Effects of deforestation and cultivation on the physical and chemical characteristics of soils under natural forest in Eastern Amazonia

Abstract

At the CPATU-EMBRAPA Experimental Center of Capitão Poço, Pará, Eastern Amazonia, the effects of clearing and annual cropping on soil physical and cbemical characteristics were studied. ln this area soils are predominantly medium-textured latosols and podzolic latosols, according to the Brazilian soil classification. The selected adjacent sites fall in a sequence from native to freshly burnt, forest, to cultivated for one and five years, for two years and fallow for three years. ln each position on the sequence a pedon was described, sampled, and compared to three reference pedons under native forest, characterized according to drainage intensity. Burning itself had only slight effects, except on the litter material, which was entirely substituted by ash and charcoal residues. With increasing duration of cultivation, the thickness of the surface A11 horizon decreased, as a result of increasing density and cohesion. Mottling increased, and reached the A3 horizon, whereas the flocculation degree, which expresses the proportion of non water-dispersible clay particles, decreased. Residues burnt above ground desappeared rapidly; afier five years cropping soil organic carbon content decreased by 14% in the whole profile, and 24% in the 0-20cm soillayer. Burning was found to bring about an increase of 2.5 units of surface soil pH, and to release exchangeable bases, which progressively migrate, resulting in a decrease of exchangeable aluminium, even in the deeper soillayers, and afier five years of cropping. Afier three years of fallow following two years of cropping, the vegetation underwent regeneration; organic residues were produced, soil biology was stimulated and there was a progressive return to initial soil characteristics.