Black carbon in density fractions of anthropogenic soils of the Brazilian Amazon region

Frequent charcoal findings together with black carbon concentrations in the soil organic matter (SOM) of up to 35% provided evidence that black carbon is important for the SOM stability in Terra Preta soils. This paper aims t o investigate whether black carbon is additionally stabilised by organo-min eral complexation. For this purpose black carbon was analysed in density fr actions using benzenecarboxylic acids as molecular markers. Density fractio ns were also studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. Concentrations and total amounts of black carbon were g enerally highest in the light fraction indicating that a major part of blac k carbon is not chemically stabilised but intrinsically refractory. On the other hand, a large part of black carbon was also found in the heavier frac tions, where it was partly embedded within plaques of iron and aluminium ox ides on mineral surfaces. The major part of black carbon in the medium frac tion seemed to be organo-mineral complexed because we found amounts of blac k carbon in this fraction by wet chemical analysis but not by scanning elec tron microscopy and energy dispersive X-ray spectroscopy. The spectroscopic analysis can only detect particulate black carbon. Black carbon was partic ularly enriched in 30-40 cm soil depth, and in all fractions of Terra Preta soils compared to adjacent Oxisols. The occurrence of particulate black ca rbon together with potsherds in the subsoil horizons of Tetra Preta soils i ndicate that this might be due to turbation processes or the soils were cov ered by earthworm or termite activities. Further research, however, is need ed to clarify the transport mechanisms of black carbon into deeper soil hor izons. (C) 2000 Elsevier Science Ltd. All rights reserved.