RECENT DEVELOPMENTS IN RESEARCH ON NITROGEN FERTILIZERS FOR RICE

Rice uses nitrogen fertilizers inefficiently, however, especially urea, which accounts for 85% of nitrogen fertilizers produced in Asia. This paper examines studies conducted in 11 Asian countries by the IRRI and the IFDC to determine the causes of nitrogen losses and ways to reduce them. Since these losses cannot be measured directly, a method was devised for measuring them indirectly by measuring losses in the balance of an introduced stable isotope, 15N (15N balance technique). The main nitrogen loss mechanisms were identified as ammonia volatilization and, to a lesser extent, nitrification-denitrification. The potential for ammonia volatilization was measured by studying the urea and ammonium-nitrogen concentrations in the floodwater after fertilizer application. It was found that reducing these concentrations is likely to reduce losses due both to volatilization and to surface runoff. Recent IRRI measurements of nitrous oxide fluxes from wetland soils give evidence that nitrification-denitrification does occur and appears peculiar to flooded soils. IRRI also studied the effect of deep placement of urea supergranules (USG) on root and algal growth patterns. Deep placement provided a bonus of nitrogen to the soil-plant system while not disturbing the natural algal flora as broadcast nitrogen fertilizers do. Research to increase nitrogen fertilizer efficiency shows that both deep placement of USG and conventional placement of slow-release fertilizers such as sulfur-coated urea (SCU) increase rice yields compared with split-application techniques. Data from 117 trials in Asia show a US$4-7 return for every extra dollar spent on labor for deep USG application or the cost of SCU. Individual test results, however, vary from site to site and season to season, suggesting the need for further research before a specific fertilizer or management practice is adopted, particularly since the trials were conducted at irrigated sites with good water control -- markedly different from the majority of Asian rice fields. A 17-item bibliography (1954-80) is appended.