Deeply buried organic soil rich in ancient carbon from fire ash and vegetation poses a previously unrecognised greenhouse-gas threat, say scientists.
Disturbance of the soils through erosion, agriculture, deforestation, mining and other human activities risks releasing the carbon into the atmosphere, they claim.
Formed thousands of years ago, the soil layers lie many feet below the present-day Earth's surface.
US lead researcher Dr Erika Marin-Spiotta, from the University of Wisconsin-Madison, said: "There is a lot of carbon at depths where nobody is measuring.
"It was assumed that there was little carbon in deeper soils. Most studies are done in only the top 30 centimetres.
"Our study is showing that we are potentially grossly underestimating carbon in soils."
The scientists studied Brady soil, created between 13,500 and 15,000 years ago in what is now Nebraska, Kansas and other parts of the US Great Plains.
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