Nitrogen uptake and grsan yield efects of double-cropping rice at different nitrogen application rates in the first crop of ratoon rice

Nitrogen uptake and grain yield effects of double-cropping rice at different nitrogen application rates in the first crop of ratoon rice were studied．The results show that the N uptake of double-cropping rice plants exhibits a parabola curve with N application rates in the first crop of ratoon rice ．The N absorption rate is the highest at the panicle development phase in the first crop，the second highest is at the late tillering stage and the grain filling stage of the first crop and during the period from harvest stage of the first crop to full heading stage of the second crop of ratoon rice ．Among the total N uptake of rice plants．the ratio of uptake of soil N decreases as N application rate enhances and the uptake of fertilizer N shows a parabola curve with the increasing of N application rates ．At the grain filling stage，it is the main N source transported from vegetable parts to grains ．The relationship between the dry matter accumulation of double-cropping rice and the N application rates in the first crop is also a parabola curve，in that the net assimilation rate(NAR)significantly decreases when the N application is over limitation，in turn leading to reduced crop growth rate(CGR)and net dry matter accum ulation．Grain yield has a closely significantly positive correlation with the total dry matter accum ulation，specially a very close correlation with the net dry matter accum ulation of the mi ddle-late stages in the first crop．The grain yield of the second crop also shows a very close corelation with the net dry matter accumulation of the mi ddle-late stages in the first crop．The net dry matter accum ulations recorded in the middle-late stages and the grain yield in the first and second crops are the highest after applying N 225-300kg／hm² in the first crop and suitable bud fertilizer in the rationed rice，in-dicating that the grain yield of the first crop is 10913-11300kg／hm² and that of the second crop of ratoon rice is 7565-7878kg／hm²．