At ancient sites across the Amazon River basin, mysterious plots of unusually rich soil dot (点缀) the landscape. Scientists have long debated the origin of this "dark earth", which is darker in color than surrounding soils and richer in carbon. Some people think this dark earth formed naturally, but the latest research has shown that modern Kuikuro people in Brazil create similar soil around their villages on purpose, which adds evidence to the idea that long-ago Amazons deliberately made such soil too. Perron, an earth scientist at MIT, reviewed interviews of Kuikuro people conducted by a Kuikuro filmmaker in 2018 and found that Kuikuro villagers actively make dark earth by using ash, food bits and controlled burns. "When you plant in hilly land, the soil is weak," explained elder Kanu Kuikuro in one of the interviews. That is why we throw the ash, manioc peelings and manioc pulp. When comparing soil samples from ancient and modern sites, researchers found "striking similarities" — both were far less acidic than surrounding soils and contained higher levels of plant-friendly nutrients.
Analyses also revealed that dark earth holds twice the amount of carbon as surrounding soils on average. Scans (扫描) of the Xingu region suggest that the area is dotted with dark earth, and that hold as much as about 9 million tons of carbon — the annual carbon emissions of a small, industrialized country. "This number could roughly equal the annual carbon emissions of the United States when all dark earth across the Amazon is taken into consideration," Perron says.
Figuring out the true value of carbon stored in the Amazon's dark earth will require more data. Still, the research has significant influences on the Amazon's future. The technique highlights how ancient people were able to live in the Amazon by developing sustainable farming that doubled as a carbon-storing technique. With more and more greenhouse gases entering the atmosphere, it could also provide a blueprint for developing methods of sustainably locking atmospheric carbon in the soil, helping fight climate change.
