March 3rd US Global Change Seminar: Ecological and Climatic Consequences of Human-Induced Changes in the Global Nitrogen Balance"
tsocci at usgcrp.gov
Wed Feb 26 11:16:42 EST 1997
U.S. Global Change Research Program Second Monday Seminar Series
Ecological and Climatic Consequences of Human-Induced Changes
in the Global Nitrogen Balance
How large an impact are humans having on the global nitrogen cycle? How
did this imbalance come about? What are the implications and effects of
perturbing the global nitrogen balance for the environment and for society?
Do these results have any implications for forests? Are nitrogen-enriched
ecosystems likely to exacerbate global warming by becoming a net source of
CO2? What can one expect in the near future in terms of nitrogen sinks or
impacts related to this nitrogen imbalance?
Monday, March 3, 1997, 3:15-4:45 PM
Rayburn House Office Bldg., Room B369, Washington, DC
Dr. Jerry Melillo, Associate Director for the Environment (Designee),
Office of Science and Technology Policy, The White House, Washington, DC.
Dr. William H. Schlesinger, James B. Duke Professor of Botany, Department
of Botany, Duke University, Durham, NC.
Dr. David Tilman, Distinguished McKnight University Professor of Ecology,
University of Minnesota, St. Paul, MN.
Although 80% of the Earth's atmosphere is made up of molecular
nitrogen (N2), only a small but very important fraction of this nitrogen is
converted to a form that can be used by plants and animals, a form known as
"fixed" nitrogen. Until recently, this "fixed" atmospheric nitrogen has
been thought of as beneficial to all living things. However, industrial and
other human derived sources of fixed nitrogen have now doubled the rate
that is now available.
This global overload of fixed nitrogen, despite being one of
nature's essential life-giving and life-limiting nutrients, now poses a
suite of very serious environmental concerns. For example, too much
nitrogen can result in: 1) loss of commercially important fish stocks and
ecosystems by promoting algal blooms which result in oxygen deprivation and
reduced sunlight in coastal and aquatic ecosystems; 2) local extinction of
terrestrial plant, animal, and microbial species, thereby reducing
biodiversity and ecosystem health; 3) an increase in the greenhouse gas,
N2O, which is contributing to global warming; and 4) an increase in the
concentration of nitric oxide, which contributes to acid rain and smog.
The Global Nitrogen Cycle: Natural and Humanly-Altered Conditions
Molecular nitrogen (N2) is the most abundant gas in the Earth's
atmosphere. However, in order for nitrogen to be useful to life it must
first be transformed, naturally, into forms that are useful to living
organisms, a process known as "nitrogen-fixation." Life depends on "fixed"
nitrogen that can be absorbed by plants and subsequently used by animals
linked together in the "food chain." The amount of nitrogen available at
any one time and place has a direct effect on the growth of plants--on land
and in the sea-- because fixed nitrogen is an essential life-giving and
life-limiting nutrient. Thus, the health of the biosphere is strongly
dependent upon the availability of nitrogen in a chemical form that is
useful to life.
Under natural conditions, a small amount of nitrogen is fixed
through chemical processes such as lightning. A much larger amount is
fixed by biological processes involving nitrogen-fixing bacteria in the
soil and on the roots of certain plants. Once plants die, however, this
fixed nitrogen is subsequently returned to the atmosphere by the
decomposition of dead tissue by bacteria and is then recycled for later
To enhance the availability of nitrogen for living things such as
food and fiber products, humans produce nitrogen in the form of fertilizer.
With the growth of agriculture, half of all industrial nitrogen fertilizer
used in human history has been applied since 1984. In addition to their
intentional creation of fixed nitrogen, humans also inadvertently produce
fixed nitrogen through the burning of fossil fuels. On a global scale, the
fixation of nitrogen by humans now roughly equals the amount of nitrogen
that was formerly made available naturally to life by the combined activity
of all bacteria on land. In other words, our society has now doubled the
amount of fixed nitrogen available to all living things.
Growing amounts of fixed nitrogen are showing up in remote
locations, leading to significant impacts. The concentration of nitrous
oxide (N2O) in the Earth's atmosphere, is rising at about 0.3%/yr. Perhaps
even more ominously, N2O has roughly 200 times the global warming potential
of carbon dioxide, and remains in the Earth's atmosphere for approximately
150 years, thus making it a long-lived and potent problem. Nitrous oxide is
also implicated in the loss of stratospheric ozone.
Increases in the emissions of nitric oxide (NO) due to the
combustion of coal and oil also contribute directly to higher levels of
acid rain and ozone (smog) in the lower atmosphere. Atmospheric deposition
of nitrogen is the largest single source of human-derived nitrogen in the
eastern U.S. coastal waters. There is now 10-20 times more nitrogen
entering coastal rivers in the northeastern U.S. and northern Europe than
in pre-industrial times.
Excess nitrogen flushed from fertilized farmlands, sewage treatment
plants, and fossil fuel combustion ultimately ends up in streams, rivers,
and coastal waters, where it provokes and enhances the growth of
microscopic plants that form the base of the food chain upon which more
complex and larger plants and animals later feed. However, during the
prolific, nitrogen-driven growth and life cycle of these marine and aquatic
microscopic plants, they tend to cloud the waters, thus shutting out
essential sunlight for other plants. Furthermore, upon the death of these
microscopic and larger plants, their once living tissue is consumed by
bacteria which proliferate due to the excess nitrogen, and deplete the
surrounding water of oxygen necessary for the metabolic processes of marine
and aquatic animals, including important commercial fish and shellfish
Impacts of Nitrogen Deposition on Terrestrial Ecosystems
Since 1982, Dr. Tilman and his colleagues have been engaged in an
experiment in which fixed nitrogen was systematically added to 207 plots of
grassland and savanna throughout Minnesota. In each instance, nitrogen was
added at rates which have been observed in a variety of locations around
the world so as to mimic and replicate real levels of nitrogen deposition
in a variety of places. The results of this 12-year experiment reveal that
high levels of nitrogen can have a number of serious impacts on these and
other ecosystems, including:
1. The addition of nitrogen to these grasses caused major changes in plant
species composition, insect species composition, and soil fungal
composition. Higher nitrogen levels led to decreased abundances of native
plants and to increased abundances of non-native plants, especially certain
2. The addition of nitrogen caused a significant decrease in plant species
diversity. The lowered diversity in these experimental grassland sites
corresponded with lowered stability of primary productivity in the face of
a major disturbance such as a flood or drought.
