Denitrification as an N₂O sink

Abstract

The strong greenhouse gas nitrous oxide (N₂O) 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 N₂ by denitrifying organisms. A potential strategy to reduce N₂O emissions would be to enhance the activity of N₂O reductase (NOS) in the denitrifying microbial community. A survey of existing literature on denitrification in wastewater treatment systems showed that the N₂O reducing capacity (V_maxN2O→N2) exceeded the capacity to produce N₂O (V_maxNO3→N2O) by a factor of 2–10. This suggests that denitrification can be an effective sink for N₂O, potentially scavenging a fraction of the N₂O 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 N₂O 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.