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DURHAM, N.C. -- Scientists cannot account for all the extra carbon that fossil fuel burning and land clearing should have pumped into Earth's atmosphere. But a Duke University geochemist's studies of nuclear bomb test residues hint that much of this "missing" carbon is hiding in the soils.
"It is the first time that the soil has been identified as a major player," said Kevin Harrison, who made his deductions by plotting how levels of radioactive carbon-14 changed in soils as a result of atmospheric bomb testing of the 1950s and 60s.
Harrison's studies also suggest how much of this hidden carbon may be released to the atmosphere when soil is plowed, and how much may be captured by the soil when former agricultural land is replanted in trees.
Carbon-14 can be created when cosmic rays or nuclear blasts cause single protons to be ejected from atoms of atmospheric nitrogen, Harrison said in an interview. Like normal carbon, this "radiocarbon" form can then be oxidized to form atmospheric carbon dioxide (CO2) , which plants use to make sugars in the photosynthetic process.
Whether it is radioactive or not, the carbon in CO2 is incorporated into plant tissues. And it eventually finds its way into the soil when plants die and decompose.
After that, microbes will cause carbon in "active" components of soil to be oxidized back into CO2 , he said. As carbon dioxide, it can then be re-released into the atmosphere. But the rest of the carbon remains bound in "passive" soil components, where it can stay for many centuries.
The trouble is that without reasurable benchmarks, scientists have no way to determine how long carbon will remain soilbound. Nor can they estimate the rates that carbon is entering or leaving the dirt. Harrison reasoned that the data from the bomb testing era, which ended in 1964, could provide such guidelines.
Harrison began investigating carbon's "turnover times" in soil as a graduate student at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. His Ph.D. adviser, Lamont geologist Wallace Broecker, had challenged him to look for the location of the missing carbon.
That question is an important one both for scientists and policy makers. Many expect increasing levels of atmospheric CO2 from human activities will trap enough extra solar heat in a "greenhouse effect" to significantly raise global temperatures and perturb Earth's climate. Some believe the first signs are already here.
"We know how much carbon is accumulating in the atmosphere and in the oceans, but a large part of it is unaccounted for," Harrison said. "We decided it has got be going into the land. And the biggest pool of carbon on the land is in the soil.'"
Based on measurements around the world, various scientists have estimated that about 1.5 trillion tons of carbon currently reside in the soil. "But how much of this carbon is exchanging with the atmosphere on an annual basis?," he said. "And are the amounts increasing or decreasing? Nobody knew. Using bomb radiocarbon measurements, I was the first to actually quantify that."
There is no way to tell how long normal carbon has been present in the soil. But carbon-14 isotope is different, both because it is radioactive and because its radioactivity decays at a known rate. Passive components of soil, for example, should contain far less of it. That's because those have far less interchange with an atmosphere that is preferentially rich in carbon-14.
Harrison also had access to yearly carbon-14 soil records collected during and after the nuclear testing period. And, by using samples of 1850 wood as a standard, he was able to estimate how radiocarbon levels have changed in the environment since the dawn of the industrial age. After that, carbon dioxide emissions caused by human activity began significantly increasing.
Using all these tools, Harrison could directly measure changes in carbon-14. And, by extension, he could use these changes as benchmarks to estimate how quickly carbon turns over in various kinds of soils.
In a 1993 article in the journal Science, Harrison, Broecker and Swiss scientist Georges Bonani used his method to explain a phenomenon that other scientists had already noted: farmed land has a lower carbon content.
They found plowed soils have lower percentages of carbon-14. That, they concluded, is because plowing mixes radiocarbon-poor passive soils, which predominate deeper underground, with radiocarbon-rich active soils nearer the surface. The net effect is an accelerated release of CO2.
The Science paper estimated that 10 years worth of fossil fuel carbon dioxide emissions could become bottled up in soils "if agricultural soil across the globe could be engineered back to its original carbon content."
In another 1993 report, published in the journal Global Biogeochemical Cycles, the same authors use his method to estimate that 31 billion tons of extra carbon may have entered the soil since 1850 as a result of increasing industrial age CO2 emissions.
By 1994, Harrison had moved to Oak Ridge National Laboratory in Tennessee. And in 1995, he received a National Science Foundation fellowship funding postdoctoral research at Duke.
In 1995, he also coauthored a Global Biogeochemical Cycles paper with Oak Ridge scientist Wilfred Post as well as Daniel Richter, a soil scientist at Duke's Nicholas School of the Environment.
That study used carbon-14 measurements in a South Carolina forest to suggest that carbon is stored at exceptionally high rates in former agricultural land that has been allowed to return to woods.
And, in a 1996 article in the journal Radiocarbon, his latest, Harrison found that between 0.5 and 0.7 trillion tons of carbon may be going into soil. He concluded that may locate between 45 and 65 percent of the "lost" CO2, using various scientists' estimates.
Harrison, who is about to assume his first faculty position at Boston College, acknowledged that more "real world" research is needed to address another nagging uncertainty. No one really knows what the future carbon dioxide "fertilization factor" will be.
He explained that as atmospheric concentrations of carbon dioxide continues to build, scientists suspect the extra gas will cause at least some plants to grow more robustly. If that is borne out, this so-called CO2 fertilization could cause even more carbon to find its way into the soil.
On the other hand, increasing amounts of CO2 are also expected to raise global temperatures. "When you increase the temperature, reactions tend to go faster," he noted. "So you'll increase the microbial oxidation of soil organic material and you'll lose carbon from the soil.
"The unknown question is: what happens to the nutrients that are associated with the soil organic matter? Are they incorporated into the microbes? Or can the plants utilize them? If the plants can utilize them, then warming could still stimulate carbon storage. Though you would be losing carbon from the soils, you would more than make that up in the increased vegetation."
As an extension of his radiocarbon research, Harrison will continue to collaborate with Oak Ridge and Duke scientists in an effort to answer such questions. For example, he is participating in the long-range Forest-Atmosphere Carbon Transfer and Storage (FACTS-I) experiment at Duke Forest, an extensive research reserve near Duke's campus.
A collaboration with the Brookhaven National Laboratory, FACTS-I is the world's first fully replicated study that can simulate how an entire forest ecosystem responds to the kind of atmospheric CO2 concentrations that are expected in the future.