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

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.

摘要：在整个亚马逊区域，人们已识别出一类肥力提升的深色土壤，并将其命名为亚马逊黑土（Amazonian Dark Earths, ADE）。这类独特土壤以发育有深色外观的人为表层、富含炭屑与人工遗存为典型特征，与周边非ADE土壤截然不同。亚马逊黑土展现出诸多非凡特性：即便处于亚马逊地区极利于有机质氧化与养分淋溶的气候条件下，历经多年耕作仍可维持深色外观与较高养分水平。亚马逊黑土被视作亚马逊生物群落中的肥力岛屿，同时也是农业与环境可持续性、碳封存、养分生物有效性、粮食安全以及人类学研究的重要模式载体。然而，关于人类活动对其改造的程度、其形成机制以及分类学体系中的归属问题，仍有诸多悬而未决的疑问。因此，本研究旨在对巴西朗多尼亚州南部区域的ADE土壤剖面进行识别、描述与特征解析，并依据巴西土壤分类系统（Brazilian Soil Classification System, SiBCS）与世界土壤资源参比基础（World Reference Base, WRB）完成其分类，以期为相关研究提供参考。本研究共描述并采样了5个土壤剖面：P1至P3位于草地牧场，P4与P5分别覆盖原生林与免耕作物种植区。研究对这些剖面的形态学、物理学与化学属性，以及铁、铝、硅、钛氧化物含量与可提取态铁形态进行了系统表征。其人为表层呈现深色外观，含人工遗存与炭屑，质地偏砂性，以团粒结构为主，层理界面多为突变或清晰过渡，边界则以波浪状与不规则状居多。所有人为表层的pH值、钙离子（Ca²+）、镁离子（Mg²+）含量、交换性盐基总量（S）、阳离子交换量（CEC）以及有机碳（C）均处于较高水平。相较于亚马逊地区的非ADE土壤，所有人为表层的磷含量均更高，其中P1、P2与P3的表层土壤磷含量达到峰值。依据巴西土壤分类系统（SiBCS），P1被归类为Cambissolo，P2为Argissolo，P3、P4与P5则归为Latossolos。除P4外（其梅利奇-1浸提法（Mehlich-1）提取态磷含量低于30 mg·kg⁻¹），所有供试土壤均满足SiBCS中人为表层的判定标准，研究还提出了亚类级别的新分类单元。在世界土壤资源参比基础（WRB）分类体系中，P4与P5被归为铁铝土（Ferralsols），其余剖面则被归为人为土（Anthrosols）。采用加速器质谱法对炭屑碎片进行的放射性碳定年（C¹⁴）结果显示，其年代介于940±40至1230±60 a BP之间。