Using Fixed-Potential Electrodes To Quantify Iron And Manganese Redox Cycling In Upland Soils

Abstract

Changes in metal redox in soils can exert strong controls on carbon mineralization but are difficult to measure in real time. Recently, potentiostatically poised electrodes (fixed-potential electrodes) have been demonstrated as promising for measuring the rate of oxidation and reduction at a specific reduction potential in situ in riparian soils but are yet untested in upland soils. Here for the first time, we explored the fine-scale temporal fluctuations of redox of both iron and manganese in response to environmental conditions. We used three-electrode systems with working electrodes fixed at 100 mV (vs. SHE) in 2019 and added 400 mV in 2020 at 50 cm and 70 cm in the valley floor soil of a headwater watershed at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO). Electrodes fixed at 100 mV mimic iron oxides and at 400 mV mimic manganese oxides, and real-time reduction and oxidation rates can be calculated from measured changes in the electric current over time. Alongside the electrodes, soil porewater chemistry, pCO2, pO2, groundwater level, and precipitation were measured. Results indicate that fixed-potential electrodes successfully detected temporally fine-scale fluctuations in metal redox state, which was confirmed by the coordinated datasets. Water table fluctuations at the electrode depth drove metal reduction, and rainfall events stimulated oxidation reactions in the vadose zone. For the first time in soils, we directly measured the frequency, period, and amplitude of oxidation and reduction events. All of these are key variables that control the biogeochemical impact of metal oxide redox in terrestrial systems. At the SSHCZO we observed multi-day reduction or oxidation events with a return interval of 5 – 10 days, controlled by precipitation frequency. Such measurements with fixed-potential electrodes hold promise for accurately exploring the fast-changing biogeochemical impacts of metal redox in upland soils where such reactions have been difficult to quantify.