CO2 Research Gets to the Root of Things

A new study indicates that the potential for soils to soak up atmospheric carbon dioxide is strongly affected by how long roots live. In forests with long-lived roots, soil absorption of carbon might thus not act to ameliorate global warming from excess human-caused carbon dioxide.

The study, by researchers at Duke, Argonne National Laboratory, Oak Ridge National Laboratory and the University of Illinois at Chicago, was reported in the Nov. 21 issue of the journal Science. The research was funded by the Department of Energy.

The new study used a novel technique to measure the longevity of roots -- the source of some of the carbon that would be transferred by decay into the soil -- in trees growing in forest plots infused with a computer-controlled flow of carbon dioxide. The flow was metered to maintain the higher carbon dioxide levels predicted to occur in the middle of this century. Such an increase in carbon dioxide, caused by the burning of fossil fuels and clearing of the world's forests, underlies the global warming that scientists widely believe to have already begun.

The scientists' measurements revealed that the roots of loblolly pine, but not of sweetgum trees growing in simulated mid-century air at two experimental sites remained intact far longer and transferred less carbon dioxide into soils than scientists had expected.

Co-author William Schlesinger, dean of Duke's Nicholas School of the Environment and Earth Sciences, called the root study results "a huge change from dogma, which says that these roots turn over all the time. This really says the roots can last quite a while.

"A lot of people have counted on forest soils to soak up carbon dioxide from the atmosphere by more rapid plant growth, but this study's implication is that may be an unfulfilled expectation," Schlesinger said.

Lead author of the paper was Schlesinger's former post-doctoral fellow, Roser Matamala, now at Argonne National Laboratory. The other authors were Richard Norby of Oak Ridge National Laboratory, Miquel Gonzalez-Meler, another former Duke postdoctoral researcher now at the University of Illinois at Chicago, and Julie Jastrow, also at Argonne.

Some policy makers expect that the surge of human-produced CO2 will boost plant growth enough to remove much of the extra gas from the atmosphere, Schlesinger said. The assimilated carbon dioxide, converted into sugars during photosynthesis, would thus be stored in plant tissue for long periods, ameliorating the gas's potential impact on predicted global warming. Under this scenario, significant amounts of residual carbon would ultimately be sequestered in soil particles when roots and other tree parts decay, he said.

To test how a CO2-enriched atmosphere will actually affect the environment, the researchers bathed test plots within a growing loblolly forest near Duke with computer-controlled levels of the gas expected in the air worldwide by mid-century. Norby and his colleagues performed the same experiments in plots of sweetgum-dominated woodlands in eastern Tennessee.

At both the Duke and Oak Ridge test sites the extra carbon dioxide is released from arrays of tower-mounted valves that are computer-controlled to ensure reliable concentrations regardless of wind direction.

During the first three years of these continuing seven-year experiments, the extra CO2 boosted overall pine growth by 25 percent and sweetgum production by 21 percent, according to the "Science" report.

However, carbon tracer measurements revealed that the fine roots of the trees at the Duke site lasted significantly longer than plant biologists had previously estimated, implying that they are replaced less often. At the same time, the fine roots in the Oak Ridge experiment were found to have shorter lifetimes and much more of the extra carbon was transferred to the soil.

According to Schlesinger, a biogeochemist and ecologist, the carbon tracer approach used in the study gives scientists a more accurate way to estimate the age of roots because it documents how long the carbon actually resides in root tissue. The fact that growing roots are so hard to study without killing them or disturbing their growth has led scientists to overestimate how much carbon from extra doses of carbon dioxide might end up in the soil, he said.

"These long root lifetimes suggest that root production and turnover in forests have been overestimated and that sequestration of anthropogenic (human-produced) atmospheric carbon in forest soils may be lower than currently estimated," wrote the paper's authors.

Schlesinger said that prior to the new study scientists at the carbon dioxide enrichment site in Duke Forest assessed root growth by inserting tiny video cameras into tubes implanted in the forest soil. But that approach had its limitations, because "the act of putting in tubes means the environment is different in the soil than it would be without the tubes," he explained.

Using CO2 gas embedded with a special tracer to follow the movements of carbon is a "very elegant" new way to investigate root growth and endurance without altering natural conditions, he said. He plans to include the results of the new study in his teaching, he added.