Anthropogenic nitrate attenuation versus nitrous oxide release from a woodchip bioreactor

Nitrogen loss via overland flow from agricultural land use is a global threat to waterways. On-farm denitrifying woodchip bioreactors can mitigate NO3- exports by increasing denitrification capacity. However, denitrification in sub-optimal conditions releases the greenhouse gas nitrous oxide (N2O), swapping the pollution from aquatic to atmospheric reservoirs. Here, we assess NO3--N removal and N2O emissions from a new edge-of-field surface-flow bioreactor during ten rain events on intensive farming land. Nitrate removal rates (NRR) varied between 5.4 and 76.2 g NO3--N m(-3) wetted woodchip d(-1) with a mean of 30.3 +/- 7.3 g NO3--N m(-3). The nitrate removal efficiency (NRE) was similar to 73% in ideal hydrological conditions and similar to 18% in non-ideal conditions. The fraction of NO3--N converted to N2O (rN(2)O) in the bioreactor was similar to 3.3 fold lower than the expected 0.75% IPCC emission factor. We update the global bioreactor estimated Q(10) (NRR increase every 10 degrees C) from a recent meta-analysis with previously unavailable data to >20 degrees C, yielding a new global Q(10) factor of 3.1. Mean N2O CO2-eq emissions (431.9 +/- 125.4 g CO2-eq emissions day(-1)) indicate that the bioreactor was not significantly swapping aquatic NO3- for N2O pollution. Our estimated NO3--N removal from the bioreactor (9.9 kg NO3--N ha(-1) yr(-1)) costs US$13.14 per kg NO3--N removed and represents similar to 30% NO3--N removal when incorporating all flow and overflow events. Overall, edge-of-field surface-flow bioreactors seem to be a cost-effective solution to reduce NO3--N runoff with minor pollution swapping to N2O.