Further theories are mentioned by Smith (1980), which all did not hold against later investigations.
It is now widely accepted that these soils were not only used by the local population but are a product of indigenous soil management as proposed by Gourou (1949).
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Additionally, the horizons which are enriched in organic matter, are not only 10-20cm deep as in surrounding soils, but may be as deep as 1-2m (average values probably around 40-50cm)!
Therefore, the total carbon stored in these soils can be one order of magnitude higher than in adjacent soils.
Other images and text belong to their respective owners. "Terra Preta de Indio" (Amazonian Dark Earths; earlier also called "Terra Preta do Indio" or Indian Black Earth) is the local name for certain dark earths in the Brazilian Amazon region.
These dark earths occur, however, in several countries in South America and probably beyond.
These soils are therefore highly fertile (Lehmann et al., 2003).
Fallows on the Amazonian Dark Earths can be as short as 6 months, whereas fallow periods on Oxisols are usually 8 to 10 years long (German and Cravo, 1999).
Further, the potential for enhancing sequestration by active management of black C could be established with important linkages to energy production and land use (see biochar soil management).
In addition to their high soil organic matter contents as mentioned above, Amazonian Dark Earths are characterized by high P contents reaching 200-400 mg P/kg, and higher cation exchange capacity, p H and base saturation than surrounding soils (Sombroek, 1966; Smith, 1980; Kern and Kämpf, 1989; Sombroek et al., 1993; Glaser et al., 2000; Lehmann et al., 2003; Liang et al., 2006).
This is in part due to the varied features of the dark earths throughout the Amazon Basin.