“Breaking the “enzymatic latch”: Mechanisms underlying greenhouse gas fluxes from tropical rainforest soils”
(Introduction by Whendee Silver)
Humid tropical forest soils harbor a globally-significant quantity of carbon (C) yet simultaneously support the highest organic matter decomposition rates of any terrestrial biome. Resolving the biogeochemical mechanisms regulating soil C turnover and greenhouse gas production from these ecosystems is critical for understanding their feedbacks to climate change. Fluctuating oxygen (O2) availability in humid tropical soils may provide one important regulator of decomposition; the “enzymatic latch” hypothesis proposes that anaerobic conditions suppress decomposition by constraining the activity of microbial enzymes. Here, we investigated the impact of spatial and temporal variation in soil oxygen (O2) coupled to the sequential reduction and oxidation of iron (Fe) as a mechanism of organic mater decomposition. The microbial respiration of iron oxide minerals (Fe(III)) under anaerobic conditions generates reduced Fe (Fe(II)), which oxidizes to Fe(III) in the presence of O2. We found that Fe(II) oxidation can stimulate decomposition via two mechanisms: generation of reactive oxygen species that oxidize organic matter, and fluctuations in pH that solubilize C. Field rates of phenol oxidative activity scaled linearly with Fe(II) concentrations, and laboratory CO2 fluxes were stimulated greater than two-fold by Fe(II) oxidation. The decomposition of 13C-labeled lignin under fluctuating O2 conditions was equivalent to that under static aerobic conditions, likely driven by Fe redox cycling. Periodic O2 deprivation in humid tropical ecosystems is unlikely to impose an “enzymatic latch” on decomposition; rather, a fluctuating redox environment may actually stimulate decomposition. This scale-emergent property of redox fluctuations helps explain the rapid decomposition of biochemically-recalcitrant compounds in tropical soils, and suggests the importance of rainfall dynamics in controlling soil C cycling in a changing climate.