Abstract:A venting method was adopted to determine the effect of nitrogen topdressing stages on ammonia volatilization (AV) from topdressing fertilization in irrigated and rainfed wheat fields. The results show that under rainfed conditions, the AV from topdressing fertilization in wheat fields is mainly in the period from 5 to 25 days after the fertilization. With the delay of nitrogen topdressing stage from the pseudo stem erection stage (SE, GS30) to the jointing stage (JT, GS32), the peak value of the AV rate is increased and the occurrence time of the peak is ahead. With the delay of nitrogen topdressing stage to the booting (BT, GS41), the peak value of the AV rate is reduced. The rates of AV loss are 24.84%-25.32%, 25.42%-25.50% and 14.77%-16.62% in the SE, JT and BT treatments, respectively. Under 60 mm irrigation amount, the nitrogen fertilization stage has less effect on the AV, the AV rates from the topdressing fertilization are not changed significantly, and the N loss from the accumulated AV is N 0.40 to 0.55 kg/ha, only accounting for 0.36% to 0.49% of topdressing N. Under the rainfed conditions, there is a significant positive correlation between 0-10 cm layer soil NH4+-N concentration and the AV rate, and under irrigated conditions there is a significant positive correlation between 10-20 cm layer soil NH4+-N concentration and the AV rate. Soil temperature and rainfall are the important influence factors of AV. In addition, AV is connected with aeration status of field soil surface. Multiple spike type wheat is helpful to reduce wheat field AV loss.