Temporal Dynamics in Soil Oxygen and Greenhouse Gases in Two Humid Tropical Forests

Soil redox plays a key role in regulating biogeochemical transformations in terrestrial ecosystems, but the temporal and spatial patterns in redox and associated controls within and across ecosystems are poorly understood. Upland humid tropical forest soils may be particularly prone to fluctuating redox as abundant rainfall limits oxygen (O 2 ) diffusion through finely textured soils and high biological activity enhances O 2 consumption. We used soil equilibration chambers equipped with automated sensors to determine the temporal variability in soil oxygen concentrations in two humid tropical forests with different climate regimes. We also measured soil trace gases (CO 2 , N 2 O, and CH 4 ) as indices of redox-sensitive biogeochemistry. On average, the upper elevation cloud forest had significantly lower O 2 concentrations (3.0 ± 0.8%) compared to the lower elevation wet tropical forest (7.9 ± 1.1%). Soil O 2 was dynamic, especially in the wet tropical forest, where concentrations changed as much as 10% in a single day. The periodicity in the O 2 time series at this site was strongest at 16 day intervals and was associated with the hourly precipitation. Greenhouse gas concentrations differed significantly between sites, but the relationships with soil O 2 were consistent: O 2 was negatively related to both CO 2 and CH 4 and positively related to N 2 O. These results are among the first to quantify the temporal and spatial scale of variability in soil redox in humid tropical forests, and show that the timing of precipitation plays a strong role in biogeochemical cycling on the scale of hours to weeks.