No abstract

(6) RESUMO Na Amazônia brasileira é comum a ocorrência de sítios arqueológicos. Frequentemente, observa-se que as ações humanas nesses sítios promoveram modificações significativas em muitas das características físicas, morfológicas e químicas dos solos desses ambientes, tornando-os muito diferentes dos solos adjacentes, especialmente nas áreas de terra firme da região. Embora muitos estudos tenham sido conduzidos visando compreender a magnitude dessas modificações e seus reflexos na gênese dos solos influenciados, muitas questões precisam ser mais bem compreendidas, sobretudo as relacionadas com a mineralogia desses solos. Este estudo teve como objetivo avaliar as características químicas, a composição mineralógica das frações argila e areia e o grau de pedogênese de cinco perfis de solos com horizonte A antrópico (Au), em ambientes de terra firme e várzea, localizados na Bacia Sedimentar do Amazonas, entre Coari e Manaus-AM. Amostras de solos foram coletadas, preparadas e submetidas às análises químicas de rotina e caracterizadas quanto aos teores de carbono oxidável e aos teores de Si, Fe, Al e P; adicionalmente, efetuou-se a identificação dos minerais presentes nas frações argila e areia, por meio de difratometria de raios X. Os resultados mostraram que as modificações promovidas pela atividade humana levaram à melhoria da fertilidade do solo, resultando em solos com acidez moderada, elevados teores de Ca 2+ e de P disponíveis e baixos teores de Al 3+ trocável.

Amazonian dark earths (ADEs) are fertile soils created by pre-Columbian Amerindian societies of the Amazon Basin. However, it is still not clear whether these soils were produced intentionally to improve infertile Amazonian upland soils or if they resulted from the accumulation of organic matter from sedentary settlements. This study characterizes the ADEs found in the naturally fertile alluvial floodplains of the Amazon River in the Central Brazilian Amazon according to total, exchangeable, and available contents of elements and organic carbon in soil profiles. ADEs contained higher levels of available elements and total P, Ca, Zn, and Cu. High total Cr, Ni, Co, and V content in these soils indicate that mafic minerals contributed to their composition, while higher contents of P, Zn, Ba, and Sr indicate anthropic enrichment. The presence of ADEs in floodplain areas strongly indicates non-intentional anthropic fertilization of the alluvial soils, which naturally contain levels of P, Ca, Zn, and Cu higher than those needed to cultivate common plants. The presence of archaeological sites in the floodplains also shows that pre-Columbian populations lived in these regions as well as on bluffs above the Amazon River.

SUMMARYLarge areas of Plinthosols with ferruginous materials such as plinthite and/or petroplinthite are fairly common in the Brazilian Amazon basin. This work was carried out to investigate the chemical behavior, mineralogical composition and weathering stage of four representative soil profiles with plinthite and petroplinthite, in Iranduba, AM (Central Amazon). Three well-drained soil profiles at high elevations were studied (P1, Plinthic Vetic Ferralsol; P2 and P3, Vetic Endopetric Plinthosol) and a contrasting poorly drained soil (P4 Haplic Plinthosol), located at low elevation. After profile descriptions, soil samples were collected from each horizon, air-dried, sieved (2 mm), and analyzed for particle-size distribution, pH, exchangeable cations (Al 3+ , Ca 2+ , Mg 2+ , K + , and Na + ), as well as available P and total organic carbon (TOC) content. The minerals present in the clay and sand fractions, as well as in the ferruginous materials were identified by X-ray Diffraction (XRD). The weathering stage of these soils was assessed by means of Ki and Kr indexes, and the amounts of free and amorphous Fe and Al oxides by using dithionite citrate bicarbonate (DBC) and ammonium oxalate dissolution procedures, respectively. The results showed that all soils were extremely unfertile, with pH levels ranging between strong and moderate acidity, very low sum of bases and organic matter content, and of available P. The mineralogy of the soil CHEMICAL PROPERTIES AND MINERALOGY OF SOILS WITH PLINTHITE AND PETROPLINTHITE... profiles was very similar, mainly of the well-drained soils, with predominance of kaolinite and quartz in the clay and sand fractions, respectively. In the poorlydrained P4, 2:1 clay particles were also observed. These profiles can be considered highly developed according to the Ki index, however, the Ki value of P4 was higher, indicating that this soil was less developed than the others. In summary, these profiles with plinthite and petroplinthite can be characterized as highly developed and infertile soils and are, with exception of P4, well-drained.Index terms: Plinthosol, soil fertility, XRD, weathering degree, Central Amazon. RESUMO: ATRIBUTOS QUÍMICOS E MINERALOGIA DE SOLOS