Amazonian Dark Earths in Rondônia State: Soil properties, carbon dating and classification

Abstract

ABSTRACT Throughout the Amazon region, dark-colored soils with increased fertility are identified and referred as Amazonian Dark Earths (ADE). These unique soils are characterized by an anthropic surface horizon with dark colors, presence of charcoal and artifacts, in contrast with surrounding non-ADE soils. The ADEs show extraordinary properties such as the capacity of maintaining the dark colors and high nutrient levels after years of farming, even under the highly favorable climatic conditions for oxidation of organic matter and weathering of nutrients of Amazon region. The ADE are considered islands of fertility in the Amazon biome, as well as very important models for research, in terms of agricultural and environmental sustainability, carbon sequestration, nutrient bioavailability, food security, and for anthropological studies. However, there are many unanswered questions regarding the extent of human alterations, how they were formed, and their classification in the taxonomic systems. Therefore, the objectives of this study were to identify, describe, and characterize soil profiles of ADEs, located in the Southern region of Rondônia State, Brazil, and to contribute to their classification according to the Brazilian Soil Classification System (SiBCS) and the World Reference Base (WRB). Five soil profiles were described and sampled; P1 to P3 were under grass pastures, P4 and P5 under forest and crops with no-till, respectively. The morphological, physical, and chemical attributes, as well as contents of Fe, Al, Si, and Ti oxides and extractable iron forms were characterized. The anthropic horizons exhibited dark colors, artifacts, charcoal, sandier textures, predominantly granular structures, abrupt or clear transitions, and mostly wavy and irregular boundaries. Values of pH, Ca 2+ , Mg 2+ , exchangeable bases (S), CEC, and C were high in all anthropic horizons. Compared to non-ADE soils in the Amazon region, phosphorus contents were superior in all anthropic horizons, with the highest values in surface horizons of P1, P2, and P3. According to SiBCS, P1 was classified as Cambissolo, P2 as Argissolo , and P3, P4, and P5 as Latossolos. Except for P4, where values of extractable P by Mehlich-1 are lower than 30 mg kg -1 , all soils fulfilled the requirements for the anthropic horizon in the SiBCS, and new classes at the subgroup level were proposed. In the WRB, P4 and P5 were classified as Ferralsols. The other profiles were classified as Anthrosols. The radiocarbon dates (C 14 ) of charcoal fragments, obtained using a mass accelerator, showed ages between 940 ± 40 and 1230 ± 60 years BP.

Keywords

• Terra Preta de Índio
• Anthrosols
• Soil Taxonomy