Salinity is one of the key factors that affect biological nutrient removal (BNR) performance for wastewater treatment. Higher salinity or salinity dynamic could also stimulate nitrous oxide (N2O) production during nitrification. This study investigated the nitrification process and N2O production under salinity shock (7.5 and 10 g NaCl/L) and salinity acclimation (7.5 and 10 g NaCl/L) conditions in the activated sludge bioreactors. N2O conversion pathway affected by salinity (shock effect or long-term effect) was quantified with sole ammonia addition and mixed ammonia and nitrate addition, using stepwise inhibition and N-15 isotope labeling methods, respectively. The results showed that with addition of sole ammonia, the N2O conversion ratios were higher in salinity shock tests (2.35 and 2.86 times, respectively, in 7.5 and 10 g NaCl/L) and acclimation tests (1.62 and 2.05 times, respectively in 7.5 and 10 g NaCl/L) than that in no salt addition tests. Stepwise inhibition and N-15 isotope labeling methods both revealed the salt-induced N2O was mainly via nitrifier denitrification in salinity shock tests (due to the temporally nitrite accumulation) and via nitrifier denitrification and nitrification coupled denitrification in salinity acclimation tests (due to elimination of nitrite oxidation bacteria and enrichment of Nitrosomonas (ammonia oxidation bacteria) in salinity acclimation systems). Additionally, in the presence of mixed ammonia and nitrate, N-15 isotope labeling trials revealed that the contribution of heterotrophic denitrification to N2O production did not vary in salinity shock tests but lowered in salinity acclimation tests. (C) 2014 Elsevier B.V. All rights reserved.