Soil redox dynamics are a powerful driver of many biogeochemical processes in terrestrial ecosystems. High clay or organic soils, coupled with warm temperatures, high soil moisture, and high biological activity can lead to conditions where oxygen (O2) consumption exceeds diffusive resupply. We show how soil O2 availability varies in space and time across a range of tropical forests, and how these patterns interact with carbon (C), nitrogen (N), phosphorus (P), and iron (F)e biogeochemistry. Very low redox events (< 4% soil O2) occurred in most ecosystems and ranged from 3 to 30% of the total sample set over an eight year period. Aerating events (>18% soil O2) were also variable in space and time, accounting for 2 to 49% of the measurements. Low to moderate soil O2 events were coupled with dissimilatory nitrate reduction to ammonium (DNRA), denitrification, Fe reduction, P solubilization, and methanogenesis. Our analyses indicate a particularly important role for Fe reduction in highly weathered, Fe-rich tropical soils. When coupled with labile C inputs typical of the high net primary productivity in these ecosystems, Fe(III) reduction was rapid and accounted for significant rates of P solubilization. Fe(II) production occurred at approximately 500 µg/g/d under anaerobic conditions and P was released at a rate of 5 µg/g/d. Fe(III) reduction also fueled considerable C oxidation. Average rates of soil CO2 production were similar under aerobic, anaerobic and fluctuating redox conditions. Using a simple model and measured soil CO2 effluxes we estimate that Fe(III) reduction accounts for 10 to 43% of heterotrophic respiration in upland humid tropical forests. These data highlight the importance of understanding soil redox dynamics in upland terrestrial ecosystems, and their control on key biogeochemical processes.
