A new hypothesis for the origin of Amazonian Dark Earths

Abstract

Amazonian Dark Earths (ADEs) are unusually fertile soils characterised by elevated concentrations of microscopic charcoal particles, which confer their distinctive colouration. Frequent occurrences of pre-Columbian artefacts at ADE sites led to their ubiquitous classification as Anthrosols (soils of anthropic origin). However, it remains unclear how indigenous peoples created areas of high fertility in one of the most nutrient-impoverished environments on Earth. Here, we report new data from a well-studied ADE site in the Brazilian Amazon, which compel us to reconsider its anthropic origin. The amounts of phosphorus and calcium—two of the least abundant macronutrients in the region—are orders of magnitude higher in ADE profiles than in the surrounding soil. The elevated levels of phosphorus and calcium, which are often interpreted as evidence of human activity at other sites, correlate spatially with trace elements that indicate exogenous mineral sources rather than in situ deposition. Stable isotope ratios of neodymium, strontium, and radiocarbon activity of microcharcoal particles also indicate exogenous inputs from alluvial deposition of carbon and mineral elements to ADE profiles, beginning several thousands of years before the earliest evidence of soil management for plant cultivation in the region. Our data suggest that indigenous peoples harnessed natural processes of landscape formation, which led to the unique properties of ADEs, but were not responsible for their genesis. If corroborated elsewhere, this hypothesis would transform our understanding of human influence in Amazonia, opening new frontiers for the sustainable use of tropical landscapes going forward.

Beyond total carbon: conversion of amazon forest to pasture alters indicators of soil C cycling

“…It is also possible that relic forest-derived SOC persists as a refractory pool of pyrolyzed C generated by burning of forest biomass during initial deforestation. Pyrolyzed C can have a half-life 10-to 100-fold greater than native SOC (Singh et al 2014) with turnover rates estimated at the centennial scale (Spokas 2010;Singh et al 2012), and is therefore considered a major contributor to longterm SOC storage, including in the Amazon basin via ancient natural fires (Silva et al 2021), despite being a minor fraction of total SOC in most ecosystems today (Lehmann et al 2008;Reichstein et al 2013).…”

Evidence confirms an anthropic origin of Amazonian Dark Earths

“…Table 1 Data used in Figure 1 based on Silva et al 2 , Viers et al 20 , Braadbaart et al 12 , Cílová and Woitsch 22 , Vassilev et al 23 , Huisman et al 24…”

“…The elevated fertility of the dark and often deep A horizon of ADEs is widely regarded as an outcome of pre-Columbian human influence 1 . Controversially, in their recent paper Silva et al 2 argue that the higher fertility of ADEs is principally a result of fluvial deposition and pre-Columbian peoples just made use of these locales rather than contributing to their enhancement. Soil formation is inherently complex and often difficult to interpret, requiring a combination of geochemical data, stratigraphy, and dating.…”

Reply to: Evidence confirms an anthropic origin of Amazonian Dark Earths

“…However, it remains unclear how these persistent patches of high fertility could have been created in nutrient-impoverished tropical landscapes. In a recent study 1 , we report new data from one of the best-studied ADE sites in Brazil which warrant a revision of its origin story. We found large amounts of phosphorus (P) and calcium (Ca) correlated with 16 trace elements that indicate exogenous sources rather than in situ deposition, an inference that is supported by neodymium (Ne), strontium (Sr) and carbon (C) isotope signatures.…”