Abstract: Nitrogen contents in the waters and sediments collected from three fish ponds dominated with grass carp but slightly different in polyculture model (Model I, containing 250 Grass carps, 35 Silver carps, 40 bighead craps, and 15 Crucian carps; Model II, 250 Grass carps, 25 Silver carps, 20 bighead craps, 20 paddlefishes, and 15 Crucian carps. Model III,250 Grass carps, 35 Silver carps and 15 Crucian carps) were analyzed for comparison. Results show that in waters that the mean total nitrogen content of Model Ⅰ, Model Ⅱ, Model Ⅲ was 1.251 mg?L^-1, 1.001 mg?L^-1, and 1.228 mg?L^-1, respectively; and the mean ammonium nitrogen content, 0.391 mg?L^-1, 0.345 mg?L^-1 and 0.319 mg?L^-1, respectively; the content of nitrate showed a decreasing order of Model I > Model II > Model III, and the content of nitrite followed a different order, i.e. Model Ⅱ> Model Ⅰ> Model Ⅲ. And in sediments, the mean total nitrogen content of Model I, Model II and Model III was 0.793 mg?g^-1, 0.910 mg?g^-1 and 0.963 mg?g, respectively; and the mean ammonium nitrogen content, 0.005 mg?g^-1, 0.006 mg?g^-1 and 0.004 mg?g^-1, respectively. TN in water was significantly lower in Model II than in Models I and III (P <0.05), and the transformation of nitrogen nutrients in the sediment was quicker too. Obviously, Model II is the most conducive to transformation and utilization of Nitrogen nutrients in the waterbody.

Key words: grass carp, nitrogen, polyculture model, pond