Using fixed-potential electrodes to quantify iron and manganese redox cycling in upland soils

Although metal redox reactions in soils can strongly affect carbon mineralization and other important soil processes, little is known about temporal variations in this redox cycling. Recently, potentiostatically poised electrodes (fixed-potential electrodes) have shown promise for measuring the rate of oxidation and reduction at a specific reduction potential in situ in riparian soils. Here for the first time, we used these electrodes in unsaturated soils to explore the fine-scale temporal redox fluctuations of both iron and manganese in response to environmental conditions. We used three-electrode systems with working electrodes fixed at 100 mV (vs. SHE) and 400 mV at 50 cm and 70 cm in the valley floor soil of a headwater watershed. Electrodes fixed at 100 mV to mimic iron oxides and at 400 mV to mimic manganese oxides allowed real-time reduction and oxidation rates to be calculated from temporal variations in the electric current. Electrode measurements were compared to soil porewater chemistry, pCO(2), pO(2), groundwater level, resistivity measurements, and precipitation. The fixed-potential electrodes recorded fluctuations over timescales from minutes to weeks. A consistently negative current was observed at 100 mV (interpreted as oxidation of Fe), while the 400-mV electrode fluctuated between negative and positive currents (Mn oxidation and reduction). When the water table rose above the electrodes, reduction was promoted, but above the water table, rainfall only stimulated oxidation. Precipitation frequency thus drove the multi-day reduction or oxidation events (return interval of 5–10 days). These measurements represent the first direct detections of frequency, period, and amplitude of oxidation and reduction events in unsaturated soils. Fixed-potential electrodes hold promise for accurately exploring the fast-changing biogeochemical impacts of metal redox cycling in soils and represent a significant advance for reactions that have been difficult to quantify.