FERTILIZATION PRACTICES AND SOIL VARIATIONS CONTROL NITROGEN-OXIDE EMISSIONS FROM TROPICAL SUGAR-CANE

Nitrogen (N) fertilization of agricultural systems is thought to be a major source of the increase in atmospheric N2O; NO emissions from soi ls have also been shown to increase due to N fertilization. While N fe rtilizer use is increasing rapidly in the developing world and in the tropics, nearly all of our information on gas emissions is derived fro m studies of temperate zone agriculture. Using chambers, we measured f luxes of N2O and NO following urea fertilization in tropical sugar can e systems growing on several soil types in the Hawaiian Islands, Unite d States. On the island of Maui, where urea is applied in irrigation l ines and soils are mollisols and inceptisols, N2O fluxes were elevated for a week or less after fertilization; maximum average fluxes were t ypically less than 30 ng cm(-2) h(-1) NO fluxes were often an order of magnitude less than N2O. Together, N2O and NO represented from 0.03 t o 0.5% of the applied N. In fields on the island of Hawaii, where urea is broadcast on the surface and soils are andisols, N2O fluxes were s imilar in magnitude to Maui but remained elevated for much longer peri ods after fertilization, NO emissions were 2-5 times higher than N2O t hrough most of the sampling periods. Together the gas losses represent ed approximately 1.1-2.5% of the applied N. Laboratory studies indicat e that denitrification is a critical source of N2O in Maui, but that n itrification is more important in Hawaii. Experimental studies suggest that differences in the pattern of N2O/NO and the processes producing them are a result of both carbon availability and placement of fertil izer and that the more information-intensive fertilizer management pra ctice results in lower emissions.