Depth dependence of black carbon structure, elemental and microbiological composition in anthropic Amazonian dark soil

Abstract

“Terras Pretas de Índio” are anthropic Amazonian soils rich in pyrogenic black carbon, which might be responsible for the soil long-term stability and high fertility. This black carbon, produced by the Indians while handling their residues, became a model material for agriculture and environment. The key question to answer for artificially reproducing the desired agricultural properties of the Terra Preta de Índio is whether the black carbon structure found today in these soils is the same as produced by the ancient Indians, or whether its structure results from long-term complex physical, chemical and biological activities in the soil. To address this question, this work investigates the depth dependence of the properties from a soil collected from the Balbina site, in Presidente Figueiredo, Amazonas State, Brazil. The black carbon structure and the soil composition are investigated, with special emphasis on the poorly studied microbiological composition (fungi, bacteria, arbuscular mycorrhizas). The comparative analysis between the properties from shallower (newer) and deeper (older) soil strata indicates that, while soil composition exhibits depth dependence, the pyrogenic black carbon structure does not. This finding suggests that this model material should be reproducible by repeating the pyrolysis conditions utilized in their production.