Clorimuron-ethyl (CE) has been used wldely in modern agriculture,but little is known regarding the influence of CE on ammonia-oxidizing bacteria (AOB) and archaea (AOA) populations in soils.In this study,microcosm incubation of aquic brown soil was conducted for 60 d.Associated changes in the population sizes of AOB and AOA in response to CE application with or without urea fertilization were examined via quantitative real-time PCR (qTR-PCR) assays of amoA,nirS and nirK genes.Compared with untreated control,the application of CE alone had no strong effects on soil pH,but substantially increased soil NH4+-N and NO3--N levels via inhibition of nitrification and denitrification,respectively.Urea fertilization temporarily increased soil pH in the first 7 days and constantly increased NH4+-N and NO3--N levels throughout the 60-d incubation.Compared with urea fertilization alone,CE-combined urea fertilization decreased soil NO3--N level.The abundance of AOA amoA gene was greater than that of AOB amoA gene in all treatments,both significantly suppressed by CE application in a dose-depended manner.Urea fertilization generally increased AOB and AOA amoA gene abundances.The stimulation effects of urea fertilization on AOA and AOB amoA gene abundances were strongly suppressed by CE application.This study indicated that CE application substantially suppressed soil nitrification via inhibition of AOB and AOA populations regardless urea fertilization,and that AOB and AOA inhabiting separate ecological niches with different NH4+ levels played various roles in N cycling.