Nitrogen Economy and Canopy Carbon Dioxide Assimilation of Tropical Lowland Rice

Increased irrigated areas, availability of short-duration modern rices (Oryza sativa L.) and herbicides, and increasing labor cost have motivated Asian farmers to shift from transplanting to direct seeding of lowland rice. Information on cultural requirements of direct seeded lowland rice in Asia is limited. Thus, two field experiments were conducted during the dry season on a Vertic Tropaquept soil to evaluate various cultural practices of direct seeded rice, and to compare the growth characteristics of direct seeded and transplanted rice. Direct seeding increased vegetative growth and shortened crop duration by more than 1 wk. Broadcast seeded rice gave the highest tiller number, followed by row seeded and hill-wise dibbled rice. Relative tillering rate was significantly correlated with relative growth rate. Greater vegetative growth of direct seeded rice decreased tissue N concentration which in turn impeded sink formation. Grain yield of direct seeded rice was significantly correlated with spikelet number per panicle but not with panicle density, while the reverse trend was observed in transplanted rice. Band placement of fertilizer N gave significantly higher canopy CO₂ assimilation rates and grain yield than did conventional fertilizer application methods in both direct seeded and transplanted rice. By increasing N availability to the plant, band placement allowed the use of a onethird to two-thirds N split application which increased plant N concentration during the reproductive stage, particularly in direct seeded rice. Leaf area index (LAI) was higher in direct seeded than in transplanted rice. Canopy photosynthesis was limited by LA1 during the vegetative growth stage and by foliage N concentration during the reproductive stage. Contribution from the Agron. Dep. (IRRI). Supported by the German Agency for Technical Cooperation (GTZ), West Germany.