Correlation of the amount of nitrogen accumulated in the aboveground biomass at panicle initiation and nitrogen content of soil with the nitrogen uptake by lowland rice during the period from panicle initiation to heading

To estimate the site-specific optimal amount of nitrogen (N) to be applied as top-dressing (N top-dressing) at panicle initiation, we examined the effects of the amount of N accumulated in the aboveground biomass at the panicle initiation stage (N,), the total N content of soil and the amount of mineralizable soil N (Nm) on the amount of soil N taken up by the plants during the period from panicle initiation to heading (Ns) and the recovery ratio of applied chemical N during this period (J). We analyzed the data obtained in paddy fields with a similar soil type and weather condition in the growth period from 1999 to 2001. The growth of rice plants was regulated by the amounts of N applied before transplanting and at panicle initiation. The relative N uptake rate during the period from panicle initiation to heading (RNR) in the plots without N top-dressing was explained about 89% by multiple regression of N the total N of soil and Nm. Ns in the plots with N top-dressing that was estimated using RNR showed a significantly negative partial correlation with N the total N of soil and Nm. J in the plots with N top-dressing showed a significantly negative partial correlation with N but a significantly positive partial correlation with Nm. These results suggest that the fertilizer N should be applied considering the contribution of N the total N content of soil and Nm to Ns and J to fit the site-specific N demand of rice.