The North China Plain is one of the most important bases for grain production. In the present agricultural production, farmers apply excessive fertilizer for high yield. Therefore, fertilizer use efficiency has become very low and environmental pollution as a result of it has also become more and more serious. The sustainable development of local agricultural economy and ecological environment has been greatly threatened. This paper reported our research findings on nitrogen processes in agro-ecosystems based on long-term (since 1978) fertilizer field experiment in Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences. Nitrogen processes and the related environmental effects on the agro-ecosystem in the North China Plain were analyzed. This included nitrogen transformation characteristics in soil-plant systems, nitrogen fluxes and their transformation mechanisms, comprehensive nitrogen management and regulation, etc. Eight long-term field experiments related to nitrogen addition to the soil were conducted since 1978. Based on the field experiments, improvements in soil fertility, high production and high efficiency, recycled nutrient applications, agro-ecological processes and response mechanisms and feedback to climate change were studied. The research methods for nitrogen processes were fairly improved and innovated. Based on in situ observation system of soil denitrification in high nitrogen background concentrations, the mechanisms of soil denitrification in indoor incubations, total quantity of nitrogen removal in situ denitrification and production compositions were also studied and quantified. Meanwhile, system errors of denitrification rate measured by acetylene inhibition method were quantified. Furthermore, soil gas determination techniques in deep soil profile were established. Therefore, the mechanisms of N_2O production/consumption at the soil-atmosphere interface were expanded into the deep soil profile. The depth-dependent contribution of N_2O flux in the soil profile to soil surface gas exchange was quantified based on studies on N_2O production mechanisms, diffusion and reduction processes in the subsoil. The state of nitrogen balance in agro-ecosystem in the region was estimated based on studies on nitrogen conversion mechanisms, nitrogen fluxes and the related environmental effects. Different ways of nitrogen loss in agro-ecosystem were compared and the prefer approaches for controlling nitrogen loss and increasing fertilizer use efficiency put forward.