Ammonia volatilization from winter wheat cropland in Northern China based on a literature analysis

Northern China is the main intensive agricultural area for winter wheat-summer maize production in China. In the past 30 years, chemical nitrogen (N) fertilizer was the main source of N input in the farmland ecosystem and plays a key role in crop production and soil quality. Due to the excessive application of chemical N fertilizer and frequent irrigation, fertilizer N usage efficiency was low and was also lost into the environment and this caused many negative environmental pollutions. Among the fates of fertilizer N applied, ammonia volatilization was an important gaseous N loss, and it was one of the main sources of atmospheric ammonia and significantly contributed to the formation of atmospheric pollutant PM2.5. Besides, compared with the acid soil in southern China, the proportion of ammonia volatilization from calcareous soil after N fertilization in northern China was higher. In the past, although many experimental studies have been carried out on ammonia volatilization in northern China Plain, few have systematically investigated the relationship between ammonia volatilization and fertilizer N used, and the efficacies of different ammonia collection methods, i.e., sponge absorption method and vacuum suction method. In this study, the works of literature on ammonia volatilization during the winter wheat season in northern China published from 1980 to 2018 were collected and the methods of regression analysis and T-test were adopted. Among the 31 papers collected in this study, 17 adopted the sponge absorption method and 14 adopted the vacuum suction method to measure the ammonia volatilized after N fertilization during the winter wheat season. The results showed that under farmer's conventional N fertilization level, the cumulative total ammonia and net ammonia volatilization was exponentially (y=2.64e0.006 6x)and power-functionally(y=0.004 8x1.358 9) correlated with the fertilized N rate, respectively. Correspondingly, the average proportion of net ammonia volatilization to total chemical N applied was (6.27±0.98)%. Under the same N rate at the basal and topdressing stages, the corresponding ammonia volatilization was 58.7% and 41.3% of the total ammonia volatilization in the whole wheat season, indicating the higher contribution of N volatilization at the basal stage, and the importance of mitigation the ammonia volatilization for winter wheat season. At the fertilization rate of 180 kg/hm2 during the winter wheat season, the ammonia volatilization determined by the vacuum suction method and the sponge absorption method was similar. At the fertilization rate of <180 kg/hm2, the ammonia volatilization determined by the sponge absorption method was 0-5% higher than that of the vacuum suction method and at 180-400 kg/hm2, the vacuum suction method was 0-6.9% higher than that of the sponge absorption method, although the statistic differences between these two methods were not significant. This study also found the priming effect of the ammonia volatilization due to chemical N fertilization was 21.8%±3.57%; the net ammonia volatilization determined without considering of priming effect was significantly higher than that determined with considering of priming effect. The mitigation of ammonia volatilization in the winter wheat season should focus on optimizing the N fertilization rate, mainly at the basal fertilization stage, and combined with other farming measures such as drip irrigation and soil mulching). During the field study of ammonia volatilization caused by chemical N fertilization, the priming effect and the overestimate of ammonia volatilization should not be neglected. Differences of NH3 volatilization via the sponge absorption method and the vacuum suction method should also be taken into consideration.