Overuse of N fertilizers either in mineral or in organic form will inevitably lead to N accumulation in soil where it is applied. In upland soil, nitrification of N is dominant, when it is grown with vegetable crops with a shallow and small root system in particular. The accumulated high content of nitrate N in the upper layer will certainly be leached down to the groundwater, bringing NO₃^- concentration above limit (50 mg/L). In fields grown with food and other economic crops such as wheat, corn or cotton with deeper and larger root systems, no extra NO₃^- will be left with the current fertilization rate during the growing season in particular, excluding the risk of polluting the groundwater.
About 80 % of the N in the surface water in the Taihu Lake region is in NH₄⁺-N form and most likely comes from human, livestock and poultry excreta, rather than seasonally applied mineral N fertilizer, as is identified by δ¹⁵ N method. So the key to protection of the water environment quality is to control non-point source pollution, like domestic sewage and livestock and/or poultry manure.
Phosphorus is the most contributive factor of water eutrophication in the temperature zone. Overuse of P fertilizers, in mineral or organic form, will build up soil P. According to the soil survey and field study, among all sources of rural and agricultural non-point source P pollution, domestic sewage, livestock and poultry manure (liquid or solid) and fish culture are the major ones, contributing about 90%, while P in runoff and drainage from the field entering the water system accounts for less than 10%. Soil P load to water is about 2~3 folds higher in the wheat season than in the rice season. Much higher accumulation of soil P in vegetable field than in food crops field. Therefore, it is suggested that vegetable fieldsshould be moved from the near suburban areas to further rural area, meanwhile the area for winter wheat could be reduced from this region. On other hand that the area for wetland rice should be kept as much as possible in summer, and in winter season, oil rapeseed crop, winter green manure crops or fallow with rice straw mulch are suggested considering food security, and environment friendly sustainable use of the soil resources.