Ammonia emissions from paddy fields are underestimated in China.

Excessive nitrogen (N) fertilizers are often used in China, and a large proportion of the N can be lost as ammonia (NH3 ). However, quantifying the NH3 emission from paddy fields is always affected by large uncertainties due to different measuring methods and other factors such as climate. In this study, using a standardized method, we measured the NH3 emissions in three typical annual rice cropping systems: single rice, double rice and rotation with other crops. The measurements were conducted for 2 years with a total of 3131 observations across China. Results showed that NH3 emissions accounted for 17.7% (14.4-21.0%) of the N applied under current farm practice, which was 33.1% (10.6-52.6%) higher than previous estimates. Nitrogen application rate was the dominant factor influencing NH3 emission rate, which exponentially increased with the N fertilizer rate (p < .001). Total NH3 emissions from paddy fields were estimated at 1.7 Tg N yr-1 in 2013 in China, several times the amount of N lost through leaching or runoff. This suggests that mitigation measures for non-point source pollution from cropland should take into account not only the N lost to water, but also to air, thereby improving air quality.