In the State of Rondônia, Brazil, soil characteristics of forest and converted pasture areas were compared (four age classes). Adjacent areas of undisturbed natural forest were also surveyed for comparison. Based on the analysis of TM/LANDSAT images obtained between 1987 and 1997, three sites were selected for soil sampling. Two of these sites belong to the class "Argissolo Vermelho-Amarelo" (PVA - Ultisol), and the other to the "Luvissolo Crômico" (TC - Alfisol). The soil samples were collected at a depth of 20 cm, and fertility and texture analyses were carried out to obtain data on pH, exchangeable bases, available phosphorus, aluminum and base saturation, and organic matter values. Soil fertility in the dystrophic soil of low natural fertility (PVA) was strongly influenced by the incorporation of ashes derived from the burning of the primary forest and by the remaining vegetation in the first years after pasture implantation. Base saturation was higher in the two PVA pasture areas than in the forest, even after more than 10 years of use. On the contrary, the aluminum rate was lower in pastures after conversion in all investigated age classes. The values of available phosphorus dropped quickly after reaching a peak and returned to practically the original level after over 10 years of pasture utilization in both studied soil types. The eutrophic soil (TC) was less influenced by the effect of the ashes than the distrophic soil (PVA) and presented elevated nutrient leaching, due to factors related to the low clay content and the marked local topography.
land use; carbon; shifting cultivation; nutrient dynamics