Soil redox plays a key direct role in regulating biogeochemical transformations of carbon (C) and nitrogen (N) in terrestrial ecosystems, and can play an indirect role in phosphorus (P) cycling through iron (Fe) reduction and oxidation.  Upland humid tropical forest soils may be particularly prone to experiencing fluctuating redox conditions as abundant rainfall limits oxygen (O₂) diffusion through finely textured soils and high biological activity enhances O₂ consumption. Few field studies have examined the temporal and spatial variability in soil O₂ concentrations, and related these to C, N, and P biogeochemistry. We used buried equilibration chambers (0-15 cm depth), to measure carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) concentrations and continuous O₂ sensors to measure O₂.  At the end of the experiment we harvested chamber soils for Fe and P analyses.  Our goals were to determine the spatial and temporal variability in soil O₂ dynamics in relation to climate, greenhouse gas emissions, Fe reduction, and soil phosphorus (P) availability. 
Results/Conclusions

Soils from both sites experienced long periods with low bulk soil O₂.  On average, the upper elevation cloud forest site had significantly lower O₂ concentrations (3%) compared to the lower elevation site (8%).  Soil O₂ was dynamic, especially at the lower elevation Tabonuco forest site, where concentrations changed by as much as 10% in a single day.  On average there was a periodicity in O₂ concentrations at two-week intervals at the lower elevation site with significant coherence to the timing of precipitation.  Soil O₂ concentrations were not correlated among individual chambers.  At both sites soil O₂ was significantly negatively correlated with CO₂. Carbon dioxide concentrations reached over 10% during anoxic periods.  While there were no simple relationships between CH₄ or N₂O and O₂ at either site, the gas concentrations at the two sites differed dramatically: at the lower elevation site N₂O was 40 ppm with CH₄ only 4 ppm while at the upper elevation site CH₄ concentrations were several orders of magnitude higher than N₂O, reaching over 10%.  At both sites there were weak negative relationships between reduced Fe concentrations and soil O₂, suggesting other factors such as labile C pools or Fe mineralogy are likely to be important variables for predicting patterns. Our results show that soil redox is highly dynamic in space and time in humid tropical forests, which feeds back on patterns in C, N, and P biogeochemical cycling.