Linkages Between Reducing Conditions and Soil Carbon Dynamics: Scaling from Microsites to a Humid Tropical Forest Landscape.

While oxygen (O₂) availability and the prevalence of reducing conditions are widely acknowledged to control soil carbon (C) stocks in wetlands, they have received much less attention in terrestrial soils. I will synthesize recent studies linking redox dynamics to soil C pools and fluxes across multiple spatial and temporal scales in the Luquillo Experimental Forest, Puerto Rico. This humid tropical forest landscape is characterized by high spatial and temporal variability in soil O₂ that drives periodic reduction and oxidation of iron (Fe). We found that reduced Fe (Fe(II)) was the single best predictor of surface soil C concentrations at the landscape scale (R² = 0.98), likely reflecting the importance of reducing conditions in suppressing decomposition rates in forests that experience increasing rainfall. Similarly, turnover times of mineral-associated C derived from radiocarbon measurements increased with Fe(II) in samples from a catena spanning a single forest type. However, at finer spatial and temporal scales, we found that O₂ fluctuations linked to Fe reduction and oxidation are associated with high rates of decomposition. Redox fluctuations increased the relative contribution of lignin to soil respiration as determined using stable isotope tracers. Periods of hypoxia also increased the activities of extracellular decomposition enzymes relative to aerobic controls. These phenomena may be linked to microbial Fe reduction, which could enhance decomposition by solubilizing mineral/organic complexes and liberating adsorbed enzymes. Across the landscape, we found that different proxies for short-range-order Fe showed both positive and negative relationships with soil C, indicative of multiple interacting relationships between C and Fe contingent on O₂ dynamics. Ongoing work aims to unify these disparate mechanisms to predict impacts of changing precipitation on soil C stocks in these ecosystems.