Oxygen (O(2)) availability and diffusivity in wetlands are controlling factors for the production and consumption of both carbon dioxide (CO(2)) and methane (CH(4)) 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 CH(4)/CO(2) dynamics in situ, we compare high-resolution subsurface O(2) concentrations, weekly measurements of subsurface CH(4)/CO(2) concentrations and near continuous flux measurements of CO(2) and CH(4). Detailed 2-D distributions of O(2) concentrations and depth-profiles of CO(2) and CH(4) 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), CO(2), and CH(4). In drained conditions, O(2) consumption exceeded CO(2) production, indicating CO(2) dissolution in the remaining water-filled pockets. CH(4) emissions were negligible when the oxic zone was >40 cm and CH(4) 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.

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Northern peatland methane (CH 4 ) budgets are important for global CH 4 emissions. This study aims to determine the ecosystem CH 4 budget and specifically to quantify the importance of Phalaris arundinacea by using different chamber techniques in a temperate wetland. Annually, roughly 70±35% of ecosystem CH 4 emissions were plant-mediated, but data show no evidence of significant diurnal variations related to convective gas flow regardless of season or plant growth stages. Therefore, despite a high percentage of arenchyma, P. arundinacea-mediated CH 4 transport is interpreted to be predominantly passive. Thus, diurnal variations are less important in contrast to wetland vascular plants facilitating convective gas flow. Despite of plant-dominant CH 4 transport, net CH 4 fluxes were low (-0.005-0.016 μmol m −2 s −1 ) and annually less than 1% of the annual C-CO 2 assimilation. This is considered a result of an effective root zone oxygenation resulting in increased CH 4 oxidation in the rhizosphere at high water levels. This study shows that although CH 4 , having a global warming potential 25 times greater than CO 2 , is emitted from this P. arundinacea wetland, less than 9% of the C sequestered counterbalances the CH 4 emissions to the atmosphere. It is concluded that P. arundinacea-dominant wetlands are an attractive C-sequestration ecosystem.