In the predominantly oxic, upland soils, periods of high wetness trigger anaerobic processes such as iron (Fe) reduction within the soil microsites, with implications for organic matter decomposition, the fate of pollutants, and nutrient cycling. In fluctuating O conditions, Fe reduction is maintained by the re-oxidation of ferrous iron, which renews the electron acceptor, Fe, for microbial Fe reduction. To characterize such processes, it is fundamental to relate the redox cycling of iron between the two redox states to the hydro-climatic conditions. Here, we link iron cycling to soil moisture variability through a model of iron-redox dynamics and find the hydrologic regime that maximizes Fe reduction, under non-limiting organic carbon availability. Away from the optimal cycle, the duration of the oxic or the anoxic phase limits the regeneration of Fe or its reduction rate, respectively. We relate the average duration of the oxic and anoxic intervals to the frequency and mean depth of precipitation events that drive the dynamics of soil moisture, effectively linking iron cycling to the hydrologic regime. We then compare a tropical (Luquillo CZO) and a subtropical (Calhoun CZO) forest to provide insights into the soil moisture control on iron-redox dynamics in these ecosystems. The tropical site maintains a high potential for iron reduction throughout the year, due to quick and frequent transitions between oxic and anoxic conditions, whereas the subtropical site is strongly affected by seasonality, which limits iron reduction to winter and early-spring months with higher precipitation and lower evaporative demand.

Temporal redox fluctuations alter the pools of reducible FeIII and greenhouse gas emissions in humid upland soils. However, it is less clear how the characteristics of these fluctuations (length, frequency, amplitude) impact biogeochemical rates. We hypothesized that anaerobic rates of FeIII reduction and CH4 emissions are sensitive to the length of soil oxygen deprivation. To test this hypothesis, we exposed a surface soil from the Luquillo Experimental Forest to three lengths of O2 perturbation during repeated redox oscillations: an anoxic interval of 6 d with oxic intervals of 8, 24, or 72 h. We found that shorter oxic intervals resulted in more anaerobic FeIII reduction, while longer oxic intervals stimulated higher anaerobic CH4 emissions (CO2 fluxes did not change). We propose that short O2 pulses stimulate Fe reduction by resupplying the FeIII electron acceptor, but do not last long enough to inhibit microbial Fe reducers; conversely long O2 pulses suppress microbial iron reducers to a greater extent than methanogens leading to enhanced CH4 emissions. Thus, the length of periodic oxidant exposure selectively enhances less thermodynamically favorable anaerobic processes by modulating the competitiveness of dominate anaerobic bacteria, which is important for regulating greenhouse gas emissions in redox dynamic soils.