Bruno Glaser, Ludwig Haumaier, Georg Guggenberger & Wolfgang Zech

Abstract Many soils of the lowland humid tropics are thought of being too infertile to support sustainable agriculture. However, in the Brazilian Amazon region, within the landscape of infertile soils, patches of sustainably fertile soils known as Terra Preta do Indio occur. These soils not only contain higher levels of nutrients such as nitrogen, phosphorus, potassium, and calcium, but also higher stocks of stable soil organic matter. Frequent charcoal findings and highly aromatic humic substances suggested that residues of incomplete combustion of organic material (black carbon) are a key factor for the persistence of soil organic matter in these soils. Our investigations showed that Terra Preta soils contained up to 70 times more black carbon than the surrounding soils. Due to its polycyclic aromatic structure black carbon is chemically and microbially stable and persists in the environment over centuries. Oxidation during this time produces carboxylic groups on the edges of the aromatic backbone, which increases the nutrient holding capacity. We conclude that black carbon could act as a significant carbon sink and is a key factor for sustainable and fertile soils especially in the humid tropics. There is strong evidence that permanent or semi-permanent agriculture itself created sustainably fertile Terra Preta soils known.