Nitrification　failure　of　wastewater　treatment　plants　(WWTPs)　in　cold　season　calls　into　investigations　of　the　functional　ammonia-oxidizing　microorganisms　(AOMs).　In　this　study,　we　report　the　abundance　of　ammonia-oxidizing　archaea　(AOA),　bacteria　(AOB)　and　complete　ammonia-oxidizing　(comammox)　Nitrospira　in　23　municipal　WWTPs　in　cold　season,　and　explore　the　correlations　between　AOMs　abundance　and　their　relative　contribution　to　nitrification.　The　copy　numbers　of　AOA　and　AOB　amoA　gene　ranged　from　2.42　x　10(7)　to　2.47　x　10(9)　and　5.54　x　10(6)　to　3.31　x　10(9)　copies/g　sludge,　respectively.　The　abundance　of　amoA　gene　of　Candidatus　Nitrospira　inopinata,　an　important　strain　of　comammox　Nitrospira,　was　stable　with　averaged　abundance　of　8.47　x　10(6)　copies/g　sludge.　DNA-based　stable　isotope　probing　(DNA-SIP)　assays　were　conducted　with　three　typical　WWTPs　in　which　the　abundance　of　AOA　was　lower　than,　similar　to　and　higher　than　that　of　AOB,　respectively.　The　results　showed　that　considerable　C-13-assimilation　by　AOA　was　detected　during　active　nitrification　in　all　WWTPs,　whereas　just　a　much　lesser　extent　of　C-13-incorporation　by　AOB　and　comammox　Nitrospira　was　found　in　one　WWTP.　High　throughput　sequencing　with　C-13-labeled　DNA　also　showed　the　higher　reads　abundance　of　AOA　than　AOB　and　comammox　Nitrospira.　Nitrososphaera　viennensis　was　the　dominant　active　AOA,　while　Nitrosomonas　oligotropha　and　Nitrosomonas　europaea　were　identified　as　active　AOB.　The　results　obtained　suggest　that　AOA,　rather　than　AOB　and　comammox　Nitrospira,　dominate　ammonia　oxidation　in　WWTPs　in　cold　season　despite　the　numerical　relationships　of　AOMs.　(C)　2018　Elsevier　Ltd.　All　rights　reserved.