Alaska scientists find Arctic tundra yields surprising carbon loss

09/24/04

Carbon loss from soils exceeds storage by plants

Fairbanks, Alaska – Institute of Arctic Biology (IAB) ecologists Donie Bret-Harte and Terry Chapin and colleagues working in northern Alaska discovered that tundra plants and soils respond in surprisingly opposite ways to conditions that simulate long-term climate warming.

Their findings are published in the September 23, 2004 edition of the leading science journal Nature and are featured in the journal's News and Views section.

Bret-Harte, Chapin, lead author Michelle Mack of the University of Florida, Gainesville, and colleagues set out to investigate whether the commonly held assumption that a warming climate will lead to bigger plants that can store more carbon and thereby reduce atmospheric carbon dioxide was indeed a silver lining in the global warming cloud that some people had hoped for.

Apparently not.

"The broadest implication of this research is that climate warming could lead to a much greater release of carbon dioxide to the atmosphere and a greater positive feedback to further warming than we originally thought," Bret-Harte said.

In the experiment, conducted at IAB's Toolik Field Station, researchers measured the amount of carbon and nitrogen in plants and soils from plots of tundra that have been continually fertilized since 1980 – a condition thought to simulate the increased nutrient availability expected as a result of a warmer climate. The plots are part of a 20-plus-year project by Terry Chapin of IAB, and Gus Shaver of The Ecosystems Center at the Marine Biological Laboratory in Massachusetts.

"One of the greatest values of IAB's Toolik Field Station is that it provides opportunities for long-term uninterrupted research in a pristine environment. We could never have gotten the results we did without such a long-term experiment," said Bret-Harte.

"The connection between fertilization and warming is that warmer temperatures should stimulate decomposition of dead plant material, releasing carbon to the atmosphere and nitrogen to plants. Nitrogen limits plant growth in most terrestrial ecosystems, said Bret-Harte"

"What's really surprising about this result is that we didn't expect that this big loss of carbon from the soils would be stimulated by nitrogen alone. Everyone had assumed increased decomposition would be caused by increased temperatures, and the main effect of increased nitrogen would be to stimulate plant growth and store more carbon. We expected that fertilization by itself would lead to increased carbon storage."

"Instead, nitrogen seems to stimulate decomposition and promote carbon dioxide release to the atmosphere from the soils," Bret-Harte said.

The researchers found that although the aboveground portion of tundra plants doubled their productivity under fertilization and, as expected, stored more carbon, the losses of carbon and nitrogen from the deep-soil layers was substantial and more than offset the increased carbon stored in the aboveground plants and plant litter.

Because more than one-third of the world's soil carbon is stored in northern ecosystems – boreal forest and Arctic tundra – and is equivalent to two-thirds of the carbon found in the atmosphere, the loss of deep-soil carbon could mean an even greater increase in atmospheric carbon dioxide concentrations than is caused by fossil fuel burning.

"The paradigm is that decomposers (microbes) are always limited by carbon availability and almost never limited by nitrogen availability, but this project suggests that we don't understand decomposition as well as we thought we did. Better understanding of decomposition is necessary to be able to predict what will happen with climate warming in northern ecosystems."

Source: Eurekalert & others

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