We have compared the effect of15N urea and15N ammonium sulfate on the process of stabilization in organic matter and mineralization in three soils of central Amazonia, classified as Oxisol, Ultisol, and low-humic gley. During the first month of incubation in the laboratory, the nitrogen fertilizer amendment slightly increased mineralization of organic nitrogen in both acid soils (Oxisol and Ultisol). In these soils, 60% of the nitrogen added was recovered as NH4* after 2 months incubation, while in the low- humic gley soil, 50% of the applied nitrogen was nitrified. The incorporation of15N fertilizer in the organic matter of the latter soil was higher (30%) than in the acid soils. Incorporation in the acid soils, though low, was more important for N derived from urea than ammonium sulfate. For the Oxisol, nitrogen incorporation in organic matter values varied between 12% and 19%, and 7-9% for the Ultisol. © 1995 Taylor & Francis Group, LLC.
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