In a yellow Latosol from the Central Amazon rain forest, the amount of organic carbon within a 5-m depth is 240 t/ha, of which 120 t/ha are concentrated in the first 1 m of soil. Organic matter reflects soil conditions, and reacts quickly to external influences. This happens in the case of agricultural use, which frequently implies burning of the forest phytomass and litter. Lysimeter studies show that burning and subsequent decomposition of above-ground organic residues result in a loss of water-soluble nutrients, mainly nitrate and mineral cations, together with soluble organic anions. Even though a well-managed pasture of Brachiaria humidicola was installed following burning, the organic-matter content of the 0-0.20-m soil layer decreased by 25% in two years, because the decomposition of humus of forest origin was faster than the input of humus of pasture origin. After this time, these two processes were reversed, and soil organic-matter content was restored in about eight years. Natural carbon-13 measurements show that the proportion of soil carbon of grass origin was about 20% after two years, and 50% after eight years of pasture.
