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 by the agricultural use of fertilizers. Some scientists have 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 from the greenhouse effect.
But that rosy scenario seems unlikely in the wake of work by scientists at the University of Toronto and the University of Minnesota. Writing in the Dec. 6 issue of Science, Toronto's David Wedin and Minnesota's David Tilman report little reason for optimism. 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 nonnative 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.
The two researchers spent 12 years studying the effects of experimentally added nitrogen in 162 plots in three Minnesota grasslands that are part of the National Science Foundation (NSF)-supported Long Term Ecological Research network. The grasslands were naturally low in nitrogen and varied in such factors as total soil carbon and plant species composition. The researchers supplied the fields with other nutrients that might have limited plant growth and controlled soil pH in order to isolate the effects of experimentally manipulated amounts of nitrogen.
"We added nitrogen at rates spanning what's deposited from the atmosphere in Minnesota to those of the Ohio Valley, right up through those 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. The rate in Minnesota is now about 0.7 to 1 gram of nitrogen--that's roughly equal to three-quarters of a teaspoon of pure ammonium nitrate fertilizer--per square meter per year. The Ohio Valley and the lower Great Lakes region receive about two or three times this amount, and parts of Europe receive up to 10 times this amount."
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. Fields of warm-season prairie grasses, which were best able to retain added nitrogen and store carbon, were also most vulnerable to loss of species diversity and changes in plant species composition. Also, fields bearing a heavy nitrogen load were able to retain lesser proportions of the nutrient. Most of the lost nitrogen leaked into groundwater as nitrate, an important pollutant and human health threat across the Midwest.
"Whether we're concerned about tying up carbon dioxide or reducing groundwater nitrate pollution, the experiment shows that our native prairies do a better job," said Wedin. "Unfortunately, they are almost all gone in the Midwest. We should preserve the pieces of prairie left and start replanting them wherever we can."
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.
"We won't know how general these results are until the experiment is repeated in forest ecosystems because they store more carbon than grasslands," said Tilman. But Tilman and Wedin conclude that in grassland ecosystems, nitrogen loading is a major threat that leads to loss of diversity, greater abundance of nonnative 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."
The work was supported by NSF and the Natural Sciences and Engineering Research Council of Canada.
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