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.
