Oxygen Dynamics and Redox Sensitive Biogeochemical Cycling in Upland Ecosystems.

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 the dominant greenhouse gases 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.

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 carbon (C) and nitrogen (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.

Fluctuating redox dynamics have the potential to stimulate considerable redox sensitive biogeochemistry. In these iron (Fe)-rich soils, Fe redox cycling was an important driver of phosphorus (P), N, and C fluxes. Fe reduction produced CO₂ fluxes equivalent to over 30 % of total soil respiration in these sites. Reducing events released labile P into solution and may help explain perceived low P availability measured under oxidizing conditions in these ecosystems. Fe reduction coupled with ammonium oxidation under anaerobic conditions led to the production of dinitrogen (N₂), nitrite (NO₂^-) and nitrate (NO₃^-) via a processed termed Feammox. Both NO₂^- and NO₃^- were subsequently denitrified to N₂O in some incubations. Feammox provides a new mechanism for N₂ and N₂O production in soils. Iron oxidation in turn had the capacity to produce free radicals that were shown to degrade recalcitrant C compounds, mimicking oxidative enzyme activity. Our results indicate that redox dynamics within and across upland sites can drive locally and regionally important biogeochemical cycling.