Sequential P Fractionation of Relict Anthropogenic Dark Earths of Amazonia

Abstract

Dark earths which are rich in small artifacts can be found in a wide range of environmental settings in the Amazon Basin. While various conflicting theories have been proposed about the origin of these soils (Smith 1980), it is now widely accepted that they formed under anthropogenic influence (Sombroek 1966; Smith 1980; Woods et al. 2000). These relict Anthrosols are often the only testimony of pre-Columbian settlements and can provide important information about the former inhabitants (Vacher et al. 1998). Amazonian Dark Earths have high soil organic matter and nutrient contents such as P (Sombroek 1966; Smith 1980; Kern and Kampf 1989), and are therefore highly fertile. Phosphorus availability is the most important constraint to crop production in central Amazonia (Lehmann et al. 2001a) and high P contents are the primary reason for the high production potential of these Anthrosols. Farmers value these soils in many areas of Amazonia, and some authors claim that continuous cropping is possible for 40 years or more without fallowing (Petersen et al. 2001). Whether the so-called terra preta de indio soils were intentionally created for agricultural purposes or whether they formed as a result of habitation is still under debate (McCann et al. 2001). In order to understand the origin of these soils, it is important to know what type of organic input caused the high organic matter and P contents, and under which conditions.