Amazon soil may store billions more tonnes of carbon than once thought

Nutrient-rich "dark earth" soil may store an amount of carbon nearly equivalent to annual CO2 emissions in the US, a finding that suggests the Amazon sequesters far more carbon than previously known.

Rich soil in the Amazon cultivated over centuries by Indigenous communities may store billions of tonnes of carbon, suggesting that the rainforest plays an even larger role in stabilising the global climate than previously thought.

The Xingu Indigenous Territory in the Amazon may contain over 900 square kilometres of dark earth
Leo F Freitas/Getty Images

The soil, known as “terra preta” or “dark earth” for its distinctive colour, is formed by people spreading ash and other organic waste around settlements. It is more fertile than the region’s typically sandy, nutrient-poor soils, and stores around double the carbon. In some areas, dark earth may contain as much carbon as the plants growing above it.

Archaeologists have identified dark earth deposits at sites across the Amazon basin, but the total amount of such soil currently present in the region remains unknown.

Samuel Goldberg at the University of Miami in Florida and his colleagues used satellite imagery to predict the areas with dark earth across the 2.6 million-hectare Xingu Indigenous Territory in the south-east Amazon. In this region, dark earth cultivation, including by modern Kuikuro farmers, has been well documented.

Because the soil is hidden from satellites by tree cover, the researchers looked at differences in the colour of the tree canopy. They found that undisturbed trees growing on dark earth have higher amounts of chlorophyll in their leaves and can reflect more infrared light.

The researchers trained a machine learning algorithm on satellite imagery data on forest colour from sites known to contain dark earth. “You can’t see it, but the satellite can,” says Goldberg. This let them predict spots with dark earth throughout the entire region.

The algorithm predicted that at least 3.4 per cent of the region, or 91,000 hectares, contained dark earth, the majority of which was unknown sites. The algorithm also had low error rates when predicting known dark earth sites. The researchers estimate this human-modified soil stores 9 million tonnes more carbon than it would otherwise.

The results from Xingu can’t reliably be extrapolated to the whole Amazon basin without more regional data. But if dark earth is present to the same extent across the more than 5.5-million-square-kilometre basin, dark earth in the Amazon could store billions of tonnes of carbon – an amount approaching the annual CO2 emissions of the US, says Goldberg.

“[Our survey] is huge relative to what you can do with a shovel at an archaeological site, but tiny relative to the entire Amazon,” he says.

Journal reference:

 Nature Sustainability DOI: 10.1038/s41893-024-01399-3