水稲

A pot experiment was conducted to evaluate the efficiency of ammonium- and nitrate- nitrogen contained in irrigation water during the reproductive growth period of paddy rice （Oryza sativa L. cv. Hinohikari） that did not receive topdressed nitrogen at the panicle formation stage. lrrigation of water containing a low level of nitrogen （7mgNL^-1） did not increase yields so much compared to topdressed nitrogen. lrrigation of water containing a high level of nitrogen （14mgNL^-1） caused substantial increases in yields with higher concentrations of grain nitrogen. To prevent the accumulation of nitrogen in grains, irrigation of Water containing high levels of nitrogen during the reproductive growth period is not recommended whether topdreSsed nitrogen is applied at the panicle formation stage or not. lrrigation of water containing the high level of nitrogen during the early cultivation period and water containing the low level of nitrogen during the reproductive growth period resulted in the greatest yields of grains with acceptable nitrogen concentrations owing to larger vegetative growth. Because ammonium-nitrogen was recovered effectively both in grains and straws compared to nitrate-nitrogen, ammonium-nitrogen showed greater effects on grain yields and nitrogen content than nitrate-nitrogen.