(213a) Novel Field Deployable Sensors to Monitor How Dynamic Hydrology Shapes Nutrient and Element Transformations in a Great Lakes Coastal Estuary: A Two-Year Study

The propensity for redox processes to occur is often predicted using measured or assumed redox potential (E_h). However, process measurements often deviate from predictions based on the traditional, thermodynamic “redox tower” paradigm. Previous work has shown that poorly measured, granular scale spatial and temporal soil heterogeneity causes apparent departures from thermodynamic exclusion principles at bulk scales, with consequences for large-scale biogeochemical cycling which are not currently resolved in ecosystem, regional, or global scale models.

To help study these processes at a smaller scale, we developed a 3D printed graphite electrode array to perform zero resistance ammetry (ZRA) measurements to measure redox disequilibria at various spatial scales in benthic sediments. This allowed us to detect the distributions, extents, and kinetics of biogeochemical processes occurring in near real-time. ZRA was used to measure electrical current that arose from microbially-induced redox disequilibrium. Prior to deployment in the field, we characterized the 3D printed electrodes using cyclic voltammetry and chronocoulometry. We also used the 3D printed graphite electrodes in laboratory-scale experiments mimicking various posions on the redox ladder using a split cell setup containing poorly crystalline Fe(III) (hydr)oxide-rich soils. When both chambers were incubated under oxic or anoxic conditions, no current was observed. However, when one chamber was oxic and the other anoxic, no Fe(III) reduction was observed in the oxic chamber and Fe(II) accumulated in the anoxic chamber. The redox disequilibrium generated between chambers was accompanied by electron transfer from the anoxic to the oxic chamber detected as ZRA.

We then refined the design, construction, and deployment of a paired ZRA and Eh multi-sensor system to detect electrochemical signals across and directly beneath the sediment-water interface at nested scales (sub-millimeter to decimeter) at a shallow location (< 10 cm surface water) in the Old Woman Creek wetland. During the summers of 2022 and 2023, we deployed these sensors and collected concurrent data on dissolved oxygen dynamics, surface and pore water nutrient concentrations, greenhouse gas fluxes (chamber measurements), and soil geochemistry. Future work will use this data to demonstrate the feasibility of integrating microsite electrochemical and redox variability to the ecosys ecosystem scale model for improved representation of soil redox processes in spatially variable and temporally fluctuating systems. Field data shows the success of a vertically stacked ZRA electrode at monitoring microbial processes at the small-scale.