Saturday, 15 July 2006

Soils with Archaeological Dark Earth in Caxiuanã (Eastern Amazon): Pedologic Structuring, Mineralogy, Chemical Composition, Fertility and Influence of the Crop.

Marciléia S. Carmo1, Marcondes L. Costa1, and Dirse C. Kern2. (1) Federal Univ of Para, Trav. Monte Alegre, 1247- Jurunas, Belém-PA, 66030-370, Brazil, (2) Museu Paraense Emílio Goeldi, Av.Perimetral, 1901, Belém, Brazil

The area of Caxiuanã is located close to Marajó Island in Eastern Amazon. One of its peculiarities is the wide occurrences of dark earth soil, called Archaeological Dark Earth (ADE). The relationship between ADE and the prehistoric occupation is indicated by the great abundance of ceramic fragments and lithic artifacts. The dark color is due to richness of organic matter, 7% in ADE's of Caxiuanã compared with 2% in Yellow Latosols (Falesi, 1984; Kern & Costa, 1997). Besides that, they show high concentrations of Ca, Mg, Mn, P, Zn and C in comparison with other Amazon soils (Kern, 1988; Kern & Kämpf, 1989; Kern, 1996). Manioc (Manihot esculenta Crantz) is mainly cultivated for subsistence in those soils. Due the high fertility of those soils and its almost continued use for agriculture, a multi-disciplinary research to evaluate the impact of that activity on those soils has began: the reason of this continuity, why this doesn't occur with other Amazon soils, as well as to evaluate the transfer of some geochemical signatures of these soils to the consuming populations for long time, by alimentary diet, especially through manioc. The human population of Caxiuanã display relatively elevated values of Hg in its hair after analysis of the Institute Evandro Chagas (IEC) and of Museu Emílio Goeldi, arousing the interest for this research. The values of Hg determined in ADE's are also above the background. One site and two crops of manioc were selected to carry out this study based aon the analysis of geochemical data by Kern (1996): Raimundo site and its adjacent area (AA) with ADE without crop (ADE/WtC), Conceição plantation of manioc with ADE (ADE/WC) and adjacent areas Martinho with manioc (AA/WC). The soil profiles are constituted by quartz, kaolinite, goethite, hematite and anatase, with tendency to the domain of quartz in the A-horizons with ADE. While the ADE soil profiles look similar, diverging partly at depth, they are distinguished from Yellow Latosols by their larger depth or thickness of the A-horizon and by their sandy texture. The pH values in AA vary from 3.81 to 4.28, while in ADE 4.22 to 6.27, therefore the soils in the AA they are relatively more acid than ADE. In the soils profiles without ADE, AA/WtC and AA/WC, the pH tends to increase with the depth, while in the soil profiles with ADE, ADE/WtC or ADE/WC, it decrease with the depth. The chemical analyses show that the two sites with ADE are chemically similar to each other, but they diverge in composition of other sites with ADE in Caxiuanã and other areas, in that they are much richer in SiO2 and consequently poorer in Al2 O3, that is to say, poorer in kaolinite and richer in quartz. That mineralogical and chemical composition is comparable with most of the Latosols and Podzols of Amazon. They stand out, however, by their average contents of CaO and MgO, which vary significantly when compared with Amazon soils, which are poor in those elements. The medium concentration of organic matter (OM) in the A1 and A2 horizons was 9.1% and 5.5% in ADE, without and with crop of manioc, respectively. The dark soils studied here present similar pedologic profiles to other ADE of the area, and of other parts of the Amazon, in what concerns the ceramic material. The concentrations of the elements as Ca, Mg, Mn, Zn and P total are relatively high when compared in general to the soils of Amazon, and when compared to ADE of Caxiuanã. However the values of available P (14.6 mg/kg) are very low, due the continued use of ADE for manioc cultivated, same form that the total values of K2O. Therefore it is probable that the use of ADE for agriculture is progressively exhausted its reservations of nutrients, as expected, however in a less intense way than the common soil of Amazon. Possibly this is controlled by the content and type of organic matter.

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