N fertilizers have been exclusively used in China since 1970s, especially in some agriculture-intense areas (Zhu et al., 2010).
However, N use efficiency (NUE) in China is 10%-20% lower than that of some developed countries (Ju and Zhang, 2003).
On the other hand, the over-fertilization causes environmental problems, such as nitrate contamination and eutrophication of water bodies.
The annual application of N is 25 000 000 tons, which is three times as the average applicationrate globally.