Optimal hydrologic regime for regenerating FeIII electron acceptors for
iron reduction in upland soils
Abstract
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.