TRANSFORMATIONS OF NITROGEN FERTILIZERS IN BRAZILIAN AMAZONIA SOILS

We have compared the effect of N-15 urea and N-15 ammonium sulfate on the process of stabilization in organic matter and mineralization in t hree soils of central Amazonia, classified as Oxisol, Ultisol, and low -humic gley. During the first month of incubation in the laboratory, t he nitrogen fertilizer amendment slightly increased mineralization of organic nitrogen in both acid soils (Oxisol and Ultisol). In these soi ls, 60% of the nitrogen added was recovered as NH4+ after 2 months inc ubation, while in the low-humic gley soil, 50% of the applied nitrogen was nitrified. The incorporation of N-15 fertilizer in the organic ma tter of the latter soil was higher (30%) than in the acid sails. Incor poration in the acid soils, though low, was more important for N deriv ed from ur ea than ammonium sulfate. For the Oxisol, nitrogen incorpor ation in organic matter values varied between 12% and 19%, and 7-9% fo r the Ultisol.