The sensitivity of soil O2 and redox biogeochemistry to landscape position and climate

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. 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. Drought conditions dramatically altered soil O2 dynamics in both time and space, and showed that redox drivers differed by topographic position. Both ridges and slopes produced higher CO2 fluxes than valleys. 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.