Denitrification activities and N2O production under salt stress with varying COD/N ratios and terminal electron acceptors

The effects of sodium chloride (NaCl) on nitrogen removal and nitrous oxide (N2O) production during denitrification were investigated under various COD/N ratios and in the presence of different terminal electron acceptors. High NaCl salinity hindered the denitrification rate, and the degree of inhibition was dependent on the influent COD/N ratio and terminal electron acceptor. Specifically, with nitrate as terminal electron acceptor, NaCl shock induced relatively higher nitrite accumulation at lower COD/N ratios than at higher COD/N ratios. Without the addition of NaCl, N2O did not accumulate, even when no external carbon source was added. Conversely, at 20 g NaCl/L salinity, N2O accumulated, regardless of the COD/N ratios and terminal electron acceptor. Overall, a low COD/N ratio combined with higher NO2-–N levels stimulated more N2O accumulation, particularly in response to high salt levels, and the salinity was found to be the most important factor that strongly influences the N2O production. In the absence of an external carbon source, glycogen rather than poly-β-hydroxybutyrate (PHB) was the main carbon source responsible for endogenous denitrification. Microbial inhibition by salt stress primarily occurred via osmotic stress rather than via ionic toxicity and oxidative stress.

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• Effect of NaCl salinity on N2O production by denitrification was studied.
• Salt stress inhibited denitrification rates.
• The degree of inhibition depends on the influent COD/N and terminal electron acceptor.
• High salinity combined with low COD/N resulted in a sharp increase in N2O production.
• Microbial inhibition by salt stress (⩽20 g NaCl/L) occurred via osmotic stress.