Yield Response to Urea and Controlled-Release Urea as Affected by Water Supply in Tropical Upland Rice

Abstract

ABSTRACT Nitrogen (N) deficiency and water stress are major abiotic constraints upon the production of upland rice (Oryza sativa L.) in tropical Asia. It is, therefore, imperative to quantify the effect of N input on yield and to identify factors affecting the variability of yield response to N under various water supply conditions in order to develop efficient methods of N and crop management. Field experiments were conducted in Bihar, India, over four years to analyze the effect of N rate with split application of soluble urea, and a N sources, split application of soluble urea, and a single application of polyolefin resin-coated controlled-release urea (PCU) on the grain yield, apparent N recovery fraction (ANR), economic N use efficiency (grain yield per N uptake or GYNUE), and root growth. The yield responded linearly to increasing N rate in split urea, but along a different slope in each year. The yield response to N rate was variable among years due to variations in ANR and GYNUE. A significant relationship between the ANR and yield gain by N application across years indicated the importance of improving ANR to maximize the cost-benefit ratio. A comparison of split urea and PCU indicated that ANR could be improved more by the application of PCU than by split urea, especially under a high water supply from rainfall or irrigation. This is probably due to less N having been lost due to runoff and leaching when PCU was used. GYNUE was relatively stable across N treatments, but showed significant year-to-year variation, caused by variations in both the biomass N use efficiency (BNUE) (aboveground dry weight per N uptake) and harvest index (HI) as affected by rainfall conditions. The application of N increased root length from the surface to a below-45 cm depth, which may have contributed to the water exploitation under water stress.