Electrochemical monitoring of shallow soils in the Susquehanna Shale Hills Critical Zone Observatory for real-time measurement of metal redox reaction dynamics

Abstract:

Metals in soils can be used as electron acceptors for anoxic respiration and as electron donors for lithotrophic microbes. These reversible redox reactions influence carbon mineralization and fixation, and they are regulated by variations in soil moisture and resultant oxygen dynamics. Recent work from the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) in Central Pennsylvania confirms the importance of water content to soil iron redox, indicating widespread iron redox cycling in response to precipitation. In this study, we tested the ability of electrochemical monitoring to serve as a real-time nondestructive measurement of dynamic metal redox reactions in the valley floor of a shale watershed at the SSHCZO. Graphite electrodes were installed at 50 cm and 70 cm and poised at 100 mV and 400 mV (vs. SHE) to mimic iron oxides and manganese oxides, respectively. Current through the electrode systems was measured continuously, with positive current indicating the working electrode was receiving electrons as a proxy of metal reduction and negative current indicating the electrode was delivering electrons similar to metal oxidation. This chronoamperometric monitoring was partnered with adjacent measurements of soil porewater chemistry, pCO₂, pO₂, moisture content, temperature, and precipitation. The electrochemical data were consistent with all other measurements, indicating periods of anoxic respiration following precipitation events and elevated soil moisture as well as the oxidation of reduced metals as the water table receded and oxygen became available. These electrode systems are presently being used for cyclic voltammetry measurements to reveal whether discrete redox signatures can be identified from the electrodes fixed at these two distinct potentials. This study is the first time that fixed potential electrodes have been used successfully to track fine temporal scale fluctuations in microbially-mediated iron and manganese redox in situ in upland soils.