Soil samples from sites 4, 10 and 15 year-old pastures and one reference native forest, from Paragominas-Pará (Brazil), were used to determine the carbon (C) and nitrogen (N) contents and the natural abundance of ¹³C isotope, with the objective of quantifying the C and N budgets during replacement of the forest by pastures. With pasture establishment, C and N contents were increased. After 4 and 15 years of pasture, the C content increased to 0.28 kg.m-2 and 0.86 kg.m-2 respectively in the 0-30 cm soil layer. The 10 year pasture remained stable (0.02 kg.m-2 decreased) in relation to the natural forest, but decreased by 0.15 kg.m-2 in the 0-10 cm layer. In the 0-30 cm layer, organic matter in the natural forest site was 2.6 kg.m-2 stable C and 2.1 kg.m-2 biodegradable C. After 10 and 15 years under pasture, biodegradable C represented 1.3 kg.m-2 and 1.0 kg.m-2, corresponding to 27.7% and 21.3% of the original forest C, while the stable C represented 2.6 kg.m-2.
organic matter; pastures; Amazon forest; <FONT FACE=Symbol>d</font>13C; carbon; nitrogen
