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

Abstract

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 15N-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 4h following dosing, such that NO3--limiting conditions (<2mg/L of NO3-N) for denitrification prevailed over the final 44h 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 NO 3- removal, suggesting that other transformation processes, such as plant uptake, were responsible for most of the NO 3- removal. Although considerable increases in 15N-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 N 2O sink) when NO3- levels are low. These results argue for the use of engineered bypass flow designs to regulate NO 3- loading to wetland denitrification buffers during high flow events and thus enhance retention time and the potential for NO 3--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. © CSIRO 2008.