RICE PLANT-GROWTH AND NITROGEN ACCUMULATION FROM A MIDSEASON APPLICATION

Nitrogen (N) fertilization in rice (Oryza sativa L.) is extensive thro ughout the world, but fertilizer N recovery is generally low. Split fe rtilizer applications that coincide with plant demand have been sugges ted as a method of improving fertilizer N efficiency. However, the eff ectiveness of split applications has not been established. Furthermore , there is little information available on plant N accumulation after a midseason application. The purpose of this study was to measure plan t dry matter; root growth, and N accumulation after a midseason N appl ication and to determine the length of time during which midseason N i s accumulated by the plant. 'Cypress' rice was drill-seeded in a Crowl ey silt loam soil (fine, montmorillonitic, thermic Typic Albaqualf) an d urea-N was broadcast at 101 kg N ha(-1) preflood. Microplots enclose d by retainers were established prior to panicle initiation (PI), and N-15-labeled urea was topdressed at PI into the floodwater within each microplot at 67 kg N ha(-1). Microplots were harvested at 1 day after topdress (DAT), 3 DAT, 7 DAT, 14 DAT, and at 90% heading (35 DAT). Dr y matter production was not affected by the midseason N application an d increased linearly from the time of midseason application until 90% heading. Root growth at the time of the midseason application was exte nsive and roots could be seen at the soil surface. Root length density was greatest in the top 7.5 cm of the soil profile and decreased with depth. Most accumulation of midseason N occurred within 7 DAT. Both m idseason N and native N in the plant increased during this period. Abo ut half of the midseason N was accumulated by the crop, probably becau se of the extent of the root system. This approximates N recovery from preplant or preflood N applications. Nitrogen loss was probably due t o ammonia (NH3) volatilization. Nitrogen accumulation by the plants co ntinued throughout the duration of the experiment. This study shows th at N broadcast into the floodwater at PI is quickly and efficiently ut ilized.