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 N 2 O; NO emissions from soils have also been shown to increase due to N fertilization. While N fertilizer use is increasing rapidly in the developing world and in the tropics, nearly all of our information on gas emissions is derived from studies of temperate zone agriculture. Using chambers, we measured fluxes of N 2 O and NO following urea fertilization in tropical sugar cane systems growing on several soil types in the Hawaiian Islands, United States. On the island of Maui, where urea is applied in irrigation lines and soils are mollisols and inceptisols, N 2 O fluxes were elevated for a week or less after fertilization; maximum average fluxes were typically less than 30 ng cm −2 h −1 . NO fluxes were often an order of magnitude less than N 2 O. Together, N 2 O and NO represented from 0.03 to 0.5% of the applied N. In fields on the island of Hawaii, where urea is broadcast on the surface and soils are andisols, N 2 O fluxes were similar in magnitude to Maui but remained elevated for much longer periods after fertilization. NO emissions were 2–5 times higher than N 2 O through most of the sampling periods. Together the gas losses represented approximately 1.1–2.5% of the applied N. Laboratory studies indicate that denitrification is a critical source of N 2 O in Maui, but that nitrification is more important in Hawaii. Experimental studies suggest that differences in the pattern of N 2 O/NO and the processes producing them are a result of both carbon availability and placement of fertilizer and that the more information‐intensive fertilizer management practice results in lower emissions.