Gasping for Air: How Redox Varies in the Critical Zone and Drives Biogeochemical Cycling.

The soil oxygen (O₂) availability and associated redox dynamics are key drivers of carbon and nitrogen cycling and greenhouse gas emissions in terrestrial ecosystems. However, few studies have measured soil O₂ availability, and even fewer have related this to biogeochemical cycling over space and time. Redox dynamics are likely to play a particularly important role in humid tropical forests characterized by high rainfall, near constant warm temperatures, high biological activity, and finely textured soils, all of which contribute to periodic O₂ depletion throughout the soil profile. These ecosystems exhibit rapid C turnover and are a globally important source of the major greenhouse gases. We report on an extensive network of galvanic O₂ sensors and time-domain reflectometry along topographic gradients in a lower montane wet tropical forest in Puerto Rico (n = 105 sensors). Within the sensor field we also installed three automated surface flux chambers in each topographic zone (ridge, slope and valley). A Cavity Ring-Down Spectroscopy (CRDS) gas analyzer was used to measure pseudo-continuous fluxes of CO₂, N₂O, and CH₄. Results showed that soil O₂ concentrations decrease nonlinearly from ridges to valleys along topographic gradients. Soil moisture was the best single predictor of soil O₂ concentrations explaining over 50% of the variability in the data, even in these well-drained soils. Both ridges and slopes produced higher CO₂ fluxes than valleys (P<0.05). Daily CH₄ emissions went up to ~2000 g CH₄ ha^-1d^-1 for valleys (hot spots and hot moments). Soil O₂ dynamics also helped explain patterns in reactive Fe species and C storage, as well as pH along the catena. Our results highlight the potential for soil O₂ concentrations as an integrator of biogeochemical dynamics in variable redox environments. They also provide a mechanism for identifying and exploring the role of hot spots and hot moments in space and time.