Temporal trends in N2O flux dynamics in a Danish wetland – effects of plant‐mediated gas transport of N2O and O2 following changes in water level and soil mineral‐N availability

Temporal trends of N₂O 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 N₂O fluxes were linked to changes in subsurface N₂O and O₂ concentrations, water level (WL), light intensity as well as mineral‐N availability. Weekly concentration profiles showed that seasonal variations in N₂O concentrations were directly linked to the position of the WL and O₂ availability at the capillary fringe above the WL. N₂O 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 N₂O emission during day time and deposition during night time were observed when max subsurface N₂O concentrations were located below the root zone. Correlation (P < 0.001) between diurnal variations in O₂ concentrations and incoming photosynthetically active radiation highlighted the importance of plant‐driven subsoil aeration of the root zone and the associated controls on coupled nitrification/denitrification. Therefore, P. arundinacea played an important role in facilitating N₂O 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 N₂O emissions. Complex interactions between seasonal changes in O₂ and mineral‐N availability following near‐surface WL fluctuations in combination with plant‐mediated gas transport by P. arundinacea controlled the subsurface N₂O concentrations and gas transport mechanisms responsible for N₂O fluxes across the soil–atmosphere interface. Results demonstrate the necessity for addressing this high temporal variability and potential plant transport of N₂O in future studies of net N₂O exchange across the soil–atmosphere interface.