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23 January 2021
New hypothesis for origin of Amazonian Dark Earths

Amazonian Dark Earths (ADEs) are unusually fertile soils characterised by elevated concentrations of charcoal. Discovered decades ago in central Brazil, ADEs are regarded as a Pan-Amazonian phenomenon. Frequent occurrences of pre-Columbian artefacts at ADE sites led to their classification as soils of human origin, though it remains unclear how areas of high fertility became established in one of the most nutrient-impoverished environments on Earth.
     New data from a well-studied site in the Brazilian Amazon reveal levels of phosphorus and calcium - two of the least abundant macronutrients in the region - which are orders of magnitude higher in ADE profiles than in the surrounding soil, beginning several thousands of years before the earliest evidence of soil management for plant cultivation in the region.
     Amazonian landscapes are dominated by soils characterised by high acidity and low nutrient concentrations. Native plant species display adaptations which allow the soils to maintain some of the most productive ecosystems on Earth while limiting food production even under intensive management.
     The Amazon basin has a complex history of occupation and land use. Prior to European contact, indigenous peoples relied on more than 80 different plant species. Three main phases of human occupation are thought to have occurred: a pre-cultivation period more than 6000 years ago, an early-cultivation period 6000 to 2500 years ago, and a late-cultivation period 2500 to 500 years ago - a chronology supported by genomic, linguistic, and archaeological evidence.
     Recent findings indicate domestication of native plant species dates back more than 10,000 years in western Amazonia. Complex societies relying on soil management for agriculture occur less than 4000 years ago. Some of the earliest evidence of settlements comes from the Peruvian Andes, where records of deforestation and soil erosion exist for much of the past 6900 years. For most of the Amazon basin the earliest evidence of intensive cultivation falls between 3380 and 700 years ago. ADEs are rare near Andean settlements; the vast majority are thousands of kilometres away in the central and eastern Brazilian Amazon, where evidence of management is more recent - 2500 to 500 years ago.
     The latest research focused on a typical well-studied ADE site, thought to have originated from a large settlement less than 2000 years ago. Artefacts are found in charcoal-rich soil layers where vestiges of human faeces are also present. The current view attributes those layers to biomass burning, but experiments show this inadequate to replicate basic characteristics of indigenous ADEs. At the study site, the enrichment of micro-charcoal and mineral elements began around 7630 years ago, before the earliest evidence of soil management that characterizes the late-cultivation period. To achieve the observed levels of enrichment, large human populations would have had to actively manage soils continuously for thousands of years prior to the currently accepted chronology of settlement. The study found direct and indirect evidence that natural processes were responsible, suggesting indigenous peoples used their knowledge to settle areas of exceptionally high fertility before the onset of intensive land use in central Amazonia.

Edited from Nature Communications (4 January 2021), University of Oregon (5 January 2021)

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