Modern society pollutes the air not only with carbon dioxide, but also with large amounts of nitrogen-containing compounds released by the burning of fossil fuels and the use of fertilizers. Scientists had hoped that this extra nitrogen would spur the growth of plants and that the plants, in turn, would absorb some of the extra carbon dioxide in the atmosphere to moderate global warming.

That scenario now seems unlikely, say scientists at the University of Toronto and the University of Minnesota. In the December 6th issue of the journal Science, Toronto's David Wedin and Minnesota's David Tilman report little reason for optimism about this problem. In studying the effects of experimentally added nitrogen on prairie grasslands, they found that while low rates of nitrogen deposition encouraged plant growth and high carbon storage in fields dominated by native "warm-season" prairie grasses, the results were very different in fields dominated by non-native "cool-season" grasses. These fields lost most of the added nitrogen and showed no net storage of carbon. Further, at medium and high rates of nitrogen addition, the native prairie species went extinct, the diversity of vegetation dropped sharply, and the ability of the prairie grasslands to store carbon disappeared.

"From a global change perspective, this is the first long-term field experiment to demonstrate the tight linkages between nitrogen deposition, carbon dynamics, and plant species composition in grasslands," says Scott Collins, director of the National Science Foundation's Long-Term Ecological Research Program, which funded the research.

The two researchers spent 12 years studying the effects of experimentally added nitrogen in 162 plots in three Minnesota grasslands. "We added nitrogen at rates equivalent to what's deposited from the atmosphere in Minnesota and the Ohio Valley, right up through the amounts of highly agricultural a
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Contact: Cheryl Dybas
cdybas@nsf.gov
703-306-1070
National Science Foundation
5-Dec-1996