Trace element distributions in Brazilian Cerrado soils at the landscape and micrometer scales

Joao Jose Granate de Sa e Melo Marques, Purdue University

Abstract

Rapid agriculturalization and increased human occupation of the Brazilian Cerrado region brings with it increased needs to assess the environmental impact of man's activities. The objectives of this study were to obtain baseline information on total trace element contents of Cerrado soils, use trace element signatures to answer some unresolved questions in soil genesis, and examine the microscale distributions and coassociations of trace elements in selected soils. We sampled three major subregions of the Cerrado: (i) eastern Goiás, (ii) northwestern Minas Gerais, and (iii) the Triângulo Mineiro area of western Minas Gerais. Five soil series were sampled in each subregion, each represented by three pedons sampled at 0–20 cm and 80–100 cm depths. All sites were carefully selected to minimize anthropogenic trace element contamination. Wavelength-dispersive x-ray fluorescence spectroscopy was used to quantify total trace element contents to provide baseline information for future contamination assessment and regulatory purposes. Trace element signatures showed that soils on the South American erosion surface may have a common parent material, despite being separated by up to 500 km, and that Dystroferric Red Latosols sampled at two different altitudes, but otherwise very similar, may have formed from two distinct basaltic flows. We also analyzed thin sections from surface and subsurface horizons of five Latosols from the Triângulo Mineiro subregion using electron and synchrotron x-ray fluorescence microprobes to analyze trace element distributions at the micrometer scale. Latosols are very heterogeneous at the micrometer scale. This heterogeneity is expressed as a “hot spot” pattern, in which elements are concentrated in a few distinct features, while being homogeneously distributed throughout the soil plasma. Areas of higher trace element contents include: (i) primary silicate minerals; (ii) primary Fe and Ti oxides; (iii) secondary Fe and Mn nodules; (iv) pieces of charcoal; and (v) fragments of plant material. With exception of Si, there was no evidence to support the hypothesis that there are chemical gradients from the surface to the core of Latosol aggregates. This suggests that Latosol aggregates may be subject to disturbance and mixing more often than previously expected, which may indicate a biological origin for the aggregates.

Degree

Ph.D.

Advisors

Schulze, Purdue University.

Subject Area

Soil sciences|Environmental science|Geochemistry

COinS