Dinâmica do carbono e do fósforo em um latossolo amarelo, textura argilosa, após conversão da floresta em capoeira e pastagens, na Amazônia oriental

Abstract

Most of brazilian soils show high acidity, low content of assimilable phosphorus (P) and high fixation capacity of P. In the amazonian region, oxisols prevail: poor in nutrients, with organic matter as the main source of nutrient, these soils have a high proportion of their P in organic form. Forest conversion in food crops or pastures, using slash and burn method, induces changes in the functioning of these systems, with sometimes a decrease, and at other times an increase of the carbon and phosphorus content of soils. The aim of this work was to understand as the tropical forest conversion affects the dynamic of organic matter and of P forms in a clay oxisol of the eastern Amazonia. The first objective was to assess the effect of deforestation and soil use on organic matter content and quality (chapter 2) and to characterize and quantify phosphorus forms after this conversion of vegetation (chapter 3). In the first stage a particle-size fractionation of organic matter was made, with the obtention of the clay fraction (0-2 µm), the silt and clay fraction (0-53 µm) and the sand fraction (53-2000 µm). In the second stage, the determination of the organic carbon and the total nitrogen (Chapter 2) and of the forms of phosphorus (Chapter 3) was carried out. The results showed that the silt fraction has the highest content of carbon, for the 3 depth layers studied. However, as the clay fraction is widely predominant, there was a high relation between the soil carbon content and this fraction: most of the carbon was associated with this fraction. After the conversion of forest to pasture (of the installation of a fallow), a slight increase of the C content occurred, mainly in the sand fraction. At the same time the content of the total phosphorus of soil tended to slightly increase after conversion to pasture and as the age of pasture increase. The total organic phosphorus accounted for 21 % of the total soil phosphorus. The acid-soluble phosphorus was the prevailing form of organic P, accounting for 65 % of the total organic P, followed by the bicarbonate-soluble phosphorus, and the basesoluble phosphorus, accounting for 21 % and 15 % respectively.