Abstract
ABSTRACT Amazonian dark earths (ADEs) are fertile soils created by pre-Columbian Amerindian societies of the Amazon Basin. However, it is still not clear whether these soils were produced intentionally to improve infertile Amazonian upland soils or if they resulted from the accumulation of organic matter from sedentary settlements. This study characterizes the ADEs found in the naturally fertile alluvial floodplains of the Amazon River in the Central Brazilian Amazon according to total, exchangeable, and available contents of elements and organic carbon in soil profiles. ADEs contained higher levels of available elements and total P, Ca, Zn, and Cu. High total Cr, Ni, Co, and V content in these soils indicate that mafic minerals contributed to their composition, while higher contents of P, Zn, Ba, and Sr indicate anthropic enrichment. The presence of ADEs in floodplain areas strongly indicates non-intentional anthropic fertilization of the alluvial soils, which naturally contain levels of P, Ca, Zn, and Cu higher than those needed to cultivate common plants. The presence of archaeological sites in the floodplains also shows that pre-Columbian populations lived in these regions as well as on bluffs above the Amazon River.
Highlights
The role pre-Columbian populations played in modifying the natural conditions of the Amazon Basin has been intensely debated (Barlow et al, 2012; Levis et al, 2012; Clement et al, 2015)
There is no consensus as to whether these anthropic soil horizons were created intentionally (Arroyo-Kalin et al, 2009; Glaser; Birk, 2012). Did they result from management practices to improve the poor natural upland soils across much of the Amazon and make them suitable for agriculture? were these soils formed around houses and other occupation areas such as trash middens, rather than former farming areas (Glaser; Birk, 2012; Schmidt et al, 2014)? This question is important because it address the long-standing debate on the role of environmental factors which limited the establishment of long-term, permanent, and sedentary settlements in the Amazon (Meggers, 1996; Roosevelt, 2013)
Small or very small (< 2 mm) charcoal pieces totaling 15-40% of the RESULTS AND DISCUSSION The profiles were classified as Eutric Orthofluvic Fluvisol (Siltic, Oxyaquic) (P1, P2, and P4), Pretic Anthrosol (Hypereutric, Siltic, Fluvic, Oxyaquic) (P3) (Figure 2A-2D), Eutric Pantofluvic Fluvisol (Siltic, Oxyaquic) (P5 and P7), Gleyic Pantofluvic Fluvisol (Siltic, Ochric) (P6), and Pretic Anthrosol (Orthoeutric, Siltic, Fluvic, Oxyaquic) (P8)
Summary
The role pre-Columbian populations played in modifying the natural conditions of the Amazon Basin has been intensely debated (Barlow et al, 2012; Levis et al, 2012; Clement et al, 2015). Amazonian dark earth soils (ADEs) are among the features indicated as supporting this hypothesis (Lehmann et al, 2003a; Teixeira et al, 2009; Clement et al, 2015) These horizon soils are found across the Amazon; they are highly fertile and normally associated with archaeological sites, with deposits reaching over 200 cm deep and several dozen hectares wide (Kern et al, 2009). There is no consensus as to whether these anthropic soil horizons were created intentionally (Arroyo-Kalin et al, 2009; Glaser; Birk, 2012) Did they result from management practices to improve the poor natural upland soils across much of the Amazon and make them suitable for agriculture? Studies of the chemical composition of ADEs are important to clarify the mechanisms involved in these formations and potentially replicate this process for agricultural use; they could be used to develop waste management methods that create soil conditioners, halt land degradation, and act a model for sustainable agriculture in the humid tropics (Glaser et al, 2001)
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