PROVIDENCE, R.I. -- Preliminary results at a Duke University research forest
site exposed to the high levels of atmospheric carbon dioxide expected worldwide
in the next century suggest plants might shunt much of that extra CO2 into
groundwater to remain stored away for thousands of years.
That finding, if bolstered by further research, might not only lower predicted
impacts of global warming due to accelerated CO2 emissions by factories,
vehicles and land-clearing activities, but it might also help solve a mystery:
why there is now 29 percent less of the gas in the atmosphere than current
emissions inventories suggest should be there.
"While it wouldn't explain all of that 'missing sink,' if you push
the calculations, groundwater might explain maybe one-fifth of it,"
said William Schlesinger, a Duke botany professor and co-investigator in
the study. The study findings were prepared for presentation Wednesday at
the final day of the Ecological Society of America's annual meeting.
The ongoing doctoral research by Schlesinger's graduate student, Jeffrey
Andrews, made use of the Free Air Carbon Dioxide Enrichment (FACE) site
at Duke Forest, a research reserve administered by Duke's Nicholas School
of the Environment.
Designed by George Hendrey of the Brookhaven National Laboratory and co-directed
by Schlesinger, the Duke Forest FACE site uses rings of computer-controlled
towers to bathe entire patches of a loblolly pine forest ecosystem in air
enriched with 1 1/2 times more CO2 than is the current norm.
Years of work by Boyd Strain, another Duke botany professor, already have
demonstrated that extra CO2 spurs growth in many plants. Plants employ
the photosynthetic process to convert carbon dioxide into sugars, which
are then used to form plant tissue.
But the gas's conversion into plant tissue is only temporary. When plants
die and decay, microbes convert some of the dead tissue back into CO2. In
fact, even while the plants are still thriving their roots undergo a constant
cycle of death and replacement, releasing a steady supply of CO2 into the
soil even in the midst of the growing season.
Working during the 1995 growing season at the first FACE tower ring (there
are now seven), Andrews examined whether the extra CO2 taken up by the plants
will ultimately be re- released underground during decay.
"We started the work with the hypothesis that carbon might be going
into the soil, not because of any direct evidence but because there wasn't
anywhere else we could think it would go," Andrews said in an interview.
The experiment benefitted from the fact that the CO2 supplied to the FACE
ring is slightly deficient in carbon-13, one of several possible forms --
or "isotopes" -- of carbon. On average, there are 23 fewer atoms
of carbon-13 per every 1,000 molecules of CO2 supplied to FACE than are
present in normal atmospheric CO2, Andrews said in an interview.
While small, that difference is detectable though a laboratory procedure
known as mass spectrometry. And it allows the fate of the FACE CO2 to be
traced as it is taken up by plants inside the ring.
Andrews sunk plastic pipes into the ground within the FACE ring to allow
underground CO2 to be monitored and measured at depths up to one meter.
Working with Daniel Richter, an associate professor and a co-investigator
from the Nicholas School of the Environment, Andrews also installed instruments
to monitor the CO2 emissions from the soil.
Within a month, concentrations of FACE-supplied CO2 one meter underground
rose dramatically, Andrews said. The higher concentrations one meter down
also carried the signature of the carbon-13 deficient CO2 supplied by the
FACE towers, showing it was taken in by the plant and then released in root
decay, Schlesinger said. "The gas we found at depth could have only
come from FACE."
In a separate study prepared for the Ecological Society of America meeting,
Duke graduate student Andrew Allen found that higher atmospheric CO2 concentrations
indeed appear to boost microbe activity around plant roots -- implying enhanced
decomposition.
These findings could be significant for those hoping to forecast the future
impact of global warming, because underground CO2 tends to dissolve in groundwater
and stay dissolved until the groundwater bubbles back out to the surface.
And that re-emergence could take far longer than humans have been civilized.
"Groundwater has an age of anywhere between 1,000 and 100,000 years,
based on radiocarbon dates," Andrews said.
Since the anticipated global warming would be caused by increased atmospheric
CO2 concentrations, any natural mechanism that removes CO2 from the air
for very long periods would be like buying time.
"Our research would imply that the rate of growth in atmospheric CO2
might be slower than you would guess from the rate of emissions," Schlesinger
said.