Electron Shuttling Capacity of Solid-Phase Organic Matter in Forest Soils

Abstract:

Soil organic matter, as an electron shuttle, plays an important role in regulating the biogeochemical cycles

of metals, especially the redox reactions for iron. Microorganisms can reduce soil organic matter under

anaerobic conditions, and biotically-reduced soil organic matter can abiotically donate electrons to ferric

oxides. Such soil organic matter-mediated electron transport can facilitate the interactions between

microorganisms and insoluble terminal electron acceptors, i.e. iron minerals. Most previous studies have

been focused on the electron shuttling processes through dissolved soil organic matter, and scant

information is available for solid-phase soil organic matter. In this study, we aim to quantify the electron

accepting capacity for solid-phase organic matter in soils collected from four different forests in the

United States, including Truckee (CA), Little Valley (NV), Howland (ME) and Hart (MI). We used

Shewanella oneidensisMR-1 to biotically reduce soil slurries, and then quantified the electrons

transferred to solid-phase and solution-phase organic matter by reacting them with Fe(III)-nitrilotriacetic

acid (Fe(III)-NTA). The generation of Fe(II) was measured by a ferrozine assay to calculate the electron

accepting capacity of soil organic matter. Our preliminary results showed that the Truckee soil organic

matter can accept 0.51±0.07 mM e-/mol carbon. We will measure the electron accepting capacity for four

different soils and correlate them to the physicochemical properties of soils. Potential results will provide

information about the electron accepting capacity of solid-phase soil organic matter and its governing

factors, with broad implication on the coupled biogeochemical cycles of carbon and iron.