On-farm soil N supply and N nutrition in the rice-wheat system of Nepal and Bangladesh

On-farm research to evaluate the productivity and nitrogen (N) nutrition of a rice (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system was conducted with 21 farmers in the piedmont of Nepal and with 21 farmers in northwest Bangladesh. In Nepal, two levels of N-fertilizer (0-22-42 and 100-22-42 kg N-P-K ha(-1)) and farmers' nutrient management practices were tested in the rice season, and three levels of N (0-22-42, 70-22-42, and 100-22-42) and farmers' practices were evaluated in the wheat season. The treatments in Bangladesh included a researchers managed minus-N plot (0-22-42) and the farmers' practices. Rice and wheat yields were higher in all treatments than the 0-22-42 control plots, with the exception of rice with the fanners' practices at one location in Bangladesh. The researchers' treatment of 100-22-42 in Nepal resulted in larger yields of both rice and wheat than the farmers' practices, indicating that farmers' rates of N-fertilizer (mean 49 kg N ha(-1)) were too low. Delaying wheat seeding reduced yields in the fertilized plots in both countries, especially as N-fertilizer dose increased. Soil N-supplying capacities (SNSC), measured as total N accumulation from the zero-N plots (0-22-42), and grain yields without N additions were greater for rice than for wheat in both Nepal and Bangladesh. Higher SNSC in rice was probably due to greater mineralization of soil organic N in the warm, moist conditions of the monsoon season than in the cooler, drier wheat season. However, SNSC was not correlated with total soil N, two soil N availability tests (hot KCl-extractable NH(4)(+) or 7-day anaerobic incubation), exchangeable NH(4)(+) or NO(3)(-). Wheat in Nepal had greater N-recovery efficiency, agronomic efficiency of N, and physiological efficiency of N than rice. Nitrogen internal-use efficiency of rice for all treatments in both countries was within published ranges of maximum sufficiency and maximum dilution. In wheat, the relationship between grain yield and N accumulation was linear indicating that mobilization of plant N to the grain was less affected by biotic and abiotic stresses than in rice. (C) 1999 Elsevier Science B.V. All rights reserved.