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."