Trace elements in soils developed from metamorphic ultrabasic rocks in Minas Gerais, Brazil

Abstract

Metamorphic ultrabasic rocks are typically enriched in heavy trace elements, mostly contained within easily-weatherable silicate minerals such as talc and serpentine. In the humid tropics, such rocks can decompose rapidly, thus releasing trace elements (TE) at rates likely to create extreme soil environments that hamper plant development, but few studies are available on this issue in Brazil and most South America. We sampled eight soils formed on a 3.1 km2 steatite (a type of talc-schist) hill and its piedmont, with slopes ranging from 4 to 90%. The soils varied from shallow Entisols and Inceptisols to deep (>2 m) Oxisols, along a weathering continuum well described by increasing residual concentration of Fe oxides, which ranged from 15 to 69% total Fe2O3 (determined by X-ray fluorescence). Total Co, Cr, Ni, Ti and V were high in comparison to world soils´ ranges and averages, whereas Ba, Cu and Zr were relatively low or near average. Total concentrations of these TE except Ba were proportional to total Fe, and the same trends occurred when dithionite-extractable Fe and TE were considered. Thus, both primary (magnetite, chromite, hematite) and secondary (maghemite, hematite, goethite, amorphous phases) Fe oxides likely control soil TE contents, with much variation across TE, soils and horizons. Co, Cr and Ti were mostly comprised within the residual phases, i.e. mostly resistant primary minerals, whereas a larger part of Ba, Cu, Ni, V and Zr were included in pedogenic Fe oxides and other phases. The potentially plant-available (extracted with Mehlich-I) TE were mainly Ba, Co and Cu, and to a much lower degree, Ni. The environmental implications of the concentrations and partition of the studied TE are briefly discussed.