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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.

Temporal trends of N 2 O fluxes across the soil-atmosphere interface were determined using continuous flux chamber measurements over an entire growing season of a subsurface aerating macrophyte (Phalaris arundinacea) in a nonmanaged Danish wetland. Observed N 2 O fluxes were linked to changes in subsurface N 2 O and O 2 concentrations, water level (WL), light intensity as well as mineral-N availability. Weekly concentration profiles showed that seasonal variations in N 2 O concentrations were directly linked to the position of the WL and O 2 availability at the capillary fringe above the WL. N 2 O flux measurements showed surprisingly high temporal variability with marked changes in fluxes and shifts in flux directions from net source to net sink within hours associated with changing light conditions. Systematic diurnal shifts between net N 2 O emission during day time and deposition during night time were observed when max subsurface N 2 O concentrations were located below the root zone. Correlation (P < 0.001) between diurnal variations in O 2 concentrations and incoming photosynthetically active radiation highlighted the importance of plantdriven subsoil aeration of the root zone and the associated controls on coupled nitrification/denitrification. Therefore, P. arundinacea played an important role in facilitating N 2 O transport from the root zone to the atmosphere, and exclusion of the aboveground biomass in flux chamber measurements may lead to significant underestimations on net ecosystem N 2 O emissions. Complex interactions between seasonal changes in O 2 and mineral-N availability following near-surface WL fluctuations in combination with plant-mediated gas transport by P. arundinacea controlled the subsurface N 2 O concentrations and gas transport mechanisms responsible for N 2 O fluxes across the soil-atmosphere interface. Results demonstrate the necessity for addressing this high temporal variability and potential plant transport of N 2 O in future studies of net N 2 O exchange across the soil-atmosphere interface.

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