Abstract: Two early season hybrid rice combinations, Luliangyou 996(two-line) and Jinyou 463(three-line) were used to study the characteristics of nitrogen utilization and yield formation under different N application amounts. The results showed that with the increasing of N amount the contents of chlorophyll in leaf and the nitrogen accumulation in stem, leaf, and spike increased, the leaf area index(LAI) enlarged and the dry matter accumulation before full heading especially from tillering to full heading pro-moted, the amount and the distribution rate of N accumulation in stem and leaf increased, but N dry matter production efficiency (NDMPE) and N production efficiency (NPE) decreased. The yields of the two rice combination were mainly affected by spikelets per panicle and the number of effective panicles. Optimum N rate increased the rate of the tillers with more than the critical stem-weight (a standard dry weight of stem used for judging whether the tiller grew into effective panicle or not) and the spikelets per panicle. But, the excessive N application decreased the rate of the tillers with more than the critical stem-weight and the spikelets per panicle, resulting in the decrease of dry matter production in late growing stages especially after milky ripe. Under the fertile soil condition in this study, optimum N amount was 225 kg per hectare for Luliangyou 996 which has large spike and compact plant and 180 kg per hectare for Jinyou 463 which has smaller spike, relax plant, and strong tillering ability.

Key words: Nitrogen fertilizer rate, Early season rice, Nitrogen utilization, Yield formation