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Rooting around for missing carbon.

Call it "The Case of the Vanishing Carbon." Researchers who study carbon dioxide -- the most notorious of greenhouse gases -- can't seem to find about a billion tons of the carbon pollution emitted each year by the burning of fossil fuels. But now a team of soil scientists has gotten to a root of the problem.

Myles J. Fisher and his colleagues at the International Center for Tropical Agriculture (CIAT) in Cali, Colombia, report that deep-rooted grasses in South American pastures may transport a substantial fraction of the missing carbon into the soil. Fisher's group announced their results in the Sept. 15 Nature.

Plant biologist David O. Hall of the University of London comments that the CIAT project is the first to measure carbon storage in tropical soils. "In the past, most studies have emphasized carbon storage in above-ground biomass, in trees and such. But we've suggested that soils must be considered. We are trying to see where this carbon is because it's quite important as to what might happen in the future," Hall says.

Fisher and his coworkers studied two Colombian pastures planted with exotic species of African grass endowed with long roots. Since 1980, South American ranchers have planted roughly 35 million hectares with these varieties because they tap water and nutrients from the poor savanna soils more efficiently than native species. The researchers compared the carbon content of pasture soil to that of nearby "unimproved" savanna grasslands.

At one research site, a pasture with deep-rooted grass stored 13 percent more carbon than a neighboring savanna did. Sites planted with a combination of deep-rooted grasses and with nutrient-rich legumes contained even more carbon -- up to 36 percent over the natural grasslands.

Judging from the amount of South American pastureland planted with African grasses, the CIAT team calculates that these regions sequester from 100 million to 507 million tons of carbon each year.

If true, that would locate a fair portion of the missing carbon, says atmospheric scientist Pieter P. Tans of the National Oceanic and Atmospheric Administration in Boulder, Colo. Estimates suggest that fossil fuel combustion and deforestation release roughly 6.5 billion to 7 billion tons of carbon each year, but only about 3 billion tons remain in the atmosphere.

Scientists believe the oceans sop up roughly 2 billion tons annually, which leaves another 1 billion to 2 billion tons unaccounted for. This missing carbon must go into the forests, grasslands, and tundra spread around Earth. Tans and other scientists searching for the carbon sinks have tended to focus on tropical forests as a potential absorber, but the new report will prompt them to look at grasslands, he says.

While Fisher's study concerns savannas, he suggests that ranchers may also plant deep-rooted grasses and legumes in previously cleared sections of tropical rain forests. Although these plants can't offset the damage done by forest destruction, they will make pastures more sustainable, make the forage more nutritious, and help absorb some of the carbon released by deforestation.
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Title Annotation:accounting for carbon pollution emitted annually from the burning of fossil fuels
Author:Monastersky, Richard
Publication:Science News
Date:Sep 17, 1994
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