ACCUMULATION OF FERTILIZER N-15 BY RICE AT DIFFERENT STAGES OF DEVELOPMENT

Results from fertilizer N recovery experiments with N-15 have indicated high values of fertilizer N-15 lost by rice (Oryza sativa L.). Field experiments were conducted to evaluate uptake of fertilizer N-15 applied at preflood and at midseason and to determine the growth stage for maximum recovery of fertilizer N-15 by rice under a dry-seeded culture system. Aboveground plant portions were sampled throughout the season for total dry matter, total N, and fertilizer N-15. 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 N-15 recovered in the rice plant from the preflood N application was maximum (62%) at 21 d after application, and declined 10% by maturity. Fertilizer N-15 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 non-significant for the preflood N application. Recovery of fertilizer N-15 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 N-15 uptake by rice.