March 12, 2007

SMITHSONIAN SCIENTISTS REPORT NEW CARBON DIOXIDE STUDY

Researchers at the Smithsonian Environmental Research Center report the results of a six-year experiment in which doubling the atmospheric greenhouse gas carbon dioxide in a scrub oak ecosystem caused a reduction in carbon storage in the soil.

The scientists said these findings add a new perspective on the capacity of Earth's soils to store carbon, and a measure of caution suggesting that elevated carbon dioxide, by altering microbial communities, may turn the soil from a potential carbon sink into a carbon source. This could offset some of the gains in carbon storage in plant biomass due to increased growth at elevated carbon dioxide.

Previous studies (including the present study) have shown that plants will respond to higher carbon dioxide by increasing growth and taking up much of the excess carbon. This has led some to speculate that plants may be able to mitigate increases in atmospheric carbon dioxide and that soils, which represent the largest and most stable terrestrial carbon pool, also may serve as a sink for excess carbon.

During the course of their study, Smithsonian scientists found that the amount of carbon in the ecosystem as a whole increased. However, they also saw a consistent loss in soil carbon under high carbon dioxide conditions. The carbon dioxide loss from soils offset about 52 percent of the additional carbon that had accumulated in the plants above ground and in the roots.

"We were surprised to find that these soils were losing soil carbon despite the fact that there was more plant growth," said Patrick Megonigal, a microbial ecologist at SERC and one of the study's authors. "We thought that higher plant growth at elevated carbon dioxide would either add more carbon to soils, or at least leave it the same. We now need to consider a third possibility-the carbon already in soils will end up back in the atmosphere as a greenhouse gas."

The study will be published this week in Proceedings of the National Academy of Sciences.

Working at a long-term Smithsonian experimental carbon dioxide site in a Florida scrub oak ecosystem, the researchers compared core samples from test plots that had been exposed to six years of elevated carbon dioxide and core samples from plots exposed to ambient carbon dioxide. They also performed laboratory experiments on soils from both elevated and ambient plots to understand microbial composition and activity within each type of soil.

Their study reveals that added carbon dioxide has a so-called "priming effect," stimulating certain microbes and increasing decomposition. Soils exposed to the elevated carbon dioxide had higher relative abundances of fungi and higher activities of a soil carbon-degrading enzyme. As the fungi and enzymes decompose the organic matter in the soil, they free up stored carbon and release it through respiration as carbon dioxide. With the priming effect of added carbon dioxide, more soil decomposition results in higher respiration rates, an overall loss of carbon and an increase in the release of carbon dioxide from the soil.

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Contact:

Kimbra Cutlip
Smithsonian Environmental Research Center
443-871-1632
cutlipk@si.edu

This text derived from:
http://www.si.edu/

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