The objective of this study was to evaluate the fate of two different forms of 15N-labeled nitrogen fertilizers in an Oxisol of Central Amazonia on a cowpea crop (Vigna unguiculata L.). There was no effect on yield from the application of 30 kg N/ha. The nitrogen balance at the end of the period indicated that 23 and 25% of nitrogen applied as ammonium sulphate and urea,respectively, was storoged in the grains; 35% of nitrogen derived from ammonium sulphate and 39% from urea was found in the 0-30 cm soil layer. The losses were 36% with urea and 42% with ammonium sulphate. About 15% of nitrogen fertilizer were recovered in the soil profile below the plow layer (0-30 cm), indicating that the losses were partially due to leaching and to other losses not quantified in this study.
15N-labelled; cowpea (Vigna unguiculata L.); Oxisol; Central Amazonia