**Acknowledgements**

We are grateful to Professor Jer-Horng Wu who works in the Department of Environmental Engineering, National Cheng Kung University in Taiwan, for assisting with microbial clarification and data analysis of the N2O oxidation system. We also thank 21 Century Center of Excellence (COE) program (Japan), Ministry of Science and Technology (Taiwan) and NCKU Research & Department Foundation (Taiwan) for supporting the funding.
