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Denitrification bioreactor nitrous oxide emissions under fluctuating flow conditions

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  131597821,  2013 Kansas City, Missouri, July 21 - July 24, 2013. (doi: 10.13031/aim.20131597821) @2013
Authors:   Laura Christianson, James Hanly, Neha Jha, Surinder Saggar, Mike Hedley
Keywords:   Denitrification bioreactor nitrate nitrous oxide subsurface drainage woodchip.

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.

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