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Denitrification as an N2O sink
- Conthe, Monica, Lycus, Pawel, Arntzen, Magnus Ø., Ramos da Silva, Aline, Frostegård, Åsa, Bakken, Lars R., Kleerebezem, Robbert, van Loosdrecht, Mark C.M.
- Water research 2019 v.151 pp. 381-387
- activated sludge, ammonium, byproducts, carbon, denitrification, electrons, genes, greenhouse gas emissions, greenhouse gases, microbial communities, nitrates, nitrous oxide, nitrous-oxide reductase, oxidation, quantitative polymerase chain reaction, surveys, wastewater treatment
- 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 (VmaxN2O→N2) exceeded the capacity to produce N2O (VmaxNO3→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 = VmaxN2O→N2/VmaxNO3→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.