Researchers at the Smithsonian Environmental Research Center report the results of a six-year experiment in which doubling the atmospheric greenhouse gas carbon dioxide (CO2) in a scrub oak ecosystem caused a reduction in carbon storage in the soil.

The scientists said this response suggests a limited capacity of Earth’s ecosystems to stabilize atmospheric CO2 and slow global warming. These findings add a new perspective and a measure of caution suggesting that elevated CO2, by altering microbial communities, may turn a potential carbon sink into a carbon source.

Previous studies have shown that plants will respond to higher CO2 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 CO2 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 saw a consistent loss in soil carbon under high CO2 conditions. The CO2 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 CO2 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 CO2 site in a Florida scrub oak ecosystem, the researchers compared core samples from test plots that had been exposed to six years of elevated CO2 and core samples from plots exposed to ambient CO2. 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 CO2 has a so-called "priming effect," stimulating certain microbes and increasing decomposition. Soils exposed to the elevated CO2 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 CO2. With the priming effect of added CO2, more soil decomposition results in higher respiration rates, an overall loss of carbon and an increase in the release of CO2 from the soil.

Study authors are: Karen Carney and Bruce Huntgate, SERC post-doctoral fellows at the time of the study, who now work at the U.S. Agency for International Development and Northern Arizona University, respectively; Bert Drake, SERC plant physiologist; and Patrick Megonigal, SERC microbial ecologist.

Written from a news release by Smithsonian.