In nature irrigated rice fields fix about N 27 kg hm^-2 more than upland rice fields, which means that the crop needs less N fertilizer in irrigated rice paddy than in upland rice field and that growing rice in paddy fields may lower the production cost, save valuable resources and relieve environmental pressure. Results indicate that irrigated rice paddy is a sink of N because it gains N 2～20 kg hm^-2 in balance from N cycling with water flowin 7.5% of the non-point source N loading in South Jiangsu plain, suggesting that N runoff from rice paddy is not a major contributor of the problem. N leaching from irrigated rice paddy is less than from upland fields. Since the mid 2 1980s, the over 2 limit rate of nitrate N in well water has remained almost unchanged even though the N fertilizer application rate has dramatically increased, which implies that nitrate in well water is not directly related to N fertilization. N loss through ammonia volatilization accounted for about 6%～21% of the urea N applied in paddy field in the rice growing season, and only 3.1%～6.5% in the winter wheat season;On the other hand, N input through wet deposition was also much larger in the rice season than in the winter wheat season. Overall, irrigated rice paddy is a N sink, but not a N source to its surrounding environment. "Paddy field Ring" works as an artificial wetland and protects our environment from the view point of N cycling