The effects of operational modes (fully aerobic mode and anoxic-aerobic mode) on simultaneous nitrification and denitrification (SND) process were investigated to reveal the different characteristics of nitrogen removal and nitrous oxide (N2O) emission. Results indicated that total nitrogen (TN) removal and SND efficiencies under anoxic-aerobic mode increased by 17.8% and 10.1% in comparison with fully aerobic mode. Furthermore, N2O emission in the anoxic-aerobic SND was much lower than in fully aerobic SND. The amount of N2O emitted per cycle in the fully aerobic SND was 21.9 +/- 7.1% of the removed TN, which was three times higher than 7.0 +/- 1.6% observed in the anoxic-aerobic SND. Denitrification by nitrifiers was the main source of N2O emission in both systems. High-throughput pyrosequencing analysis revealed a greater abundance of Nitrosomonas sp. and Mesorhizobium sp. in fully aerobic SND than anoxic-aerobic SND, which were closely associated with N2O emissions. (C) 2015 Elsevier B.V. All rights reserved.