Results from fertilizer N recovery experiments with 15 N have indicated high values of fertilizer 15 N lost by rice ( Oryza sativa L.). Field experiments were conducted to evaluate uptake of fertilizer 15 N applied at preflood and at midseason and to determine the growth stage for maximum recovery of fertilizer 15 N by rice under a dry‐seeded culture system. Aboveground plant portions were sampled throughout the season for total dry matter, total N, and fertilizer 15 N. Total N accumulation when N was applied in a single preflood application was maximum at heading + 21 d and then declined at maturity. When N was applied at preflood and midseason, total N accumulation continued until maturity. Fertilizer 15 N recovered in the rice plant from the preflood N application was maximum (62%) at 21 d after application, and declined 10% by maturity. Fertilizer 15 N from the midseason N application had maximum recovery (75%) in the rice plant at 7 d after application, and declined by 18% by maturity. Accumulation of total N in the panicle between heading + 21 d and maturity for the midseason N application was significant; however, it was nonsignificant for the preflood N application. Recovery of fertilizer 15 N in the panicle did not differ between heading + 21 d and maturity for either the preflood or midseason N application. These results confirm earlier findings that sampling plants only at maturity will underestimate maximum fertilizer 15 N uptake by rice.
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Guindo et al. (1994) studied this question.