The objective of this paper is to present the soil carbon (C) and nitrogen (N) dynamics in a chronosequence made of a forest and pastures of different ages established in a Oxisol in the Western Brazilian Amazon Basin. The results of soil Carbon and Nitrogen stocks and gases fluxes were discussed. Stable 13C isotopic technique was used to calculate for a determinate age of pasture installation, the proportion of soil C remaining from the forest system and the proportion of soil C introduced by the grasses of the pasture system. The C lost from the original pool under the forest is 1.0 to 1.6 kg C m-2 concentrated during the first 5 years as pasture, and that the C fixed by the pasture (net fixation) is 1.7 to 2.3 kg C m-2 for the total period of 35 years. We agree with the assumption that cattle ranching would never be a profit-making venture as long as only the revenue from the sale of cattle is taken into account. But, now a days, the notions of taxes or refunds for C sequestration and land rehabilitation turn the management of areas that have already been converted to pasture a strategy for C sequestration
