The number of plant species worldwide may be dwindling from the effects of chronic low levels of nitrogen on terrestrial ecosystems, according to a University of Minnesota study.
Loss of biodiversity from high levels of atmospheric nitrogen has been reported in parts of Europe and the United States, but this is the first long-term study of the impact of much lower levels of nitrogen deposition over much of the developed world.
The study, conducted by David Tilman, Regents Professor of Ecology, and former university graduate student Christopher Clark, will be published in the Feb. 7 issue of Nature. Research was carried out at Cedar Creek Ecosystem Science Reserve, a field station operated by the university's College of Biological Sciences.
"Even at low levels, comparable to nitrogen deposition over many industrialized nations, we lost about one plant species in six at our test site [17 percent over 23 years]," Clark said. Rare species were more vulnerable to loss than common species.
But Clarke and Tilman also discovered some good news -- that the loss of species can be reversed. Thirteen years after addition of nitrogen was stopped, species numbers had recovered.
"Many ecosystems worldwide may be losing plant species because of nitrogen deposition from fossil fuel combustion and agricultural fertilizers," said Tilman. "But with a rapid, coordinated national and international effort, we can likely stem or reverse these losses of biodiversity."
Over the past 60 years, fossil fuel combustion and agricultural fertilizers have doubled the amount of nitrogen inputs to terrestrial ecosystems worldwide. The level could double again as nations in Asia and South America industrialize.
Nitrogen is an essential nutrient for plant growth, but too much causes a few species to flourish at the expense of their competitors. Within an ecosystem, species have different roles that contribute to the productivity and stability of the community. When some species are missing, the functioning of the ecosystem as a whole is impaired.
The study was performed in three prairie-like grassland ecosystems at Cedar Creek Ecosystem Science Reserve. All of the plots were treated with varying levels of nitrogen addition from 1982 to 1991. Treatments to half of the plots in one of the fields were stopped after 1991, but nitrogen addition has been continued in all other plots.
The study was funded by the National Science Foundation.
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