The Amazon River basin is known for its vast and lush rainforests, so it can be assumed that the Amazon soil is equally rich. In fact, soils with forest vegetation, especially in the rugged plateaus, are surprisingly infertile. Much of the Amazon soil is acidic and low in nutrients, making farming difficult.

But over the years, archaeologists have uncovered mysterious black and fertile patches of ancient soil in hundreds of places in the Amazon. This “dark world” has been found in and around human settlements dating back hundreds of thousands of years. Whether the ultra-rich soil was created on purpose or is an accidental byproduct of these ancient cultures has been a matter of debate.

Now, a study led by researchers from the Massachusetts Institute of Technology, the University of Florida and Brazil aims to resolve the debate about the origin of the dark world. Combining soil analysis, ethnographic observations, and interviews with contemporary indigenous communities, the team showed that dark soil was deliberately produced by ancient Amazonians as a way to improve soil and support large and complex societies.

“If you want to have large settlements, you need a food base. But the soil in the Amazon is quite nutrient-poor and naturally poor for growing most crops,” said Taylor Perron, the Cecil and Ida Green Professor of Earth, Atmosphere, and planetary sciences at MIT. “Humans played a role in creating the dark world and transformed the ancient environment into human populations.” “We claim that he deliberately changed it to make it a better place for us.”

And as it turns out, the dark world contains large amounts of stored carbon. As generations cultivated the land, enriching it with food scraps, charcoal, and waste, for example, the soil accumulated carbon-rich debris, keeping it locked away for hundreds or even thousands of years. So, by deliberately producing dark soil, early Amazonians may have unintentionally created soil that was a strong carbon absorber.

“Ancient Amazonians added a lot of carbon to the soil, and a lot of it is still there,” says co-author Samuel Goldberg, who analyzed the data as a graduate student at MIT and is now an associate professor at MIT. University of Miami. “This is exactly what we want from climate change mitigation efforts. Maybe we can adapt some of their local strategies on a larger scale to lock carbon in the soil, so we now know it will stay there for a long time.”

The team’s research was published on: Science Developments. Other authors include former MIT postdoctoral researcher and lead author Morgan Schmidt, anthropologist Michael Hackenberger of the University of Florida, and collaborators from several institutions in Brazil.

modern intent

In their current study, the team synthesized observations and data previously collected by Schmidt, Heckenberger, and others working with indigenous communities in the Amazon since the early 2000s, with new data collected in 2018-19.

The scientists focused their field work on the lands of the Cuikuro indigenous people in the upper Xingu River basin in the southeastern Amazon. In this region, there are modern Kuikuro villages as well as archaeological sites where Kuikuro ancestors are believed to have lived. Schmidt, then a graduate student at the University of Florida, encountered the dark soil around some archaeological sites during several visits to the area.

“When I saw this dark world and its abundance and started researching what was known about it, I discovered that it was a mysterious thing; no one really knew where it came from,” he says.

Schmidt and his colleagues began observing modern Kuikuro soil management practices. These practices include the creation of “waste environments” similar to manure heaps, waste piles and food scraps stored in certain locations around the center of the village.

After a while, these piles of garbage decompose and mix with the soil, creating dark, fertile soil that local residents then use to grow crops. Researchers also observed that Cuikuro farmers spread organic waste and ash onto remote fields, creating dark soil where they could grow more crops.

“We found that actions such as scattering ash on the ground or spreading charcoal at the base of a tree were intentional to modify the soil and increase its elemental content,” says Schmidt.

In addition to these observations, they also interviewed villagers to document Kuikuro beliefs and practices regarding the dark lands. In some of these interviews, villagers called the dark lands ‘egepe’ and described their daily practices of creating and cultivating fertile soil to develop agricultural potential.

Based on these observations and interviews with Kuikuro, it became clear that indigenous communities today are deliberately cultivating temnozem through soil improvement practices. So could the dark world found at nearby archaeological sites have been created by similar deliberate methods?

A bridge on the ground

Seeking a connection, Schmidt joined Perron’s group as a postdoctoral researcher at MIT. Together, he, Perron, and Goldberg conducted a comprehensive analysis of soils from both archaeological and modern sites in the Upper Xingu region.

They found a similarity in the spatial structure of the dark earth: Dark earth deposits were found in a radial pattern, concentrated mainly in the center of both modern and ancient settlements, and extending towards the edges like spokes of a wheel. The composition of modern and ancient dark soils was also similar and enriched with the same elements such as carbon, phosphorus and other nutrients.

“These are all elements found in the bodies of humans, animals, and plants, and they are elements that reduce the toxicity of aluminum in soil, a notorious problem in the Amazon,” says Schmidt. “All these elements make the soil better for plant growth.”

“The most important bridge between the modern and the ancient is soil,” Goldberg adds. “Because we see this overlap between the two time 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 show for the first time that ancient Amazonians deliberately cultivated the land, possibly using practices similar to those used today, to grow enough crops to support large communities.

The team went further and calculated the amount of carbon in the ancient dark world. They combined their measurements of soil samples with maps showing where dark soil was found at various ancient sites. Their estimates showed that each ancient village contained several thousand tons of carbon that had accumulated in the soil over hundreds of years as a result of the activities of indigenous people.

As the team concludes in their paper, “modern sustainable agriculture and climate change mitigation efforts inspired by the enduring fertility of the ancient dark world may be based on traditional practices still practiced by indigenous peoples of the Amazon.” Source