中文摘要：

自然状态下灌溉稻田每年比旱地要多固氮27kg hm^-2，可以减少氮肥用量，既节约农本和资源，又缓解对环境的压力。太湖流域不同类型稻田在水循环中可吸纳氮素N2～20kg hm^-2，是氮素的汇。该区平原稻麦轮作田氮素的径流流失量平均小于当年施氮量的5％，对苏南太湖地区面源污染的相对贡献率仅为7．5％，不是该区氮素面源污染的主要组成。稻田氮素向下淋失迁移的量低于麦田；太湖地区井水中硝态氮的超标率自20世纪80年代中期至今没有变化，说明该区井水中硝态氮含量高低与农业上氮肥用量没有直接联系。尿素挥发损失量稻季达施氮量的6％-21％，麦季为3．1％-6．5％；稻季氨挥发损失高于麦季；湿沉降带入土壤或水体的氮也是夏（稻）季高于麦季。总体上看，稻田向环境输出的氮少，而固定、汇集的氮多，“稻田圈”是保护环境的重要生态单元。

英文摘要：

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 flowing through the field in the Taihu Lake region. The N loss with runoff accounted for less than 5% of the N input as fertilizer, and only 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-1980s, the over-llmit 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.