Effect of irrigation management and nitrogen rate on nitrogen and water utilization of direct-seeded rice

Direct-seeded rice has advantages of less labor, lower labor strength and cost. But it meantime has different development characteristics from the transplanted rice. It is necessary to investigate the cultivation and growth of direct-seeded rice. In this study, a field experiment was conducted to investigate the effects of irrigation managements and nitrogen application rates on nitrogen and water utilization and yield of direct-seeded rice. In the experiment, a split-split plot design was set with rice varieties ('Dexiang 4103' and 'Jinnongsimiao') as the main factor, irrigation managements (shallow water irrigation, alternate irrigation with wetting and moderate drying, alternate irrigation with wetting and severe drying) as the sub-plot factor, and N rate (0 kg·hm^-2, 120 kg·hm^-2, 180 kg·hm^-2 and 240 kg·hm^-2) as the split-split plot factor. The photosynthetic rate, dry matter accumulation, nitrogen utilization, water utilization and yield of direct-seeded rice were measured at different growth stages. There was a significant interaction between irrigation management and N rate on nitrogen utilization, water utilization and yield of direct-seeded rice. Compared with the shallow water irrigation, the net photosynthetic rate at jointing stage, dry matter accumulation at jointing-heading stage, nitrogen transport amounts of stems and leaves at mature stage, nitrogen accumulation of grains at maturity stage, nitrogen agronomic efficiency and nitrogen fertilizer recovery efficiency were significantly increased in the alternate irrigation with wetting and moderate drying; however, the leaf area index at heading stage, dry matter accumulation before jointing and nitrogen accumulation in stems and leaves at mature stage were significantly decreased. The effect of N rates on nitrogen accumulation, nitrogen utilization efficiency and yield of 'Dexiang 4103' and 'Jinnongsimiao' were different. Under the shallow water irrigation, compared with nitrogen free treatment, the yields of 'Dexiang 4103' and 'Jinnongsimiao' increased by 31.79%-48.77%, 29.72%-45.36%, respectively, under treatments of applying nitrogen fertilizer. But with the N rate increase (higher than 180 kg·hm^-2), the yield of 'Dexiang 4103' was significantly decreased, and the corresponding indicators of 'Jinnongsimiao' were not significantly changed. Under the alternate irrigation with wetting and moderate drying, compared with nitrogen free treatment, the yields of 'Dexiang 4103' and 'Jinnongsimiao' increased by 32.58%-61.10%, 36.49%-48.45%, respectively, under treatments of applying nitrogen fertilizer. When N rate was more than 180 kg·hm^-2, for 'Dexiang 4103', the yield was not significantly changed, nitrogen fertilizer recovery efficiency, the nitrogen agronomic efficiency decreased with the increase of N rate. For 'Jinnongsimiao', the yield, dry matter accumulation not changed significantly, and the nitrogen accumulation at maturity stages increased significantly. Under the alternate irrigation with wetting and severe drying, compared with nitrogen free treatment, the yields of 'Dexiang 4103' and 'Jinnongsimiao' increased by 37.01%-42.88%, 30.11%-42.63%, respectively, under the treatments of applying nitrogen fertilizer. When N rate was more than 180 kg·hm^-2, the yield of two cultivars was not changed significantly, their nitrogen agronomic efficiency decreased with the N rate increaseing. The nitrogen accumulation of 'Dexiang 4103' at maturity stage increased significantly and that of 'Jinnongsimiao' was not changed significantly with N rate increasing. In summary, alternate irrigation with wetting and moderate drying was more suitable for high yield, water saving and high efficiency cultivation of direct-seeded rice. Furthermore, the highest yields of 'Dexiang 4103' and 'Jinnongsimiao' were observed under N rates of 240 kg·hm^-2 and 180 kg·hm^-2, respectively.