Properties of carbon particles in archeological and natural Amazon rainforest soils

[Display omitted] •Physical-Chemical analyzes showing comparison among carbon microstructures.•Carbon grains in amazon rainforest soils displayed sp2 nanocrystallite structure.•Terra Preta-carbons have larger number of point defects than Rio Negro sediment.•Terra Preta do Índio-carbons are richer in nutrients than Rio Negro deposits-carbon. Driving practices of Amazon native inhabitants are an example of positive feedback in carbon storage acting as a key element in soil fertility and stability. The anthropogenic Amazonian Terra Preta do Índio (Indian Dark Earth) soils are rich in pyrogenic black carbon and the Rio Negro (Black River) also exhibits plentiful apparently similar black-carbon particles. In this context, we characterized the structural, morphological and elemental properties of the long-lived stable carbon-based structures present in sediments of the Amazonian Rio Negro and in two Terra Preta do Índio soils from two different geographic regions. Optical microscopy, Raman spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques are utilized. The structural analysis displayed that these carbon structures exhibit similar nanocrystallite structures, despite their different geographic location and environment. The Terra Preta do Índio-carbons are however, more defective and the quantitative elemental analyzes indicate they have greater variety of nutrients, such as P and Ca, than Rio Negro sediment-carbons.