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

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

Gênese e classificação dos solos de uma topossequência em área de carste na Serra da Bodoquena , MS

Pouco se conhece sobre os solos dos sistemas cársticos carbonáticos no Brasil, apesar da sua importância e reconhecida fragilidade. Os objetivos deste estudo foram avaliar a gênese de solos desenvolvidos de rochas calcárias e materiais derivados e caracterizar os atributos desses, contribuindo para o Sistema Brasileiro de Classificação de Solos (SiBCS). Foram estudados três perfis de solos em uma topossequência na Serra da Bodoquena, MS, dispostos nas posições de sopé (P1), terço médio (P2) e topo (P3) de encosta suave-ondulada com pendente longa e perfil longitudinal plano-convexo-plano. Todos os perfis são profundos, com cores bruno-escura nos horizontes superficiais e avermelhada em subsuperfície, além de textura argilosa com incremento de argila em profundidade. O íon cálcio predomina no complexo sortivo, bem como a fração humina entre as frações húmicas. A micromorfologia dos solos revela feições de iluviação de argila em associação com feições de acumulações de CaCO3. Porém, em nenhum dos perfis, os teores de CaCO3 equivalente foram suficientes para o reconhecimento do horizonte cálcico e, ou, petrocálcico, ou de caráter carbonático ou hipocarbonático. Com sequência de horizontes A-Bt (P1); A-E-Bt (P2); e A-Bi (P3), os solos apresentam a seguinte ordem de evolução pedogenética: P1 > P2 > P3. De acordo com o SiBCS, os solos são classificados como Argissolo Vermelho eutrófico nitossólico (P1); Argissolo Vermelho distrófico típico (P2); e Cambissolo Háplico Tb eutrófico típico (P3). Para todos os perfis, no quinto nível aplica-se a classe de atividade de argila - Tm (atividade média), validando proposta recente do SiBCS

Genesis and Classification of Soils Containing Carbonates in a Toposequence of the Bambuí Group

ABSTRACT The Bambuí Group, formed from siliciclastic sediments deposited on an extensive epicontinental platform at the end of the Neoproterozoic era, is characterized by limestones with fine to very fine texture. Limestone-derived soils in the southeast of Tocantins state, Brazil, are less notorious than similar soils in other regions of Brazil and their characterization could contribute to the Brazilian System for Soil Classification (SiBCS). Given that little is known of these soils, despitetheir agricultural potential, the objective of this study was to characterize their properties and the processes leading to soil of genesis, and also contribute to developing the Brazilian System of Soil Classification (SiBCS). Soils profiles were located on the summit (P1), shoulder (P2), backslope (P3), and footslope (P4) of a toposequence in the municipality of Lavandeira, Tocantins. Morphological, physical, chemical, and mineralogical properties of the profiles were determined. The soils were classified as: P1 - Chernossolo Rêndzico Lítico típico (Lithic Haplustolls); P2 - Cambissolo Háplico Ta Eutrófico léptico hipocarbonático (Lithic Haplustolls); P3 - Cambissolo Háplico Carbonático léptico (Lithic Haplustepts); and P4 - Luvissolo Háplico Órtico típico (Typic Rhodustalfs). All the profiles showed high contents of silt, calcium, and magnesium, which resulted in high pH and low exchangeable aluminum content. Base saturation and calcium carbonate equivalent contents were also high, and the horizons showed dark colors due to high organic matter content, which contained humin as the dominant fraction. Mineralogical analysis of the clay fraction indicated predominance of hydroxy-interlayered vermiculite, followed by illite, kaolinite, and quartz, whereas the sand fraction exhibited reflections characteristic of the quartz mineral. Given P4 contents of calcium carbonate equivalent above 50 g kg-1 and the relevance of this feature for agriculture, we propose the inclusion of hypocarbonate as a criterion for classification of Luvissolos Háplicos Órticos at the subgroup level