Drivers of nitrous oxide accumulation in denitrification biofilters with low carbon:nitrogen ratios.

Heterotrophic denitrification is usually inhibited by insufficient carbon sources; however, the underlying mechanisms responsible for nitrous oxide (NO) accumulation within denitrification at low carbon:nitrogen (C/N) ratios have not been quantified from a molecular level. In this study, five denitrification biofilters were developed and exhibited efficiency (total nitrogen: 18.5%-92.2%; nitrate nitrogen: 42.9%-99.5%; chemical oxygen demand: 50.5%-93.7%) in remediating micro-polluted water with C/N ratios ranging between 0.65 and 3.0. A combined analysis revealed that the coupling of anaerobic ammonium oxidation (ANAMMOX) and denitrification accounted for NO accumulation in the biofilters, and the key drivers of the NO accumulation rates were qnorB/nirK, nosZ/(narG + napA), amx/(nirS + nirK), narG/bacteria and qnorB/bacteria. Our study demonstrated that genetic association was indicative of microbial processes relative to nitrogen cycling and reflected NO flux within denitrification biofilters at low C/N ratios.