Controls on long-term patterns in soil oxygen availability in upland humid tropical forests

Background/Question/Methods

Soil oxygen (O₂) availability and associated redox-dynamics have the potential to structure patterns in biogeochemical cycling in humid environments. This is well known for wetlands, but also applies to upland, well drained environments, such as humid tropical forests. These ecosystems exhibit a wide range of important redox-derived biogeochemical processes including the production and consumption of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). Few studies have measured soil O₂ dynamics in tropical forests, or determine the drivers of patterns in space and time. We report on an eight year study of soil O₂ concentrations along a tropical montane elevation gradient in the Luquillo Experimental Forest, Puerto Rico. We explored patterns in soil O₂ with rainfall, temperature, soil physical and chemical characteristics, and plant community composition. We also compared trends in soil O₂ dynamics with data on redox-sensitive biogeochemical cycling.

Results/Conclusions

Soil O₂ concentrations varied significantly along the gradient and most sites experienced periodic anoxia. Upper elevation palm forests experienced the lowest average soil O₂ availability (10.5 ± 0.2%) and the highest frequency of low redox events, with one third of the measurements at or below 3% O₂. There was significant temporal coherence among forest types and strong correlations among sites. There was also significant periodicity in soil O₂ at short (two week) and long (monthly to seasonally) time scales. The detectable seasonality in the long-term record was surprising given that these forests are generally considered aseasonal. Soil O₂ was positively correlated with bulk density, and negatively related to soil C and N concentrations along the gradient. The timing of rainfall was a good predictor of soil O₂ concentrations at short temporal scales, while the magnitude of rainfall was strongly correlated with O₂ at longer time scales. Our results indicate that the timing and magnitude of rainfall can be good predictors of the temporal dynamics of soil O₂ concentrations, while soil chemical and physical properties may help determine average soil O₂ availability within and across sites.