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.