Abstract: Nitrogen is the most important nutrition in rice production, and its function is affected by cultivar, fertilizer application and soil condition. In resent years, paddy field in Yangzi River has a trend of becoming fertile. In order to identify the optimal nitrogen application amount in high-fertility paddy field and to study the cultivar difference in nitrogen requirement, we planted 13 japonica rice cultivars with 5 nitrogen application levels in a high-fertility paddy field in China National Rice Research Institute, Zhejiang province. The results were as follows: (1) Rice yield was significantly increased by nitrogen, most japonica rice cultivars reached their highest yield when nitrogen was applied at 150–225 kg ha^-1. With the increase of nitrogen application amount, 1 000-grain weight and seed setting rate decreased, while grains per panicle and effective panicle number increased till nitrogen application amount reached 225 kg ha^-1. (2) There existed cultivar difference in optimal nitrogen application amount. According to the cultivar yield characteristics in none-nitrogen application treatment, low-yielding japonica rice cultivars were sensitive to N fertilizer, their yield increased largely even when small amount N fertilizer was applied, while the yield of high-yielding cultivars changed little when applied N fertilizer < 75 kg ha^-1; the optimal nitrogen application amount of cultivars with middle panicle number was larger than that with low (< 220×10⁴ per hectare) and high (< 220×10⁴ per hectare) panicle number. (3) Most of the cultivars released before 1980 had the highest yield when 225 kg ha^-1 nitrogen was applied, while the nitrogen needed for the highest yield in cultivars released after 1980 was different.

Key words: Japonica rice, Grain yield, Nitrogen fertilizer, Cultivar difference