Interactive effects of redox and temperature on soil biogeochemical processes

Increased temperature and rainfall are predicted in many regions associated with human-induced climate change. Wetter soils are more likely to experience periodic low redox events, and how this interacts with warming is poorly understood. This is particularly important for humid tropical forests, where warm temperatures, high rates of carbon and nutrient cycling, and fluctuating redox conditions already drive high greenhouse gas (GHG) fluxes. In these systems, reactive iron (Fe) has been shown to be an important predictor of both carbon dynamics and redox cycling. Here, we used a fully factorial incubation experiment to examine the interactive effects of temperature (8, 26, and 35 °C) and redox (oxic and anoxic) conditions on GHG emissions and Fe reduction in a tropical Puerto Rican forest soil. We found that soil carbon dioxide (CO 2 ) production increased exponentially with temperature in both oxic and anoxic mesocosms. The Q 10 value of CO 2 production was significantly higher under oxic (2.20 ± 0.11) than under anoxic conditions (1.47 ± 0.10). Soil CH 4 emissions at 26 °C were comparable or somewhat higher than those at 35 °C, indicative of a weak temperature dependency. Soil samples from 8 °C had significantly lower levels of Fe(II) than those from higher temperature, and no difference was observed between the 26 and 35 °C treatments, suggesting that Fe reduction may not be sensitive to warming in this soil. Together our results suggest that anaerobic metabolisms including anaerobic respiration, methanogenesis, and Fe reduction may be less sensitive to temperature than aerobic respiration. If this pattern holds true in other tropical soils, then anaerobic conditions can limit the loss of soil organic matter and GHGs production in a warmer, wetter climate.