Anthropogenic nitrate attenuation versus nitrous oxide release from a woodchip bioreactor

Abstract

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 ∼73% in ideal hydrological conditions and ∼18% in non-ideal conditions. The fraction of NO3−-N converted to N2O (rN2O) in the bioreactor was ∼3.3 fold lower than the expected 0.75% IPCC emission factor. We update the global bioreactor estimated Q10 (NRR increase every 10 °C) from a recent meta-analysis with previously unavailable data to >20 °C, yielding a new global Q10 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 ∼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.