Denitrification as an N2O sink

Abstract The strong greenhouse gas nitrous oxide (N 2 O) can be emitted from wastewater treatment systems as a byproduct of ammonium oxidation and as the last intermediate in the stepwise reduction of nitrate to N 2 by denitrifying organisms. A potential strategy to reduce N 2 O emissions would be to enhance the activity of N 2 O reductase (NOS) in the denitrifying microbial community. A survey of existing literature on denitrification in wastewater treatment systems showed that the N 2 O reducing capacity ( V maxN2O→N2 ) exceeded the capacity to produce N 2 O ( V maxNO3→N2O ) by a factor of 2–10. This suggests that denitrification can be an effective sink for N 2 O, potentially scavenging a fraction of the N 2 O produced by ammonium oxidation or abiotic reactions. We conducted a series of incubation experiments with freshly sampled activated sludge from a wastewater treatment system in Oslo and found that the ratio α =  V maxN2O→N2 /V maxNO3→N2O fluctuated between 2 and 5 in samples taken at intervals over a period of 5 weeks. Adding a cocktail of carbon substrates resulted in increasing rates, but had no significant effect on α. Based on these results – complemented with qPCR and metaproteomic data – we discuss whether the overcapacity to reduce N 2 O can be ascribed to gene/protein abundance ratios (nosZ/nir), or whether in-cell competition between the reductases for electrons could be of greater importance.