Soil seen as missing sink.

Over one-quarter of the gas emitted by fossil fuel combustion is missing from the atmosphere, confounding scientists' efforts to model climate change. New findings suggest that some of it may be under our feet.

"Soil is where it's at," says Jeffrey Andrews of Duke University in Durham, N.C. Speaking at the annual meeting of the Ecological Society of America last month, he reported that trees take large amounts of carbon dioxide from the air and pump it into the ground. Andrews suggests that the carbon dioxide eventually leaches into groundwater; this prevents it from quickly reentering the atmosphere, where it could exacerbate global warming (SN: 2/13/93, p.100).

"I applaud him," says Peter Curtis of Ohio State University in Columbus. "It's a potentially important new mechanism to account for missing carbon in the atmosphere." If Andrews' finding holds up, the subsurface may offer fertile ground for researchers trying to find the missing carbon sink.

Working at a forest reserve in North Carolina, Andrews dug samples 1 meter below loblolly pine trees that had been sprayed with one and a half times the normal amount of carbon dioxide. The gas usually makes up 3 percent of the soil's composition, but it reached 4 percent under the dosed trees. A stable isotope of carbon dioxide acted as a marker of the gas. "It's not leaving the carbon cycle," says Andrews. "It's going into a more slowly cycling pool." If correct, these results indicate that forests pumping carbon into groundwater could account for 20 percent of the missing carbon dioxide. Trapped in the groundwater, it can remain stable for thousands of years.