Losses of fertilizer-derived N from transplanted rice after heading

Microplot field experiments with N-15-labeled (NH4)(2)SO4 were conducted to investigate the changes in the percentage of fertilizer N-15 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 N-15 application decreased by 6% (0.23 g N m(-2))from heading + 2 d to maturity, and %FNR from the topdress N-15 application decreased by 11% (0.32 g N m(-2))from heading 4 d to maturity. In 2000, %FNR from the topdress N-15 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 N-15 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 N-15 with the topdress N-15 application was not significantly different from that with the basal N-15 application in 1999, whereas N harvest index of fertilizer N-15 with the topdress N-15 application was higher than that with the basal N-15 application in 2000. N harvest index of fertilizer N-15, that indicated the proportion of the N-15 remobilized to panicles after heading, was substantially responsible for changes in %FNR from the basal and topdress N-15 applications after heading.