Denitrification bioreactor nitrous oxide emissions under fluctuating flow conditions

Abstract. Denitrification bioreactors are able to reduce nitrate loads in agricultural drainage but incomplete denitrification may lead to the unwanted emission of nitrous oxide (N₂O), a potent greenhouse gas. Nitrous oxide fluxes from the surfaces of six pilot-scale bioreactors, operating under four different drainage flow regimes, were measured to investigate the impact of retention time, bioreactor surface type (soil vs. woodchip) and spatial location upon N₂O flux. Bioreactor surface N₂O fluxes from all treatments were small (<1.0 mg N₂O-N/m²-hr) and were comparable with published literature. Lower N₂O fluxes were consistently emitted from the soil surfaces compared to the woodchip surfaces (means: 0.05 and 0.40 mg N₂O-N/m²-hr, respectively) indicating bioreactor soil covers could potentially mitigate some N₂O. When nitrate removal was observed in the bioreactors, N₂O fluxes increased with increasing distance from the bioreactor inlet, but this trend was not statistically significant. The sum of measured emissions of N₂O to air plus the estimated N₂O lost in the bioreactor outflow accounted for less than 0.4% of the nitrate removed from the drainage water. Indirect N₂O emission factors (EF₅) for these bioreactors ranged from 0.37 to 0.89% (or 0.0037 to 0.0089 g N₂O-N/ g NO₃^--N entering the bioreactors). Although bioreactors will likely emit N₂O, on-site N₂O fluxes were much lower than those predicted when untreated nitrate was denitrified downstream. Therefore, enhanced-denitrification practices will not increase overall N₂O emissions per g of N leached and can potentially help reduce the “nitrogen cascade” associated with agricultural drainage.