This study analyzes homogenous stands of Eucalyptus grandis and Pinus caribaea var. hondurensis in different growth stages as well as fragments of natural vegetation of "Cerrado" and Atlantic Forest. The climatic, edaphic and floristic conditions of the chosen areas are representative of the native forests and forest plantations established at the Western plateau of the state of São Paulo. Soil samples (0-15 cm) were incubated under aerobic and anaerobic laboratory conditions by successive periods of 3, 3, 4, 4, 6 and 10 weeks in a total of 30 weeks. In both conditions, the accumulated amounts of mineralized N followed an exponential relationship with the incubation time. The equation Nt = N0 + b/t was better adjusted to the data than the equation proposed by Stanford & Smith (1972), Nt = N0 (1 - e-kt), where N0 is the potentially mineralizable N, Nt is the accumulated amount of mineralized N and t is the time of mineralization. This equation overestimated the values of N0 in several sites and incubation conditions, besides not modeling appropriately to the data. The use of the first equation to model the N mineralization kinetics is suggested. The N mineralization rates stabilized after 8 to 15 weeks of incubation. Under aerobic conditions, the total amounts of N0 (layer 0-15 cm) were in the average of 103 ± 53 kg ha-1 of N, and under anaerobic conditions, in the average of 281 ± 175 kg ha-1 of N. Considering that the N0 values obtained aerobically were underestimated, some of the main reasons may be: (a) the potential harmful effects of soil leaching with saline solution (CaCl2 0,01 mol L-1) on the microbial communities and (b) removal of great amount of soluble organic-N, potentially mineralizable, by leaching. The medium pools of N of the evaluated sites were estimated to be enough for 3 to 5 cultivation rotations (7 years each) of E. grandis. At the end of this period, response to N fertilization will probably be quite significant.
Eucalyptus; Pinus; Brazilian savanna; Atlantic Forest; forest soil; Nitrogen; mineralization