The study, by researchers at Argonne National Laboratory, Duke University, University of Illinois at Chicago, and Oak Ridge National Laboratory, was funded primarily by the U.S. Department of Energy Office of Science.
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 atmospheric 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 sweetgum trees growing in simulated mid-century air at two experimental sites remained intact far longer and transferred less carbon into soils than scientists had expected.
"Our data showed that fine root replacement varied from 1.2 to 9 years depending on root diameter and forest type," said Argonne environmental scientist Roser Matamala, lead author of the Science article. 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."
"Some forests would do a better job than others in taking up carbon dioxide from the atmosphere and placing it into the soil," Matamala said. "Pine forests have slow root replacement which decreases the potential to accumulate carbon in the soil in the short-term, while the fast root replacement coupled with increased root production in the sweetgum forest led to a rapid and significant increase in soil carbon".
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. The assimilated carbon dioxide, converted into carbohydrates 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.
"The major implication for greenhouse management strategies is that some forests won't transfer carbon from the atmosphere to soils at the speed we need them to do it to reduce global warming," said co-author Miquel Gonzalez-Meler at the University of Illinois at Chicago.
To test how a CO2-enriched atmosphere will actually affect the environment, the researchers bathed test plots within a growing loblolly forest near Duke and in plots of sweetgum-dominated woodlands in eastern Tennessee with addition carbon dioxide. 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 levels of the gas expected in the air worldwide by mid-century.
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 and carbon transfer to soil is slow. The fine roots in the Oak Ridge site, however, have a shorter life, and much more of the extra carbon is transferred faster to the soil.
The carbon tracer approach used in the study gives scientists a more accurate way to estimate replacement 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.
The analysis revealed that the pines showed a root carbon turnover of 4.2 years, and the sweetgums showed a carbon turnover of 1.25 years. Plant biologists had previously estimated that such roots would be replaced once every year in average. Based on this analysis, the larger roots would last even longer, said the scientists. Other carbon tracer studies confirmed that the long root turnover rates are changed by carbon dioxide levels.
"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.
Other authors are Richard Norby of Oak Ridge National Laboratory and Julie Jastrow of Argonne National Laboratory.