Spatially Explicit Linkages Between Redox Potential Cycles and Soil Moisture Fluctuations

Reduction‐oxidation cycles measured through soil redox potential (Eh) are associated with dynamic soil microbial activity. Understanding changes in the composition of, and resource use by, soil microbial communities requires Eh 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 Eh, 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 Eh dynamics, and the minimum amount of biogeochemistry needed to characterize the dynamics of electron donors/acceptors that are responsible for the patterns of Eh 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. Plain Language Summary Redox potentials measured in a set of lysimeter experiments for natural soil columns were contrasted by depth‐time computations to single out the actual controls on redox cycling. Our results show that the dynamics of soil saturation is not always directly linked to oxic/anoxic transitions generating the kind of changes in the reducing or oxidating conditions that require adaptations by soil microbial communities. In particular, the rate of change imposed to the hydrologic forcings proves to be a key and so far unrecognized actor. We believe that our results will prove important because pre‐requisites are established for understanding the adaptation of soil microbial communities in res