Study Suggests Excess Atmospheric Carbon Dioxide May Disappear Underground For Millennia

August 15, 1996

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.
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Duke University

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