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 and industrial areas of Europe," said Tilman. "Two of our nine treatments went beyond these rates to try to predict the longer-term effects of nitrogen deposition."
Tilman and Wedin found that more than half of the plant species were lost across the nitrogen addition gradient, with the greatest losses occurring at low levels of nitrogen addition -- the 1 to 5 gram range, which is comparable to current atmospheric deposition rates in eastern North America and northern Europe. Most of the lost nitrogen leaked into groundwater as nitrate, a pollutant and human health threat throughout the Midwest.
The nitrogen-driven loss of diversity and rise of "weedy" species in grasslands are comparable to the well-documented changes that occur in some lakes when phosphorus is added, the researchers said. In lakes lacking phosphorus, the addition of this nutrient -- often a result of human activities -- causes "eutrophication," a process that leads to increased growth of algae and other undesirable outcomes.
Tilman and Wedin conclude that in grassland ecosystems, nitrogen loading is a major threat that leads to loss of diversity, greater abundance of non-native species and the disruption of ecosystem functioning -- responses that are tightly linked. "We cannot preserve prairies or maintain the functioning of these and other ecosystems if we continue to pollute them with high rates of atmospheric nitrogen deposition," said Tilman. "Nitrogen pollution is a problem that will grow progressively worse as the human population rises unless we take direct steps to counter it."