NSF PR 96-78 - December 5, 1996
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No Such Luck: Nitrogen from Air Pollution Unlikely
to Moderate Global Warming
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."
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