1.Because the growing period of rice under "rice-rice-wheat" rotation,as compared with that under "rice-wheat" rotation,is greatly shortened,the amount of nitrogen mineralized in soil absorbed by rice plant is diminished to such a considerable extent that the need on fertilizer nitrogen has to be increased significantly.At the sane time,the dower rate of mineralization of soil nitrogen under low temperature in the early stage of early rice can not meet the need of higher nitrogen absorption rate of early rice at that time,and it is indispensible to increase nitrogen fertilizer applied in the early stage of early rice.