3. When nitrogen was added at low rates to plots dominated by native
prairie grasses, most of the nitrogen was retained within the ecosystem.
However, at higher rates of nitrogen addition, the native species were
replaced by non-native species, while most of the added nitrogen was
leached into the ground water. Ultimately, higher levels on nitrogen in the
ground water (in the form of nitrates) can provoke toxic algal blooms in
waterways and seriously impair the quality of drinking water.
4. The highest rates of carbon storage occurred at the lowest rates of
nitrogen addition, in direct contrast to anticipation. In instances where
high levels of nitrogen were available, the ability of the plant community
to effectively store carbon was lowered. A shift in plant species
composition was also observed to occur in instances involving higher rates
of nitrogen addition. This resulted in low rates of carbon storage because
these plant species decayed too rapidly to effectively store carbon.
5. High rates of nitrogen deposition are likely to greatly impact the
composition, functioning and stability of many terrestrial, aquatic and
marine ecosystems, with profound implications for food supplies and other
Dr. William H. Schlesinger is the James B. Duke Professor in the
Departments of Botany and Geology at Duke University. He completed his
A.B. degree at Dartmouth in l972, and his Ph.D. at Cornell in l976. He
later joined the faculty at Duke in l980. He is the author or co-author of
over 100 scientific papers, and the widely-adopted textbook
"Biogeochemistry: An Analysis of Global Change."
Currently, Dr. Schlesinger's teaching and research interests are in
ecosystem analysis, global change, and biogeochemical cycling. He is the
principal investigator for the Free Air Carbon Dioxide Enrichment (FACE)
Experiment in the Blackwood Division of the Duke Forest, a project that
aims to understand how an entire forest ecosystem (vegetation and soils)
will respond to elevated levels of CO2. He has also worked extensively in
desert ecosystems and on their response to global change. He is the
Principal Investigator for the NSF-sponsored program of Long Term
Ecological Research (LTER) at the Jornada Experimental Range in southern
Dr. Schlesinger is: a Fellow of the American Academy of Arts and Sciences;
a member of the Central Intelligence Agency's Environmental Task Force
(MEDEA); an elected official of the Ecological Society of America; and is
on the editorial boards of "Biogeochemsitry", "Global Change Biology", and
the "Encyclopedia of Global Change." Dr. Schlesinger's recent work has
been described on National Public Radio's "Morning Edition," CNN, "Discover
Magazine," "National Geographic Magazine," and in a host of national
newspapers including the New York Times and the Los Angeles Times.
Dr. David Tilman is the Distinguished McKnight University Professor of
Ecology and Director of Cedar Creek Natural History Area at the University
of Minnesota, where he has been on the faculty since 1976. Born in Illinois
and raised in Michigan, he earned his B.S. (1971) and Ph.D. (1976) in
zoology from the University of Michigan. His research interests include the
mechanisms of interspecific competition, the processes allowing the
maintenance of biodiversity, the impacts of biodiversity on population,
ecosystem stability and functioning, the causes of successional dynamics,
and mathematical theory related to these issues.
For the past 15 years Dr. Tilman has studied biodiversity and ecosystem
dynamics at the Cedar Creek Natural History Area in MN. He has published
two books in the Princeton Monograph Series, edited two books, and is an
author of more than 100 scientific papers. He was named a Guggenheim
Fellow in 1984, a Fellow of the American Association for the Advancement of
Science in 1985, Honorary Member No. 3 of the Lund (Sweden) Ecological
Society in 1985, received the W. S. Cooper Award from the Ecological
Society of America in 1989, was elected to the American Academy of Arts and
Science in 1995, was chosen as a Pew Scholar in Conservation Biology in
1995, and received the Ecological Society of America's Robert MacArthur
Award in 1996.
He is the founding editor of the Ecological Society of America's
new Ecological Issues series, and has served on the editorial boards of
"Ecology," the "American Naturalist," "Acta Oecologia" (Paris), and
"Limnology and Oceanography," and currently serves on the Board of
Reviewing Editors of "Science." His work on chaos, on the effects of
habitat fragmentation, and on biodiversity and ecosystem functioning has
received wide media coverage, including articles in the New York Times, a
Public TV documentary, and coverage in American, Canadian, British and
Australian broadcast and print news.
The Next Seminar is scheduled for Monday, April 14, 1997
Planned Topic: Economic Options and Costs for Mitigating Climate
Change: The Role of Energy Technologies
For more information please contact:
Anthony D. Socci, Ph.D., U.S. Global Change Research Program Office
Code YS-1, 300 E St., SW, Washington, DC 20546
Telephone: (202) 358-1532; Fax: (202) 358-4103
E-Mail: TSOCCI at USGCRP.GOV.
Additional information on the U.S. Global Change Research Program (USGCRP)
and this Seminar Series is available on the USGCRP Home Page at:
http://www.usgcrp.gov. Normally these seminars are held on the second
Monday of each month.
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