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Is the Mystery Carbon Hidden Under Grass?

Could a missing 2 billion tons of carbon be hidden below the world's grasslands and rangelands?

Reports from an 11-state Agricultural Research Service carbon dioxide monitoring network in the central and western United States show that rangeland soils could be storing at least a sizeable share of it.

The network, formed in 1995, had its roots in a mystery announced by global modelers in the 1980s: They couldn't account for 2 billion tons of carbon emitted annually in carbon dioxide from fossil fuel burning, deforestation and other activities.

Network scientists wondered if rangelands were the missing sink where the carbon was being stored. After all, rangelands are the largest type of natural landscape in the world, comprising about half of the Earth's land surface--including 824 million U.S. acres.

Network results so far show that just the 126 million acres of grassland in the Great Plains alone could be storing 9 million metric tons of carbon annually. ARS scientists estimate that U.S. rangeland soils have the potential to store a total of 30 to 110 million metric tons of carbon a year. That is about 5 percent of the annual U.S. emissions of carbon.

Each of the 11 sites has samplers on towers taking readings at three and six feet above the ground, every 20 minutes, around the clock.

Knowing where the missing sink is will help to manage it so it will keep performing its valuable carbon storage function. Network research shows that ranchers can do this as well with grazing as without, as long as they don't overgraze the land.

Rangeland has an advantage over forests for storing carbon because most of the carbon in rangeland is stored below ground, where fire won't release it.

For more on the ARS carbon dioxide rangeland monitoring network, see the October issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.