Nitrous oxide generation, denitrification, and nitrate removal in a seepage wetland intercepting surface and subsurface flows from a grazed dairy catchment

Little is known about seepage wetlands, located within agricultural landscapes, with respect to removing nitrate (NO3 ) from agricultural catchments, mainly through gaseous emissions of nitrous oxide (N2O) and dinitrogen (N2) via denitrification. These variables were quantified using a push–pull technique where we introduced a subsurface water plume spiked with N-enriched NO3 and 2 conservative tracers [bromide (Br ) and sulfur hexafluoride (SF6)] into each of 4 piezometers and extracted the plume from the same piezometers throughout a 48-h period. To minimise advective and dispersive flux, we placed each of these push–pull piezometers within a confined lysimeter (0.5m diameter) installed around undisturbed wetland soil and vegetation. Although minimal dilution of the subsurface water plumes occurred, NO3 -N concentration dropped sharply in the first 4 h following dosing, such that NO3 -limiting conditions (<2mg/L of NO3-N) for denitrification prevailed over the final 44 h of the experiment. Mean subsurface water NO3 removal rates during non-limiting conditions were 15.7mg/L.day. Denitrification (based on the generation of isotopically enriched N2O plus N2) accounted for only 7% (1.1mg/L.day) of the observed groundwater NO3 removal, suggesting that other transformation processes, such as plant uptake, were responsible for most of the NO3 removal. Although considerable increases in N-enriched N2O levels were initially observed following NO3 dosing, no net emissions were generated over the 48-h study. Our results suggest that this wetland may be a source of N2O emissions when NO3 concentrations are elevated (non-limited), but can readily remove N2O (function as a N2O sink) when NO3 levels are low. These results argue for the use of engineered bypass flow designs to regulate NO3 loading to wetland denitrification buffers during high flow events and thus enhance retention time and the potential for NO3 -limiting conditions and N2O removal. Although this type of management may reduce the full potential for wetland NO3 removal, it provides a balance between water quality goals and greenhouse gas emissions. Additional keywords: bromide, denitrification, N, NO3 removal, N2O, N2, wetland, SF6.