The Phylogeny trees were constructed using the representative sequences of the OTUs and reference sequences by MEGAN 5.0 and the results displayed in Fig. 4 and Fig. 5. In terms of AOB Vildagliptin (in Fig. 5), the numbers of OTUs vary substantially within two systems. The most dominant OUT (OTU1) contributed 17.1% of the reads in the fully aerobic SND and showed 97% similarity with the uncluttered Beta proteobacterium (KC193295.1), one genus frequently observed from nitrifying bacteria enriched activated sludge . The second highest OTU3 of the fully aerobic SND contributed 16.2% of the reads, these sequences affiliated with the uncultured ammonia-oxidizing bacterium (JQ865556.1) which was found in wastewater treatment systems . Altogether, the sequences which affiliated with Nitrosomonas sp. have occupied the proportion of 16.9% in the fully aerobic SND. However, the most prominent OTU1 committed 27.1% of the reads in the anoxic–aerobic SND was assigned to the uncultured ammonia-oxidizing bacterium lineage. Moreover, there were no substantial sequences belonged to Nitrosomonas-like cluster and majority of sequences were allocated to uncultured ammonia oxidizing bacteria. Nitrosomonas-like ammonia oxidizing bacteria have been postulated to perform denitrification and produce N2O emission under stress conditions . It is worth pointing that the analysis only paid attention to the sequences that occupied a large share, so Nitrosomonas sp. and Nitrosopira sp. may be ignored for the deficiency of reference information assigned by RDP method . Moreover, some Nitrosopira sp. favor acidic, high total N concentration environment including wastewater treatment plants, nitrification sludge and agricultural soil . The above-mentioned situations would occur unlikely in this experiment. Therefore, we can draw a conclusion based on the same classification that enrichment of Nitrosomonas-like cluster in the fully aerobic SND caused high N2O generation.