Nitrous　oxide　(N2O),　a　greenhouse　gas　which　contributes　to　the　destruction　of　the　stratospheric　ozone　layer,　can　be　emitted　from　nitrifying　processes　during　wastewater　treatment.　The　pathway　of　N2O　production　was　studied　using　a　lab-scale　nitrifying　reactor.　Allylthiourea　was　used　to　inhibit　NH4+　oxidation　and　provide　information　on　processes　that　happen　under　nitrifying　condition.　Our　study　confirmed　that　besides　heterotrophic　bacteria,　ammonium-oxidizing　bacteria　could　perform　denitrification　processes,　during　which　NO2-　was　the　electron　acceptor　and　NH4+　was　the　electron　donor,　with　N2　and　N2O　as　final　products.　The　relative　contribution　of　the　heterotrophic　denitrification　process　to　total　N2O　emissions　varied　from　46.1%　to　60.4%　depending　on　NO2(-)-N　addition.　Correspondingly,　21.8%　to　51.5%　of　total　N2O　emissions　can　be　attributed　to　nitrifier　denitrification.　Little　N2O　is　emitted　during　the　NO2-　oxidation　process.