Linking Soil O2, CO2, and CH4 Concentrations in a Wetland Soil: Implications for CO2 and CH4 Fluxes

Oxygen (O-2) availability and diffusivity in wetlands are controlling factors for the production and consumption of both carbon dioxide (CO2) and methane (CH4) in the subsoil and thereby potential emission of these greenhouse gases to the atmosphere. To examine the linkage between high-resolution spatiotemporal trends in O-2 availability and CH4/CO2 dynamics in situ, we compare high-resolution subsurface O-2 concentrations, weekly measurements of subsurface CH4/CO2 concentrations and near continuous flux measurements of CO2 and CH4. Detailed 2-D distributions of O-2 concentrations and depth-profiles of CO2 and CH4 were measured in the laboratory during flooding of soil columns using a combination of planar O-2 optodes and membrane inlet mass spectrometry. Microsensors were used to assess apparent diffusivity under both field and laboratory conditions. Gas concentration profiles were analyzed with a diffusion-reaction model for quantifying production/consumption profiles of O-2, CO2, and CH4. In drained conditions, O-2 consumption exceeded CO2 production, indicating CO2 dissolution in the remaining water-filled pockets. CH4 emissions were negligible when the oxic zone was > 40 cm and CH4 was presumably consumed below the depth of detectable O-2. In flooded conditions, O-2 was transported by other mechanisms than simple diffusion in the aqueous phase. This work demonstrates the importance of changes in near-surface apparent diffusivity, microscale O-2 dynamics, as well as gas transport via aerenchymous plants tissue on soil gas dynamics and greenhouse gas emissions following marked changes in water level.