Denitrification bioreactor nitrous oxide emissions under fluctuating flow conditions

摘要

Denitrification bioreactors are able to reduce nitrate loads in agricultural drainage but incomplete denitrification may lead to the unwanted emission of nitrous oxide (N_2O), 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 N_2Oflux. Bioreactor surface N_2Ofluxes from all treatments were small (<1.0 mg N_2O-N/m~2-hr) and were comparable with published literature. Lower N_2Ofluxes were consistently emitted from the soil surfaces compared to the woodchip surfaces (means: 0.05 and 0.40 mg N_2O-N/m~2-hr, respectively) indicating bioreactor soil covers could potentially mitigate some N_2O. When nitrate removal was observed in the bioreactors, N_2Ofluxes increased with increasing distance from the bioreactor inlet, but this trend was not statistically significant. The sum of measured emissions of N_2Oto air plus the estimated N_2Olost in the bioreactor outflow accounted for less than 0.4% of the nitrate removed from the drainage water. Indirect N_2Oemission factors (EF5) for these bioreactors ranged from 0.37 to 0.89% (or 0.0037 to 0.0089 g N_2O-N/g NO_3~-N entering the bioreactors). Although bioreactors will likely emit N_2O, on-site N_2Ofluxes were much lower than those predicted when untreated nitrate was denitrified downstream. Therefore, enhanced-denitrification practices will not increase overall N_2Oemissions per g of N leached and can potentially help reduce the "nitrogen cascade"associated with agricultural drainage.