The system of shifting cultivation is one of the main agriculture models used in the State of Rondônia, as well as in other areas of the Brazilian Amazon. This paper aimed at na evaluation of the nutrient balance in the forest ecosystem after cutting down and burning the primary vegetation. After felling and burning the forest, an experiment was conducted to compare burned areas without cultivation (burned) to burned areas with cultivation of rice (cultivated). Furthermore, these areas were compared with an area of primary vegetation (forest) used as reference. Burning consumed 36.3% of the initial biomass and produced 5.5 Mg ha-1 of ashes with significant amounts of nutrients, mainly Ca, Mg, and K. To clear areas by means of burning turned out to be a practice of low efficiency, because only a small percentage of the original biomass was converted into ashes and the greatest part remained in the form of residues. Even with the restitution of nutrients such as Ca, Mg, K by rain, there was a considerable removal of N, P, K, Ca, Mg, and S, on the one hand by direct action of the fire, on the other through the carrying off of the ashes by the wind, and/or the removal by the crop. Results indicated that nutrient loss in the burned uncultivated area was greater than in the burned cultivated one, demonstrating the importance of soil covering to keep the elements in the system. At the end of the culture cycle, the residual effect of the ashes was still noticeable in the system, clearly expressed by the P, K, Ca, and Mg values which were higher than those measured in the control area (forest).
Amazonia; shifting agriculture; residual effect; forest conversion
