Enhancing the relative abundance of comammox nitrospira in ammonia oxidizer community decreases N 2 O emission in nitrification exponentially.

Comammox Nitrospira and canonical ammonia-oxidizing bacteria (cAOB) generally coexist in activated sludge. In present study, the effect of comammox Nitrospira on N ₂ O production during nitrification of activated sludge was investigated. Comammox Nitrospira and cAOB were separately enriched in two nitrifying reactors, with respective relative abundance of approximately 98% in ammonia oxidizer community. The N ₂ O emission factor (EF) of nitrification in comammox Nitrospira dominated reactor was 0.35%, consistently lower than that (2.2%) in cAOB dominated reactor. When increasing the relative abundance of comammox Nitrospira in ammonia oxidizer community, the N ₂ O EF of nitrification decreased exponentially, which suggested that comammox Nitrospira not only decreased N ₂ O production directly but also might have reduced N ₂ O yield by cAOB. When cAOB dominated the ammonia oxidizer community of sludge, decreasing pH to 6.3, lowering DO to less than 0.5 mg/L, and increasing nitrite concentration enhanced N ₂ O EF dramatically. When comammox Nitrospira became the dominant ammonia oxidizer, however, the N ₂ O EF correlated to nitrite insignificantly and a low DO of 0.2 mg/L and weakly acidic pH (6.3) decreased N ₂ O EF by approximately 70% and 60%, respectively. These results imply that enhancing the relative abundance of comammox Nitrospira in sludge is an effective way to reducing N ₂ O emissions and can also offset the promoting effects of acidic pH, low DO, and high nitrite concentration on N ₂ O production during nitrification.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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