Plants seen as unpredictable carbon sponge.

Changes in land use--especially in forests, croplands, and fallow areas--appear to play a far bigger role than anticipated in determining how much carbon growing plants will remove from the air, a new study finds.

If confirmed, the new analysis could influence what measures nations will be encouraged to take in controlling carbon as part of their adherence to the Kyoto Protocol, a treaty to limit climate change.

Carbon dioxide is a principal greenhouse gas. Two years ago, a much publicized Princeton University study concluded that vegetation within the continental United States can, like a sponge, sop up virtually all of the carbon dioxide spewed by domestic fossil fuel burning (SN: 11/21/98, p. 332).

The new study not only challenges the Princeton estimates but also finds that carbon-absorption rates are unpredictable. They "can vary by 100 percent or more from year to year," observes David Schimel of the Max Planck Institute for Biogeochemistry in Jena, Germany. During droughts, U.S. vegetation may collectively become a carbon emitter, his group reports in the March 17 SCIENCE.

Schimel coordinated research groups working with three different computer models that simulate the use of carbon by natural and agricultural ecosystems. The groups integrated into their models a wealth of locally collected U.S. data for the period 1980 to 1993. This information included not only local climate, ground cover, precipitation, and atmospheric carbon concentrations but also descriptions of how people were managing the land. It contained data on fertilizer use, crop-planting and harvest dates, irrigation, and even tillage intensity.

The scientists saw large decreases or increases in carbon storage in areas where, for example, fields were fallowed or lands were reforested.

Overall, the new study concludes, the United States' absorption of atmospheric carbon dioxide appears to total about 0.3 gigaton per year. That's far below annual U.S. carbon emissions of 1.5 to 1.6 gigatons, and close to what Richard A. Houghton of the Woods Hole (Mass.) Research Center found last year using a very different approach (SN: 7/24/99, p. 54).

The analyses by Schimel's team also contradict a prevailing view of the fertilizing effect of carbon dioxide. Carbon is a basic building block of plants. Some studies have suggested that the amount of carbon that an ecosystem retains depends on the element's availability. As atmospheric carbon climbs, more plant matter would grow and sop it up.

The new study suggests, however, that land-use practices are at least as important as climate or carbon fertilization--and perhaps more significant than both together--in determining how big a carbon sink U.S. ecosystems provide.

Houghton says that this apparently heightened importance of land-use changes should spur more research. To date, he notes, federal agencies have devoted little funding to study the role of land-use practices in modifying the effects of carbon-dioxide pollution.

Song-Miao Fan of Princeton holds to his group's view that the United States sops up at least 1 gigaton of carbon annually. He says, however, that the new study does "highlight the continuing uncertainty of the nature of the Northern Hemisphere's carbon sink."