Abstract:To evaluate reasonably the impact of nitrogen fertilizer on rice-based ecosystem, the main biological characteristic parameters of rice plants and population dynamics of predators in rice canopy were determined in paddy fields applied with 0, 100 and 200 kg/hm2 nitrogen fertilizer, respectively, and the predation capacity of generalist predators on leaf- and planthoppers was also estimated by monitoring the predated number of alternative prey, pupae of fruit fly, Drosophila sp., in a special predation trap. The results showed that plant height, total leaf area and dry straw weight of rice plants in field with 200 kg N/hm2 were markedly higher than that in field with 100 kg N/hm2, however, no obvious difference between two fields was found in the number of tillers and yield of rice. Predators in rice canopy in all three tested fields were dominated by Tetragnatha sp., Araneus sp., Cyrthorhinus lividipennis, Micraspis sp. and Agriocnemis sp. The most abundant Tetragnatha sp. was sampled at booting stage of rice, while the number of Araneus sp. declined and Cyrthorhinus lividipennis, Micraspis sp. and Agriocnemis sp. increased with the growth of rice. Significantly abundant Tetragnatha sp. and Micraspis sp. were recorded in the field without nitrogen fertilizer than those with nitrogen fertilizer. Both number of spider webs and total web area on rice canopy at booting stage were strongly decreased with the increase of nitrogen fertilizer used in the fields. Predation capacity of generalist predators on pupae of fruit fly at booting and milky stages of rice significantly decreased in the fields applied with nitrogen fertilizer compared to that field without nitrogen fertilizer, and the lowest predation capacity was found in the field with 200 kg/hm2 nitrogen fertilizer. It implies that excessive application of nitrogen fertilizer in paddy field maybe weaken the natural control of generalist predators on insect pests.