Nitrogen balance in agro-ecosystem in China from 1978 to 2005 based on DSS

A nutrient balance decision support system (DSS) in agro-ecosystem was constructed. Based on DSS, the status quo and historical trend of nitrogen balance was analyzed in both county level and province level. The results showed that: 1）nitrogen surplus displayed an upward trend from 1978 to 2005, which increased from 10.55 million tones in 1978 to 28.67 million tones in 2005. Increase rate reached 64.7×104 t/a; 2）there were significant inter-provincial differences. In general, the nitrogen surplus in main cereals producing provinces such as Shandong, Henan and Sichuan kept a long-term high level. Contrarily, the provinces located in northwest China or southwest China such as Xinjiang and Tibet had a low nitrogen surplus for a long time. The long-term and increasing nitrogen surplus of agro-ecosystem in major cereal product areas of China showed that nitrogen input, mainly from chemical fertilizer, was far more than nitrogen demand for crops. The surplus nitrogen accumulated in field will discharge into water body and atmosphere by many different processes, which will cause the waste of nitrogen resources and potential environmental risk.