Bacillus rnethylotrophicus strain L7, exhibited efficient heterotrophic nitrification-aerobic denitrification ability, with maximum NH4+-N and NO2--N removal rate of 51.58 mg/L/d and 5.81 mg/L/d, respectively. Strain L7 showed different gaseous emitting patterns from those strains ever described. When (NH4Cl)-N-15, or (NaNO2)-N-15, or (KNO3)-N-15 was used, results of GC-MS indicated that N2O was emitted as the intermediate of heterotrophic nitrification or aerobic denitrification, while GC-IRMS results showed that N-2 was produced as end product when nitrite was used. Single factor experiments suggested that the optimal conditions for heterotrophic nitrification were sodium succinate as carbon source. C/N 6, pH 7-8, 0 g/L NaCl, 37 degrees C and a wide range of NH4+-N from 80 to 1000 mg/L. Orthogonal tests showed that the optimal conditions for aerobic denitrification were C/N 20, pH 7-8, 10 g/L NaCl and DO 4.82 mg/L (shaking speed 50 r/min) when nitrite was served as substrate. (c) 2012 Elsevier Ltd. All rights reserved.