Pollution helps weeds take over prairies

Acid rain and agricultural pollution both spew nitrogen into the air. Though plants need nitrogen to grow, a new study finds that even small additions of this fertilizing pollutant can perturb the landscape.

 In plots of Minnesota prairie to which ecologists applied nitrogen for 12 years, native grasses showed a dramatically impaired ability to compete against weeds that had immigrated from Europe centuries ago.

 The nitrogen treatment triggered the terrestrial equivalent of eutrophication -- algal blooms that result when aquatic systems are overfed, argues David A. Wedin of the University of Toronto. Just as unchecked algal growth can choke out native aquatic species, so overfertilized weeds began choking out native tall-grass prairie plants.

 Like trees and shrubs, the interloping weeds belong to a class of plants, known as C3, that employs a slightly different scheme for using carbon dioxide than the native C4 prairie grasses. Overall, C3 plants dominate in North America.

 Parsimonious in their use of nitrogen, prairie grasses can thrive where access to this nutrient is limited, Wedin observes. Adding this nutrient gave the C3 weeds -- nitrogen hogs by nature -- the toehold they needed to begin invading prairie plots, he and David Tilman of the University of Minnesota in St. Paul report in the Dec. 6 Science.

 As the weedy immigrants died, their considerable nitrogen stores spurred the growth of nitrogen-starved soil microbes that break down plant matter. These microbes, in turn, rapidly released much of the weeds' building blocks into the environment, permitting nitrogen to run off as water pollution and carbon to reenter the air.

 The naturally slow decomposition of prairie plants, in contrast, ensures that most of their constituents become stored in the earth. Indeed, this is how prairies built the U.S. farm belt's rich soils.

 The rapid decomposition of weeds should dampen the spirits of energy policy analysts, says plant ecologist Ernst-Detlef Schulze of Bayreuth University in Germany. These "technocrats" had argued that by spurring plant growth, the nitrogen in acid rain would help industrialized nations sop up more of the carbon that their fossil-fuel burning is emitting as carbon dioxide, he says. Such carbon releases threaten to trigger a global warming.

 The new study now shows the fallacy in that reasoning, Schulze says.

 He notes that the loss of prairie species provoked by adding nitrogen "in many cases caused carbon storage to be more than halved."

 Adds Tim Seastedt of the University of Colorado at Boulder, "this is a beautiful example of an ecosystem feedback," where pollution can "induce changes in species composition that alter the ecosystem to guarantee the new species will persist." Globally, however, other human activities may compensate for these changes, Seastedt notes. For instance, some Mexican C3 shrub lands are now being converted to C4 grasslands.