Background, aim, and scope A large area of water eutrophication in the Tai Lake region of China was associated with nitrogen (N) and phosphorus (P) pollution, mainly due to the discharge of untreated rural wastewater (RW) into the surface water (SW) near villages of this region. A field experiment was conducted, using irrigation of RW plus urea fertilization under equal nitrogen (N) rate, namely, black water (BW), domestic wastewater (DW), gray water (GW), SW, and SW without any N application as a control (CK), to elucidate N removal by the paddy wetland system during the rice growing season of 2007. Materials and methods SW, GW, DW, and BW were collected from the village of Liangzhu in subtropical China. Water samples were stored in darkness at 4°C in an icebox prior to analysis. Total nitrogen (TN) was analyzed by the spectrophotometric method with a continuous-flow automated analyzer. For the soil analyses, pH was measured using a pH meter with soil/distilled water = 1:5; cation exchange capacity was determined with an unbuffered salt extraction method. Redox potential (Eh) was obtained using a pH meter with a platinum electrode, whereas organic carbon (C) was determined using the Walkley and Black method. TN was calculated by acid-alkali neutralization, after the soil samples were digested with concentrated sulfuric acid (H 2 SO 4 ), distilled and then absorbed by diluted boric acid (H 3 BO 3 ), and available N was determined by alkaline-proliferation law, after the samples were hydrolyzed, reduced, and absorbed by H 3 BO 3 . Available P was determined with the colorimetric method at the wave length of 660 nm, after the samples were extracted with NaHCO 3 , and available K was extracted by unbuffered NH 4 Cl and then determined by the atomic absorption spectrophotometer. The rice plant samples were digested with H 2 SO 4 -H 2 O 2 , and N contents were analyzed by the atomic absorption spectrophotometry. Results Yield for the CK was significantly less (P≤0.05) than those of SW, GW, DW, and BW, with the yield of BW significantly greater (P≤0.05) than all the other treatments. The TN concentration of the floodwater in the paddy wetland system decreased rapidly after transplanting, rose significantly (P≤0.05) after two N topdressings, and then decreased gradually as the following sequences: SW>GW> DW>BW>CK, but tended to be stable until 15 October. Meanwhile, TN removal rates from the wastewater were significantly higher (P≤0.05) than those from the urea fertilizer. Total N load (TNL) increased significantly after two topdressings and reached the maximum value just after the first topdressing on 22 July: SW, 21.0 kg ha −1 ; GW, 19.1 kg ha -1 ; BW, 15.3 kg ha -1 ; DW, 14.3 kg ha -1 ; and CK, 0.57 kg ha -1 . Subsequently, TNL declined gradually and reached stability on 15 October. Just after the rice seedlings were transplanted, the soil released a large amount of inorganic N (26.3%-40.4%); however, after the topdressing,
