In comparison with the late-rice in double cropping system, the amount of soil nitrogen mincralized during the growth of early rice in triple cropping system was considcrably lower owing to its shorter growing period togcther with a colder climate in its early stage of growth (from the middle of May to early June). The percentage contribution of soil nitrogen to the nitrogen absorbed by the single cropping late-rice (75.9%)was significantly higher than that by early rice(57.8%),and therefore heavy application of chcmical nitrogen fertilizer, apecially at its carly stage of growth, is indispcnsable for high yield of the early rice.