Proper management of synthetic nitrogen (N) fertilizer can reduce direct N2O emission from soil and indirect CO2 emission from production and transportation of synthetic N. In the late 1990s, the average application rates of synthetic N were 212, 207 and 207 kg ha(-1), respectively, for rice, wheat, and maize in China's croplands. But research suggests that the optimal synthetic N application rates for the main grain crops in China should be in the range of 110-150 kg ha(-1). Excessive application of synthetic N has undoubtedly resulted in massive emission of greenhouse gases. Therefore, optimizing N application rates for grain crops in China has a great potential for mitigating the emission of greenhouse gases. Nevertheless, this mitigation potential (MP) has not yet been well quantified. This study aimed at estimating the MP of N2O and CO2 emissions associated with synthetic N production and transportation in China based on the provincial level statistical data. Our research indicates that the total consumption of synthetic N on grain crops in China can be reduced by 5.0-8.4 Tg yr(-1) (28-47 % of the total consumption) if the synthetic N application rate is controlled at 110-150 kg ha(-1). The estimated total MP of greenhouse gases, including direct N2O emission from croplands and indirect CO2 emission from production and transportation of synthetic N, ranges from 41.7 to 70.1 Tg CO2_eq. yr(-1). It was concluded that reducing synthetic N application rate for grain crops in China to a reasonable level of 110-150 kg ha(-1) can greatly reduce the emission of greenhouse gases, especially in the major grain-crop production provinces such as Shandong, Henan, Jiangsu, Hebei, Anhui and Liaoning.
