulate Fe-OM decomposition. The sediment samples including surface depth and capillary fringe depth were collected from ORR FRC background well FW306, Oak Ridge, TN. Water extractable DOM from the sediments has been obtained via shaking and sonication. Native microbes from FW306 groundwater will be added to the microcosms to assess the impacts of redox fluctuation on bioavailability of DOC and subsequent microbial metabolic potential. A combination of advanced analytical chemistry and molecular biology tools including XRD, EXAFS, STXM-NEXAFS, FTIR, FT ICR-MS, and metagenomics will be applied to characterize Fe-OM complexes, DOC pool, microbial community composition and metabolic potential particularly in Fe-, nitrate-, and sulfate reduction. Results from this study will provide an insight into redox controlling of Fe-OM binding and subsequent Fe and C cycling, as well as microbial mediated N and S cycling in groundwater.