Undisturbed oxisols in a central Amazon tropical forest were fertilized with ammonium, nitrate, or phosphate. Enhanced emissions of N2O were observed for all treatments with 1 day of fertilization, with the response NO3-≫NH4+≫PO43-. Approximately 0.5% of applied NO3- was converted to N2O within 2 weeks after application, with less than 0.1% of the NH4+ converted to N2O. These experiments reveal a potentially large source of N2O from microbial reduction of NO3- in the clay soils of Amazonia. ¿ American Geophysical Union 1987