Sediment cores from a newly formed urban lake (Longjinghu Lake in China's Chongqing City) were selected to study the vertical distribution characteristics of both nitrogen and phosphorus from the overlying water to porewater, and then fluxes of ammonia (NH4+ -N) and orthophosphate (PO(3-)(4)-P) from different areas of the lake were calculated using a one-dimensional transport-reaction model based on the Fick's First Law. The results showed that the mean ammonia concentrations in porewater of surface layers (0-5 cm) was 6. 13 mg-L-1 +/-3.07 mg.L-1 , higher by a factor of 10 compared with that in the overlying water. While orthophosphate contents tended to increase downwards, reaching a peak in surface sediment porewater, and then decreased in downcore, with a mean concentration of 2. 01 mg.L-1 +/- 1.05 mg L-1. Based on the porewater diffusion model, ammonia were released from sediments to overlying water, which indicated potential internal nutrient releasing risk. Although the flux of ammonia in the "initial lake area" was higher than that in the "newly submerged area", the annual load contribution from the "newly submerged area" was 85% (3.95 t.a-1). Similar to ammonia, orthophosphate was also released from sediments to overlying water in the "initial lake area", with the highest fluxes in the former Longjinghu Lake and Longjingou Lake Reservoir, reaching 7. 89 mg. ( m2 d)-1 and 6. 13 mg ( m2 .d)-1, respectively. However, in a portion of the "newly submerged area", orthophosphate in the overlying water was transported to the sediments, resulting in negative fluxes ranging from - 1.93 to -2.78 mg (m2 d) -1. The annual load of orthophosphate from the whole sediments was 0. 357 t.a-1, with 72% contributed by the "newly submerged area".