Abstract
Microplot field experiments with 15N-labeled (NH4)2SO4 were conducted to investigate the changes in the percentage of fertilizer 15N recovery (%FNR) of transplanted rice (Oryza sativa L.) with basal (4 g m−2) and topdress (3 g m−2) applications for two years. In 1999, %FNR from the basal 15N application decreased by 6% (0.23 g N m−2)from heading + 2 d to maturity, and %FNR from the topdress 15N application decreased by 11% (0.32 g N m−2)from heading − 4 d to maturity. In 2000, %FNR from the topdress 15N application decreased by 9% (0.26 g N m−2) from heading − 1 d to maturity, whereas there was no significant decline in %FNR from the basal 15N application after heading. A decrease in %FNR occurred even when total N of rice plants showed no decrease after heading. Nitrogen harvest index of fertilizer 15N with the topdress 15N application was not significantly different from that with the basal 15N application in 1999, whereas N harvest index of fertilizer 15N with the topdress 15N application was higher than that with the basal 15N application in 2000. N harvest index of fertilizer 15N, that indicated the proportion of the 15N remobilized to panicles after heading, was substantially responsible for changes in %FNR from the basal and topdress 15N applications after heading.
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