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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).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.

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Later surveys confirmed these findings (Sombroek, 1966; Smith, 1980; Kern and Kämpf, 1989).Whether they were intentionally created for soil improvement or whether they are a by-product of habitation is not clear at present.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).This is in part due to the varied features of the dark earths throughout the Amazon Basin.

The global carbon cycle has been brought to wide attention due to its importance for the global climate.

Only short fallows are presumed to be necessary for restoring fertility on the dark earths.

However, precise information is not available, since farmers frequently fallow the land due to an overwhelming weed infestation and not due to declining soil fertility.

The structural similarity of organic matter in Terra preta to biochar led scientist to assume that accumulation or purposeful application of organic carbon from incomplete combustion may have been the primary reason for the high carbon contents and fertility of these soils (Glaser et al., 2001), a theory that had been proposed by Smith (1980).

If all or some of these soils were actually created by char applications to improve soils for agriculture has still to be demonstrated.

Camargo (1941) speculated that these soils might have formed on fallout from volcanoes in the Andes, since they were only found on the highest spots in the landscape.