This study attempts to elucidate the dynamics of nitrous oxide (N2O) emission and investigate the evolution of the ammonium-oxidizing bacteria (AOB) community in a partial nitrification system producing an influent suitable for the anammox process. Based on long-term monitoring, (0.80 +/- 0.19, n = 7)% of the incoming nitrogen load was emitted as N2O. During the partial nitrification process, the N2O emission rate reached a maximum at the beginning of the aerobic period and stabilized at a low level after an initial peak. Moreover, the quantity of N2O emission increased quickly at the beginning of the cycle operation and then production slowed after 30 mm. According to polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis, the dominant AOB causing the N2O emission from the partial nitrification system were Nitrosomonas sp. Both Nitrosomonas sp. Nm33 and Nitrosomonas sp. Nm58 were enriched at high ammonia concentrations. (c) 2012 Elsevier Ltd. All rights reserved.