Caracterização química e mineralogia de solos antrópicos (terras pretas de índio) na Amazônia central

Abstract

Archeological sites are frequent in the Brazilian Amazon. The physical, chemical and morphological properties, due to anthropic influence, are quite different from surrounding soils, mainly in the uplands. Although several studies have been carried out to understand the magnitude of these modifications and their influence on the genesis of such soils, aspects such as the mineralogical properties need to be better described. This study aimed at evaluating the chemical and mineralogical properties of anthropic horizons (Au) of soils of upland and floodplain sites located in the Sedimentary Basin of the Amazon, between the cities of Coari and Manaus. Soil samples were collected and analyzed for chemical properties (pH) and for the total contents of Fe, Al, Si and P. The mineralogical compositions of the clay and sand fractions were evaluated by X-ray diffraction (XRD). The results showed that anthropic modifications lowered the acidity and exchangeable Al soil contents and enhanced the contents of available Ca and P. The total P contents in the anthropic horizons ranged from 1630 to 8840 mg kg-1 of P2O5. Kaolinite was the main mineral found in the clay fraction of upland soils, followed by gibbsite, goethite and titanium oxides. The floodplain soils showed clay fractions composed of kaolinite, goethite and 2:1 silicates. The sand fractions of all studied soils consisted predominantly of quartz. The magnetic fractions consisted of hematite and maghemite. In conclusion, better chemical properties were observed in the anthropic horizons, as well as differences in their mineralogical composition as compared to the other studied sites.