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Melting tundra would release glut of greenhouse gases, suggests study

Despite their sub-zero temperatures, ecosystems of the frozen north may actually accelerate global warming, University of Florida researchers have found.

The findings were collected in a 20-year experiment of the effects of fertilization on the arctic tundra at the Arctic Long-Term Ecological Research site near Toolik Lake, Alaska.

The 3 to 7 degree rise in temperature predicted by global climate models could cause the breakdown of the arctic tundra’s vast store of soil carbon, releasing more of the greenhouse gas carbon dioxide into the air than plants are capable of taking in, said Michelle Mack, a University of Florida ecologist (formerly of the University of Alaska Fairbanks) and one of the lead researchers on a study published in Thursday’s issue of the journal Nature.

“Our results suggest that climate warming in the arctic tundra may cause the release of much more carbon dioxide than previously expected, which has the effect of further increasing global warming,” Mack said. “This type of positive feedback will make the Earth’s climate change even more rapidly.”

The findings were collected in a 20-year experiment of the effects of fertilization on the arctic tundra at the Arctic Long-Term Ecological Research site near Toolik Lake, Alaska. The National Science Foundation and NASA provided funding for the research.

One-third of the earthís soil carbon is locked in northern latitudes because low temperatures and water-saturated soil slows the decomposition of organic matter by bacteria, fungi and other organisms, Mack said.

To understand the response of arctic ecosystems to climate change, the UF team—in collaboration with researchers at the University of Alaska and the Marine Biological Laboratory at Woods Hole, Mass.—added fertilizer, made of nitrogen and phosphorous, to the soil to simulate the release of nutrients expected as soil organic matter decomposition increases in a warmer world, she said.

The scientists hypothesized that the fertilizer would stimulate plant growth, which would remove carbon from the atmosphere and eventually add it to the soil as plants shed dead leaves and roots over time, Mack said. Thus, the whole ecosystem was thought to be gaining carbon after fertilization, she said.

“We found exactly the opposite,” she said. “Even though plants grew more, and more carbon was stored in plants and in the surface of the soil, the whole ecosystem did not gain carbon. Instead, it lost a tremendous amount from the deepest soil layers, probably because increased nitrogen accelerated the breakdown of soil carbon.”

The results could have implications for ecosystems in other regions of the world as well, said Edward Schuur, a UF ecologist who co-led the project. Places such as the northeastern U.S. and Europe, where acid rain has increased the amount of nitrogen deposited into ecosystem from the atmosphere, also could experience an increased loss of soil carbon in response to higher nitrogen inputs, he said.

“It may be that not just arctic ecosystems, but those in other parts of the world will have a similar decomposition response to increased nitrogen,” Schuur said. “Increased nitrogen levels are thought to have caused trees to grow more in many places. But it may be that these places experience the same kinds of effects below ground that we’ve noted in our experiment, where a significant loss of carbon from soil can offset carbon gains by plants.”

Few previous studies have assessed fertilization effects on soil carbon pools because these effects are difficult to detect over short periods of time, he said. The long-term nature of the experiment makes it unique among studies of arctic ecosystems, Schuur said, and makes the effects of fertilization large enough to detect.

Peter Vitousek, a professor of biological sciences at Stanford University, said the research is important. “Itís long been thought that global warming would have two opposing effects on arctic soils,” he said. “First, it would increase the breakdown of soil organic matter, releasing carbon dioxide (the major cause of warming) to the atmosphere—but second, this breakdown of soil organic matter would liberate nutrients from the soil and enhance rates of plant growth, thereby removing carbon dioxide from the atmosphere.

“This work demonstrated beautifully that there is another, even stronger, effect—an increase in nutrients also enhances the breakdown or soil organic matter,” he said. “Now we know that the overall effect of warming—which is already with us, especially in the Arctic—will be to release carbon dioxide to the atmosphere, enhancing the likelihood of further warming.”

[Read more at University of Florida]

Image of Toolik Field Station courtesy National Science Foundation | Posted 09.23.04 at 9:35 pm

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