Spatially Explicit Linkages Between Redox Potential Cycles and Soil Moisture Fluctuations

Reduction-oxidation cycles measured through soil redox potential (E-h) are associated with dynamic soil microbial activity. Understanding changes in the composition of, and resource use by, soil microbial communities requires E-h predictability under shifting hydrologic drivers. Here, 50-cm soil column installations are manipulated to vary hydrologic and geochemical conditions, and are extensively monitored by a dense instrumental deployment to record the depth-time variation of physical and biogeochemical conditions. We contrast measurements of E-h, soil saturation and key compounds in water samples (probing the majority of soil microbial metabolisms) with computations of the relevant state variables, to investigate the interplay between soil moisture and redox potential dynamics. Our results highlight the importance of joint spatially resolved hydrologic flow/transport and redox processes, the worth of contrasting experiments and computations for a sufficient understanding of the E-h dynamics, and the minimum amount of biogeochemistry needed to characterize the dynamics of electron donors/acceptors that are responsible for the patterns of E-h not directly explained by physical oxic/anoxic transitions. As an example, measured concentrations of sulfate, ammonium and iron II suggest coexistence of both oxic and anoxic conditions. We find that the local saturation velocity (a threshold value of the time derivative of soil saturation) exerts a significant hysteretic control on oxygen intrusion and on the cycling of redox potentials, in contrast with approaches using a single threshold saturation level as the determinant of anoxic conditions. Our findings improve our ability to target how and where hotspots of activity develop within soil microbial communities.