Contribution of N Fertilization to N2O Emissions From Croplands of China and Mitigation Options.

For meeting the increasing food demands, consumption of synthetic N fertilizers for crop production has increased by near 50 times in China since 1961. The total consumption reached up 26.64 Tg N in 2005, accounted for more than a quarter of the world consumption. With the increase in N consumption for agricultural production, N use efficiency decreases and negative impacts on environment, ecosystem and human health become serious. In 1995, N₂O emissions from field measurements were first published in China. Since then, the data on N₂O emissions from croplands have been accumulated substantially. It has been confirmed that flooded rice fields are also an important N₂O source and N₂O emissions from rice fields are dependent on water regime firstly and N application rate secondly. On average, N₂O emission factor of synthetic N fertilizers applied to rice fields is much smaller than those applied to uplands. By applying different methodologies, synthetic N fertilizer induced N₂O emissions and total N₂O emissions from croplands were estimated to be from 130 Gg N to 204 Gg N and from 310 Gg N to 476 Gg N, respectively in 1990¡¯s. Synthetic N fertilizer induced N₂O emission was accounted for 42-58% of total N₂O emissions from croplands. China is the unique country in the East, Southeast and South Asia that synthetic N fertilizer induced N₂O emissions dominates total emissions from croplands. Preventing excess application of N fertilizers, particularly for vegetable production, and preventing unnecessary irrigation of uplands is extremely important and practicable for mitigating N₂O emissions from croplands of China. Application of nitrification inhibitors along with N fertilizers is also an effective option. Ammonium sulphate applied to rice fields usually induces more N₂O emissions than urea and ammonium bicarbonate.