Huge amounts of carbon trapped in the soils of U.S. forests will be released into the air as the planet heats up, contributing to a "vicious cycle" that could accelerate climate change, a new study concluded.

"As the Earth warms, there will be more carbon released from soils, and that will make the Earth warm even faster," said Eric Davidson, who studies soil carbon at the Woods Hole Research Center in Massachusetts but was not involved in the new study.

Forests are an important buffer against climate change, absorbing some of the carbon-dioxide pollution released from burning fossil fuels. Fallen leaves and dead trees return carbon to the soil, which takes its brown color from the element.

But scientists have disagreed about how much of this huge store of locked-in carbon is at risk for release into the atmosphere.

"Young carbon" - such as that stored in leaves - rapidly returns to the air as microbes decompose plant matter. As the air warms, the decomposition speeds up, releasing more carbon. That process is well known.

But deeper in the soil, older carbon is locked away as "humus" - the soft, brown material that makes forest floors spongy. Some scientists have asserted that this carbon will stay locked away even as the planet warms.

To test this idea, scientists took advantage of experimental forests maintained by the Energy Department and the U.S. Forest Service in Wisconsin and North Carolina.

Since the late 1990s, scientists have blown carbon dioxide from large tanks into these forests; the gas carries a specific radioactive carbon signature, which can be easily traced.

Francesca Hopkins of the University of California at Irvine collected soil from the two forests in jars and then measured how much carbon dioxide the soil emitted as she warmed the containers. She also measured how much of the carbon dioxide was more than a decade old - meaning it had been locked away in humus for years.

She found that about one-third of the released carbon dioxide came from soils at least a decade old. As the soil heated up, that ratio stayed about the same, meaning that the older carbon was just as vulnerable to rising temperatures as the younger carbon.

"We now know for the top 15 centimeters (about 6 inches) of topsoil, most of that is going to be vulnerable to warming," Hopkins said. "It's going to increase its respiration rate as global temperatures warm."

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