Terra Preta de Indio

 

"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. They were most likely created by pre-Columbian Indians from 500 to 2500 years B.P. and abandoned after the invasion of Europeans (Smith, 1980; Woods et al., 2000). However, many questions are still unanswered with respect to their origin, distribution, and properties.

 

The origin of Amazonian Dark Earths

Already at the end of the 19th century, Smith (1879) and Hartt (1885) reported the existence of dark earths in the Amazon, which had a dark color and were highly fertile. The origin of the Amazonian Dark Earths is not entirely clear and several conflicting theories were discussed in the past. 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. Other theories included a formation as a result of sedimentation in Tertiary lakes (Falesi, 1974) or in recent ponds (Cunha-Franco, 1962). 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). 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. This is in part due to the varied features of the dark earths throughout the Amazon Basin.

 

Amazonian Dark Earths and the global climate

The global carbon cycle has been brought to wide attention due to its importance for the global climate. The Intergovernmental Panel on Global Change (IPCC, 2001) recently confirmed that the anthropogenic greenhouse effect is a reality, which we have to deal with in the future. The atmospheric CO2 has increased from 280 ppm in 1750 to 367 ppm in 1999 and today's CO2 concentrations have not been exceeded during the past 420,000 years (IPCC, 2001). The release or sequestration of carbon in soils is therefore of prime importance.
Soil organic carbon is an important pool of carbon in the global biogeochemical cycle. The total amount of organic carbon in soils is estimated to be 2011 Gt C, which constitutes about 82% of the global organic carbon in terrestrial ecosystems (Watson et al., 2000).
Amazonian Dark Earths have high carbon contents of up to 150 g C/kg soil in comparison to the surrounding soils with 20-30 g C/kg soil (Sombroek, 1966; Smith, 1980; Kern and Kämpf, 1989; Sombroek et al., 1993; Woods and McCann, 1999; Glaser et al., 2000). 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.
Furthermore, the organic matter in the dark earths is persistent since we find these elevated carbon contents even hundreds of years after they were abandoned. The reason for the high stability of the soil carbon is currently under discussion. So-called black carbon was identified as a probable reason for the high stability (Glaser et al., 2000). Further research is necessary to quantify the recalcitrance of the soil carbon over long periods of time and to evaluate techniques for creating such soils through application of black carbon (or called "biochar", see here). 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.
Important lessons can be learned from the recalcitrance of black carbon and its effects on the biogeochemistry of soils. Given the apparent ubiquity of black C established by several authors (Schmidt and Noak, 2000; Skjemstad et al., 2002), refinements of global C models and sequestration estimates may be necessary. 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).

Amazonian Dark Earths - implications for soil fertility and land use

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, pH 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). 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). 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. Continuous cropping for longer periods of time appears to be possible from a soil fertility point of view. How long a field can be continuously cropped and what can be done to prolong this period is not yet clear. Petersen et al. (2001) reported that Amazonian Dark Earths in Açutuba were under continuous cultivation without fertilization for over 40 years.
Recent efforts stimulated by Terra Preta research included the investigation of biochar (biomass-derived black carbon or charcoal) as a soil amendment to enhance nutrient availability and retention. Charcoal amendments were shown to significantly decrease nutrient leaching and increase crop growth (Lehmann et al., 2003), and the tests of slash-and-char systems were suggested as an alternative to slash-and-burn (Lehmann et al., 2002).

The following Science Brief gives a brief introduction to Terra Preta research at Cornell.

(here a version in lower quality)