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September 21, 2008 — Plants and soils act like
sponges for atmospheric carbon dioxide, but new research finds that one
abnormally warm year can suppress the amount of carbon dioxide taken up
by some grassland ecosystems for up to two years. The findings, which
followed an unprecedented four-year study of sealed, 12-ton
containerized grassland plots at DRI is the cover story in this week's
issue (September 18) of the journal Nature.
"This is the first study to quantitatively track the response in
carbon dioxide uptake and loss in entire ecosystems during anomalously
warm years," said lead author Jay Arnone, research professor in the
Division of Earth and Ecosystem sciences at DRI. "The 'lagged'
responses that carry over for more than one year are a dramatic
reminder of the fragility of ecosystems that are key players in global
The plants and soils in ecosystems help modulate the amount of carbon dioxide (CO2) in the atmosphere. Plants need CO2 to survive, and they absorb most CO2
during spring and summer growing seasons, storing the carbon in their
leaves, stems and roots. This stored carbon returns to the soil when
plants die, and it is released back into the atmosphere when soil
bacteria feed on the dead plants and release CO2.
The four-year DRI study involved native Oklahoma tall grass prairie
ecosystems that were sealed inside four, living-room-sized environment
chambers. The dozen 12-ton, six-foot-deep plots were extracted intact
from the University of Oklahoma's prairie research facility near
Norman, Okla., in order to minimize the disturbance of plants and soil
bacteria. Inside the DRI's sunlit-controlled EcoCELL chambers,
scientists replicated the daily and seasonal changes in temperature,
and rainfall that occur in the wild.
In the second year of the study, half of the plots were subjected to
temperatures typical of a normal year, and the other half were
subjected to abnormally warm temperatures -- on the order of those
predicted to occur later this century by the Intergovernmental Panel on
Climate Change. In the third year of the study, temperatures around the
warmed plots were turned down again to match temperatures in the
control plots. The CO2 flux -- the amount of carbon dioxide
moving between the atmosphere and biosphere -- was tracked in each
chamber for all four years of the study.
DRI's EcoCELL facility gave the scientists an unprecedented degree
of control over the enclosed ecosystems. Not only could they create the
same air temperature conditions from year-to-year, they could also
independently control the soil temperature in each chamber -- a key
feature that enhanced the ecological relevance of the results. Each
containerized ecosystem also sat on "load cells," the type of scales
used to weigh trucks on highways. Scientists used the scales to track
the amount of water that was taken up and lost by the plants and soil
in both normal and abnormal years. Thus, each containerized ecosystem
served as a weighing lysimeter, an instrument that's used to measure
the water and nutrients that percolate through soils.
The scientists found that ecosystems exposed to an anomalously warm year had a net reduction in CO2
uptake for at least two years. These ecosystems trapped and held about
one-third the amount of carbon in those years than did the plots
exposed to normal temperatures.
"Large reductions in net CO2 uptake in the warm year were
caused mainly by decreased plant productivity resulting from drought,
while the lack of complete recovery the following year was caused by a
lagged stimulation of CO2 release by soil microorganisms in response to soil moisture conditions," explained co-author Paul Verburg, also from DRI.
Numerous studies have found that the Earth's atmospheric CO2 levels have risen by about one-third since the dawn of the Industrial Age. CO2
helps trap heat in the atmosphere, and political and economic leaders
the world over are debating policy and economic reforms to reduce the
billions of tons of CO2 that burned fossil fuels are adding to the atmosphere each year.
"Our findings confirm that ecosystems respond to climate change in a
much more complex way than one might expect based solely on traditional
experiments and observations," said study co-author James Coleman, vice
provost for research and professor of ecology and evolutionary biology
at Rice University. "Our results provide new information for those who
are formulating science-based carbon policies."
The large collaborative study involved scientists from the DRI;
University of Nevada, Reno; University of Oklahoma in Norman, Okla.;
University of New Hampshire; the National Center for Atmospheric
Research in Boulder, Colo., and Rice University in Houston, Texas. The
study was funded by the U.S. National Science Foundation's Division of
Environmental Biology under the program, "Integrated Research
Challenges in Environmental Biology" (Grant Number: DEB0078325).
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