Organic matter of the anthropogenic dark earths of Amazonia.

The anthropogenic dark earths (ADE, Terra Preta de Índio or terra preta arqueológica, as are often referred to in Portuguese) feature unique properties that are interesting not only from a pedological point of view but have important agronomic and economic relevance. Among other properties, the organic matter (SOM) of these soils is considerably different from that of other mineral soils. Besides the quantity of SOM, its origin, composition and, consequently, function in the soil is different. Carbonized biomass (pyrogenic carbon) is a principal constituent of ADE that was, to our best knowledge, gradually incorporated into these soils, 4 to 10 thousand years ago, by native dwellers of the Amazon region. The carbon of pyrogenic origin was not only accumulated in ADE, but it also played important role in the transformation of organic matter and the humification process. As a result of this, the SOM of ADE is relatively stable, compared to the SOM of adjacent ferralsols, and at the same time it has chemical reactivity that is the major responsible for the higher cation exchange capacity and good buffer properties of these soils. These properties have very important environmental and agronomic relevance in the tropics due to organic carbon conservation in the soil and enhanced soil fertility. The ADE soils are excellent model for the development of modern soil fertility management practices