【Objective】In view of the fact that hydrosphere pollution caused by nitrogen losses with runoff from paddy fields in South China is increasingly serious, an attempt was made to explore N losing effect and potential risks of straw returning coupled with nitrogen fertilizer application and an optimum synthetic N fertilization rate in addition to full straw return. 【Method】A field experiment was conducted to explore effects of straw incorporation coupled with N fertilizer application varying in rate on rice yield and dynamics of nitrogen in paddy surface water, and to seek for an optimum fertilizer-N application rate in addition to full straw incorporation contributive to crop yield and the environment. 【Result】Results show: (1) Compared with Treatment N2 (application of synthetic N only), Treatment SN2 (Straw return plus N application at a recommended rate, N:P:K = 240:15:60) was significantly or 22.5% higher in rice yield (P <0.05). Besides, compared with Treatment SN0 (Straw incorporation with no N applied), Treatment SN1 (Straw return plus N application at a reduced rate, N:P:K = 180:15:60), SN2 and SN3 (Straw return plus N application at a conventional rate, N:P:K = 300:15:60) was significantly or 50.1%, 60.1% and 40.8%, respectively, higher in rice yield. This study showed that straw return plus N application significantly increased rice yield, but the effect reduced when too much N fertilizer was applied. Full straw return for 4 or more years in a row could replace part of the N fertilizer applied in yield raising effect; (2) TN (total nitrogen) and NH4⁺-N (ammonia nitrogen) in paddy surface water peaked in concentration in 1~2 days after urea application, then declined rapidly to a relatively low level. So the week after urea application was a period full of risks of nitrogen runoff loss. Moreover, compared with Treatment N2, Treatment SN2 reduced TN concentration in paddy surface water during the period after basal and tillering fertilizer application by 2.0% (P >0.05) and 6.1% (P <0.05), respectively. Treatment SNO reached 53.7% and was obviously higher than Treatment N2, SN1, SN2 and SN3, in DON/DTN (dissolved organic nitrogen/dissolved total nitrogen) and dominated with DON in N release. Treatment SN2 was 12 higher than Treatment N2 in DON concentration. All in all, straw return could effectively decrease the concentration of TN in surface water during the earlier rice-growing stage, while increasing the potential of DON loss; and (3) Compared with Treatment SN2 and SN3, Treatment SN1could save fertilizer-N consumption by 25% and 40% and reduce TN loss by 9.6% and 20.8%, respectively, so it’s an optimal mode that takes both yield and environmental benefits into consideration. 【Conclusion】In a word, for the region in the downstreams of the Yangtze River, especially the flood and runoff prone areas, returning straw fully coupled with 180~225 kg∙hm^-2 –N fertilization in paddy fields is recommended as a long-term farming practice. All the findings in this paper may provide a certain scientific basis for future researches on hydrological N losses as affected by straw return