Rice plant growth and nitrogen accumulation from a midseason application¹

Contribution from the Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center. Approved for publication by the Director of the Louisiana Agricultural Experiment Station as Manuscript No. 96–09–0207.

Abstract

Nitrogen (N) fertilization in rice (Oryza sativa L.) is extensive throughout the world, but fertilizer N recovery is generally low. Split fertilizer applications that coincide with plant demand have been suggested as a method of improving fertilizer N efficiency. However, the effectiveness 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 plant dry matter, root growth, and N accumulation after a midseason N application and to determine the length of time during which midseason N is accumulated by the plant. ‘Cypress’ rice was drill‐seeded in a Crowley silt loam soil (fine, montmorillonitic, thermic Typic Albaqualf) and urea‐N was broadcast at 101 kg N ha^‐1 preflood. Microplots enclosed by retainers were established prior to panicle initiation (PI), and ^l5N‐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.7DAT, 14 DAT, and at 90% heading (35 DAT). Dry matter production was not affected by the midseason N application and increased linearly from the time of midseason application until 90% heading. Root growth at the time of the midseason application was extensive 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 midseason N and native N in the plant increased during this period. About half of the midseason N was accumulated by the crop, probably because of the extent of the root system. This approximates N recovery from preplant or preflood N applications. Nitrogen loss was probably due to ammonia (NH₃) volatilization. Nitrogen accumulation by the plants continued throughout the duration of the experiment. This study shows that N broadcast into the floodwater at PI is quickly and efficiently utilized.

Notes

Contribution from the Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center. Approved for publication by the Director of the Louisiana Agricultural Experiment Station as Manuscript No. 96–09–0207.