An unexpected release of carbon dioxide.

For scientists trying to predict future climate change, the role oi vegetation and soils remains a thorny issue. Some believe the two will slow global warming by absorbing substantial amounts of carbon dioxide now accumulating in the atmosphere from the burning of fossil fuels. Others think the warming will cause plants and soils to release more carbon dioxide, exacerbating the problem. A new report from Russian scientists raises the ante on this question by suggesting that Siberian soils emit much more carbon than previously realized.

While plants store carbon dioxide through photosynthesis, microbes in the soil release this greenhouse gas by breaking down organic matter in the dirt. That much of the carbon cycle is clear during the summer, when the sun is strong and the ground surface unfrozen. But what happens when winter comes to the far northern latitudes? Biologists had presumed that microbial activity grinds to a halt when the ground freezes solid, just as photosynthesis shuts off when the sun disappears. But S.A. Zimov from the Pacific Institute for Geography in Vladivostok and his colleagues report that microbes remain busy releasing carbon dioxide long after winter arrives.

Zimov's team measured carbon dioxide emissions from the soil at a site in northeast Siberia, 100 kilometers from the Arctic ocean. Although the gas production varied substantially, depending on the place and time of measurement, the soils released an average of 13.8 grams of carbon per square meter for the three months from December through February, the scientists report in the March 20 JOURNAL OF GEOPHYSICAL RESEARCH.

"These results were certainly unexpected," says Walter C. Oechel of San Diego State University, who studies carbon dioxide emissions from tundra in Alaska. Oechel recently reported that summertime measurements in northern Alaska reveal that the carbon dioxide balance has shifted there in the last few decades (SN: 2/13/93, p. 100). Instead of storing carbon as it has for millennia, the Alaskan tundra has become a net source, releasing more carbon dioxide than it absorbs during summer. He believes that a pronounced warming since the 1970s in northern Alaska has dried the soils and increased microbial activity there, enhancing carbon dioxide emissions.

Oechel wonders whether Zimov's wintertime measurements apply to a large area of the Arctic or only the small region that was studied. He plans to visit Siberia this summer to compare measurement techniques with Zimov. Oechel's team in Alaska will also extend its measurements further into the fall than it did previously, If Zimov is correct, microbes in Arctic soils can release carbon dioxide almost year round, meaning these soils can produce much more of the greenhouse gas than scientists have assumed. In that case, Arctic soils might react to a global warming in a way that substantially enhances the problem.