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

Abstract

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 to investigate whether black carbon is additionally stabilised by organo-mineral complexation. For this purpose black carbon was analysed in density fractions using benzenecarboxylic acids as molecular markers. Density fractions were also studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. Concentrations and total amounts of black carbon were generally highest in the light fraction indicating that a major part of black carbon is not chemically stabilised but intrinsically refractory. On the other hand, a large part of black carbon was also found in the heavier fractions, where it was partly embedded within plaques of iron and aluminium oxides on mineral surfaces. The major part of black carbon in the medium fraction seemed to be organo-mineral complexed because we found amounts of black carbon in this fraction by wet chemical analysis but not by scanning electron microscopy and energy dispersive X-ray spectroscopy. The spectroscopic analysis can only detect particulate black carbon. Black carbon was particularly enriched in 30–40 cm soil depth, and in all fractions of Terra Preta soils compared to adjacent Oxisols. The occurrence of particulate black carbon together with potsherds in the subsoil horizons of Terra Preta soils indicate that this might be due to turbation processes or the soils were covered by earthworm or termite activities. Further research, however, is needed to clarify the transport mechanisms of black carbon into deeper soil horizons.