Abstract
Abstract. Nitrous oxide (N2O) emissions from a nitrifying biofilm reactor were investigated with N2O isotopocules. The nitrogen isotopomer site preference of N2O (15N-SP) indicated the contribution of producing and consuming pathways in response to changes in oxygenation level (from 0 % to 21 % O2 in the gas mix), temperature (from 13.5 to 22.3 ∘C) and ammonium concentrations (from 6.2 to 62.1 mg N L−1). Nitrite reduction, either nitrifier denitrification or heterotrophic denitrification, was the main N2O-producing pathway under the tested conditions. Difference between oxidative and reductive rates of nitrite consumption was discussed in relation to NO2- concentrations and N2O emissions. Hence, nitrite oxidation rates seem to decrease as compared to ammonium oxidation rates at temperatures above 20 ∘C and under oxygen-depleted atmosphere, increasing N2O production by the nitrite reduction pathway. Below 20 ∘C, a difference in temperature sensitivity between hydroxylamine and ammonium oxidation rates is most likely responsible for an increase in N2O production via the hydroxylamine oxidation pathway (nitrification). A negative correlation between the reaction kinetics and the apparent isotope fractionation was additionally shown from the variations of δ15N and δ18O values of N2O produced from ammonium. The approach and results obtained here, for a nitrifying biofilm reactor under variable environmental conditions, should allow for application and extrapolation of N2O emissions from other systems such as lakes, soils and sediments.
Highlights
Nitrogen (N) cycling relies on numerous biological processes exploited and altered by anthropic activities (Bothe et al, 2007)
One of the major issues related to N cycle alteration is the production of nitrous oxide (N2O), a potent ozonedepleting and greenhouse gas whose emissions exponentially increased during the industrial era (Crutzen et al, 1979; IPCC, 2014; Ravishankara et al, 2009)
Wastewater resource recovery facilities (WRRFs) contribute to about 3 % of annual global anthropogenic N2O sources, with 0 % to 25 % of the influent nitrogen loads emitted as N2O (Law et al, 2012b)
Summary
Nitrogen (N) cycling relies on numerous biological processes exploited and altered by anthropic activities (Bothe et al, 2007). One of the major issues related to N cycle alteration is the production of nitrous oxide (N2O), a potent ozonedepleting and greenhouse gas whose emissions exponentially increased during the industrial era (Crutzen et al, 1979; IPCC, 2014; Ravishankara et al, 2009). 6.7 ± 1.3 Tg NN2O in 2011; IPCC, 2014), with 0 % to 25 % of the influent nitrogen loads emitted as N2O (Law et al, 2012b). Two microbial processes are responsible for the production of N2O (nitrification and heterotrophic denitrification), with only one of these capable of consuming it (denitrification; Fig. 1a; Kampschreur et al, 2009). Nitrification is the oxidation of ammonium to nitrite (NO−2 ) via the intermediate hydroxylamine (NH2OH) conducted by ammonia oxidizers, and the subsequent oxidation of NO−2 to nitrate (NO−3 ) by Published by Copernicus Publications on behalf of the European Geosciences Union
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