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Ancient Amazonians modified the soil to create fertile “dark earth”

The vast green expanse of the Amazon rainforest stands in sharp contrast to the infertile lands that lie beneath its canopy. A large portion of Amazonian soil is nutrient-deprived and acidic, which is not conducive to agriculture.

In hundreds of sites across the Amazon, archaeologists have dug up patches of black, fertile soil. This “dark earth” has been found near ancient human settlements that date back thousands of years. 

The origin of this fertile land – whether a deliberate creation or an unintended byproduct of old civilizations – has been a subject of dispute.

Supporting large settlements

However, new research from MIT, the University of Florida, and several Brazilian institutions indicates that ancient Amazonians consciously produced this “dark earth” to boost soil quality and uphold expansive societies.

Professor Taylor Perron from MIT highlights the significant role humans played in this transformation. 

“If you want to have large settlements, you need a nutritional base. But the soil in the Amazon is extensively leached of nutrients, and naturally poor for growing most crops,” said Professor Perron. 

“We argue here that people played a role in creating dark earth, and intentionally modified the ancient environment to make it a better place for human populations.”

Powerful carbon sink 

Over centuries, the soil’s enrichment through organic waste, food remnants, and charcoal led to the accumulation of carbon-rich residues. This means that early Amazonians may have unintentionally created a powerful carbon sink. 

“The ancient Amazonians put a lot of carbon in the soil, and a lot of that is still there today,” said study co-author Samuel Goldberg, who performed the data analysis as a graduate student at MIT and is now at the University of Miami. 

“That’s exactly what we want for climate change mitigation efforts. Maybe we could adapt some of their indigenous strategies on a larger scale, to lock up carbon in soil, in ways that we now know would stay there for a long time.” 

Intentional actions

The research, published in the journal Science Advances, draws heavily on data and observations from earlier studies. 

Focusing on the Kuikuro Indigenous Territory in the southeastern Amazon, the team observed modern-day Kuikuro’s soil management practices. They found striking parallels between these practices and the ancient dark earth creation methods.

The team noted intentional soil modification activities among the Kuikuro, such as waste pile decomposition into fertile land and the scattering of ash and organic waste in fields, which led to dark earth formation.

“We saw activities they did to modify the soil and increase the elements, like spreading ash on the ground, or spreading charcoal around the base of the tree, which were obviously intentional actions,” said former MIT postdoc and lead author Morgan Schmidt.

Cultivating fertile soil

Through meticulous analysis, the research team found consistencies in the spatial arrangement of ancient and modern dark earth, further supporting the idea that ancient Amazonians purposefully cultivated the soil to sustain larger populations.

Modern and ancient dark earth was also similar in composition, containing elements such as carbon, phosphorus, and other nutrients.

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

“The key bridge between the modern and ancient times is the soil,” said Goldberg. “Because we see this correspondence between the two time periods, we can infer that these practices that we can observe and ask people about today, were also happening in the past.”

Invaluable knowledge 

By estimating carbon content in ancient dark earth, the team determined that each old village could have sequestered several thousand tons of carbon, which would be preserved over centuries.

The researchers conclude that the lessons from the Amazon’s ancient dark earth could serve as a beacon for modern sustainable farming, emphasizing the invaluable knowledge passed down by Indigenous Amazonians.

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