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 these findings add a new perspective on the capacity of Earth's soils to store carbon, and a measure of caution suggesting that elevated CO2, 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 CO2.

Previous studies (including the present study) 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 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 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 N
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12-Mar-2007