dark earth

Dark earth: Carbon-rich soils in the Amazon

The size of ancient human populations in the Amazon and the extent of their ecological impacts are hotly debated. Central to this debate is Amazonian dark earth (ADE) – anomalous anthropic soil characterized by darker colour, higher organic carbon content, and higher fertility than typical Amazonian upland soils. Archaeological evidence indicates that dark earth formed in association with human occupation, but it is uncertain what practices created dark earth, what determined its spatial extent, and whether humans created it intentionally, leading some researchers to still question whether humans created it at all. The amount of carbon distributed across dark earth sites is largely unknown, adding uncertainty to the potential climate impacts of soil carbon loss due to land-use change and global warming.

Morgan Schmidt and Sam Goldberg collaborated with Mike Heckenberger (U. Florida), colleagues in Brazil, and members of the Kuikuro Tribe of the Xingu region to investigate the origins and carbon storage of dark earth. They discovered similarities between dark earth in ancient and modern contexts, and Morgan documented contemporary indigenous practices that enrich soil. This comparison suggests that ancient Amazonians managed soil to improve fertility and increase crop productivity. These practices also sequestered and stored carbon in the soil for centuries, and Morgan and Sam calculated that the organic carbon content of some ancient dark earth sites is comparable to that of the above-ground rainforest biomass. Their results demonstrate the intentional creation of Amazonian dark earth and highlight how indigenous knowledge can provide strategies for sustainable rainforest management.

• Schmidt, M., S.L. Goldberg, (24 others), and J.T. Perron (2023). Intentional creation of carbon-rich dark earth soils in the Amazon. Science Advances 9, eadh8499. https://doi.org/10.1126/sciadv.adh8499.
• Goldberg S.L., M.J. Schmidt, J.D. Himmelstein, M. Heckenberger, B. Franchetto, H. Lima, J. Watling, B. Moraes, W. B. Dorshow, C. Fausto, K. Waura, H. Kuikuro, T.W. Kuikuro, A. Kuikuro, and J.T. Perron (2024). Widespread Amazonian dark earth in the Xingu Indigenous Territory. Nature Sustainability, 7, 1304–1312, https://doi.org/10.1038/s41893-024-01399-3.