Cloudy Future for Global Warming?

Rising atmospheric levels of carbon dioxide, blamed for global warming, may have a subterranean silver lining. Scientists report in today's Nature that carbon dioxide speeds the formation of tiny clumps of soil that act as bulwarks against erosion--and possibly as miniature storehouses for moisture and nutrients. The findings have important implications for farmers of the future, including the possibility that nutrient-poor ground could become more fertile.

In 1996, soil microbiologist Sara Wright of the U.S. Department of Agriculture in Beltsville, Maryland, found that arbuscular mycorrhizal fungi--a type of microscopic fungus that lives symbiotically with plant roots--oozes a sticky protein called glomalin. This protein binds soil particles into crumb-sized clumps, an essential feature of healthy, stable soil.

Three years later, Matthew Rillig, an ecologist at the Carnegie Institution of Washington, decided to see whether soil exposed to higher concentrations of carbon dioxide would have more glomalin and, as a result, more clumps. So he examined soil samples from two earlier California global warming studies in which scientists had exposed patches of soil to carbon dioxide levels ranging from preindustrial levels to nearly three times that much. In both cases, the clumpiness of the soil increased with exposure to carbon dioxide. "There are literally thousands of studies on the effects of elevated carbon dioxide on the ecosystems of plants," Rillig says, "but this is really the first study that has shown there can be an effect on soil structure."

The findings raise important questions about the relationship between atmospheric change and soil ecosystems, says Michael Miller, a soil ecologist at Argonne National Laboratory near Chicago. He cautions that the findings, which looked only at samples from California, may not hold true for soils elsewhere. But he wonders whether an increase in soil clumping might offset a rise in atmospheric carbon dioxide, simply by storing more carbon in the soil. "We know a lot about what's happening above ground, but we know very little about what's happening below."

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