Nitrogen Accumulation by Rice.

Consumption of rice is worldwide, and the rice-eating population is expanding rapidly. Breeding high yielding varieties has helped increase rice production. However, effective management practices have also been useful, and will probably become more vital as breeding efforts approach the genetic limits of the rice plant. Nitrogen (N) management is particularly important in reducing N losses and increasing grain yield. The objectives of this study were (1) to determine the amount of N accumulation by rice at different stages of plant development, (2) to determine if differences in N accumulation exist between rice varieties, and (3) to determine if differences in N accumulation exist between different N sources. For each objective, N accumulation as well as the associated rice plant growth was of interest. Field studies were conducted at the Rice Research Station in Crowley, LA, in 1993, 1994, and 1995. Various varieties of rice were planted in both drill-seeded and water-seeded culture. Urea and ammonium sulfate were used as N sources. Microplots and $\rm\sp{15}N$ were utilized. Nitrogen accumulation, straw, grain, and total dry matter, and root length density were measured. Nitrogen accumulation occurred throughout the growing season, while dry matter increased after panicle differentiation. Grain yields were constant from season to season, but straw yield, root growth and N accumulation varied because of air temperature. Responses were the same in drill-seeded and water-seeded culture. Most midseason N was accumulated by rice plants within 7 days after topdress. There were no differences in N accumulation among rice varieties, although grain yields differed. Modern varieties produced more grain than older varieties, and among modern varieties, semi-dwarf varieties produced more grain than tall varieties. Ammonium sulfate and urea were equally effective sources of N for rice, and S did not appear to affect those results. Microplot size and presence of a retainer were found to have an effect on experimental results. A 30 x 30 cm microplot without a retainer was found to be the best substitute for normal field conditions