Rich soils hold thousands of tonnes of carbon sequestered through centuries of indigenous practices, a new study shows. The Amazon basin is famous for its vast, lush tropical forests, so one might assume that the land in the Amazon basin would be equally fertile. In fact, the soil beneath forest vegetation, especially in hilly uplands, is surprisingly poor. Much of the soil in the Amazon is acidic and low in nutrients, making it difficult to farm.
But over the years, archaeologists have unearthed mysterious black fertile ancient soil at hundreds of sites across the Amazon. This "black soil" is found in and around human settlements hundreds or even thousands of years ago. There is ongoing debate as to whether this super-fertile soil was purposefully created by these ancient cultures or whether it was an accidental byproduct.
Now, a study led by researchers from MIT, the University of Florida, and Brazil aims to resolve the debate over the origins of black soil. The team pieced together results from soil analysis, ethnographic observations and interviews with modern indigenous communities to show that ancient Amazonians intentionally produced black soil to improve the soil and sustain their large and complex societies.
If you want to have a large settlement, you need a nutrient base. Taylor Perron, the Cecil and Ida Greene Professor of Earth, Atmospheric and Planetary Sciences at MIT, said: "But the soil in the Amazon has been heavily leached of nutrients and is naturally poor and unsuitable for the growth of most crops. Research suggests that humans played a role in creating the black soil and intentionally modified the ancient environment to make it a more suitable place for human habitation."
It turns out that black soil contains large amounts of stored carbon. As generations of farming the soil, such as enriching it with food scraps, charcoal and waste, the Earth accumulates carbon-rich residue and sequesters it for hundreds to thousands of years. So by purposefully producing black soil, early Amazonians may have also inadvertently created a powerful carbon-sequestering soil.
"The ancient Amazons added a lot of carbon to the soil, and a lot of it is still in the soil today," said co-author Samuel Goldberg, who conducted the data analysis as a graduate student at MIT and is now an assistant professor at the University of Miami.
The team's research results were published in the journal Science Advances on September 20. Other authors include lead author Morgan Schmidt, a former MIT postdoc, and anthropologist Michael Heckenberger of the University of Florida, as well as collaborators from multiple institutions in Brazil.
In the current study, the team combined observations and data collected by Schmidt, Heckenberg and others while working with indigenous communities in the Amazon since the early 2000s, with new data collected in 2018-19. The scientists' fieldwork focused on the Cuicuro indigenous territory in the upper Xingu River basin in southeastern Amazon. The area contains modern Kuikuro villages and archaeological sites where the ancestors of the Kuikuro people are said to have lived. After visiting the area several times, Schmidt, then a graduate student at the University of Florida, was struck by the deeper soil surrounding some archaeological sites.
"When I saw this black soil and how fertile it was, and started researching what people knew about it, I realized it was something mysterious - no one really knew where it came from," he said.
Schmidt and his colleagues began observing the practices of modern Cuikullo people in managing their soil. These practices include placing "trash piles" - compost-like waste and food scraps - at specific locations in the center of the village. Over time, the garbage piles decompose and mix with the soil, creating dark, fertile soil that residents can use to grow crops. The researchers also observed farmers in Cuicullo spreading organic waste and ash further into their fields, which also creates black soil, where they can then grow more crops.
"We saw activities they did to change the soil and add elements, such as spreading ash on the ground or spreading charcoal around tree roots, which was clearly intentional," Schmidt said.
In addition to these observations, they conducted interviews with villagers to document Cuikuro beliefs and practices related to the black soil. In some of these interviews, villagers referred to black soil as "eegepe" and described their daily practices of creating and nurturing fertile soil to enhance agricultural potential.
Based on these observations and interviews with the Kuikuro people, it is clear that indigenous communities today are interested in producing black soil through soil improvement practices. But could the black soil found at nearby archaeological sites have been created through similar intentional actions?
Looking for connections, Schmidt joined Perron's research group as a postdoc at MIT. Together with Perron and Goldberg, he conducted detailed analyzes of soils from archaeological and modern sites in the Pliocene region. They found similarities in the spatial structure of black soil: black soil deposits are radial, concentrated mainly in the centers of modern and ancient sites, and extend toward the edges like the spokes of a wheel. Modern and ancient black soils are also similar in composition, rich in the same elements such as carbon, phosphorus and other nutrients.
These are elements found in humans, animals and plants that reduce aluminum toxicity in soil, a notorious problem in the Amazon. All of these elements make the soil more conducive to plant growth.
Goldberg added: "The key bridge between modern and ancient times is soil. Because we see this correspondence between the two periods, we can infer that these practices that we can observe and ask people about today also occurred in the past."
In other words, the team was able to demonstrate for the first time that ancient Amazons intentionally cultivated the soil, most likely through practices similar to those used today, to grow enough crops to sustain large communities.
The research team also further calculated the carbon content in the ancient black soil. They combined measurements of soil samples with maps of black soil found at several ancient settlements. Their estimates suggest that each ancient village contains several thousand tons of carbon that has been sequestered in the soil for hundreds of years as a result of indigenous human activities.
As the team concludes in their paper, "Modern sustainable agriculture and climate change mitigation efforts are inspired by the enduring fertility of ancient black soil and can draw on traditional methods still used by indigenous peoples of the Amazon today."