DURHAM, N.C. - A long-term study has found that while trees take up in their tissues substantial amounts of the excess carbon dioxide that contributes to global warming, the accumulation of carbon into soils may be relatively slow.
The four-decade retrospective study of soil carbon in a Southeastern forest as it reestablished itself was published in the July 1 issue of Nature.
"A tremendous amount of atmospheric carbon dioxide has been reaccumulated in the regrowing forest," said Daniel Richter, an associate professor and soil specialist at Duke University's Nicholas School of the Environment, in an interview. "Carbon is accumulated not only in the wood and leaves of the trees, but also in below-ground roots and surface leaf litter. But the amount of reaccumulation is really quite surprisingly low in the topsoil itself."
Richter is lead author of the article that describes how he and his colleagues used as a tracer the elevated levels of radioactive Carbon-14 created as fallout during the open-air nuclear bomb testing era. The researchers measured where the captured carbon dioxide (CO2) went during the first 40-year life of a South Carolina experimental pine forest.
This is the first experiment of its kind that directly estimated carbon accumulation in a whole forest over several decades, he said.
Most climatologists believe that increasing atmospheric CO2 buildups from burning fossil fuels and other human activities could trap enough solar heat in a "greenhouse effect" to produce global warming in the next century. Scientists also have theorized that the world's forests may be able to take up much of that extra carbon dioxide load.
The study, a collaboration between Duke and the U.S. Department of Agriculture's U.S. Forest Service, has been underway at the Calhoun Experimental Forest, typical of many in the South, which was planted on abandoned cotton, corn and tobacco fields in 1957. This reforestation experiment was established within the Sumter National Forest by co-author Carol Wells, an ecologist now retired from the USDA's Forest Sciences Laboratory in Research Triangle Park, N.C.
The experiment is currently the longest running study of a forest system that regularly records soil changes in multiple permanent test plots and also preserves soil samples in permanent archives - now kept in a wooden cabinet near Richter's Duke laboratory.
Many scientists currently are evaluating natural processes that remove carbon dioxide from the atmosphere. They also are studying how much carbon dioxide gets released to the atmosphere by processes like converting forests into farmland and plowing carbon-rich fields to plant crops.
Researchers have long known that plants take in CO2 to create plant tissue through photosynthesis using sunlight. And they have established that some of that carbon eventually finds its way into soil after plants die and decay. But they remain unsure how much, where and how quickly captured carbon is shuttled through this complex recycling process.
According to the new article in Nature, other studies near the forest suggest that more than a century of farming activities had removed about 40 percent of the original organic carbon from the soil's upper 30 centimeters (almost one foot) before the experimental forest was planted.
In this study, Richter, Wells, Daniel Markewitz and Susan Trumbore sought to trace how much carbon dioxide was accumulated by the fast-growing loblolly pine forest planted on eight former farmland plots, and how quickly this reforestation would restore carbon to the soil.
Markewitz is a former Duke post-doctoral student now on the faculty of the University of Georgia at Athens, while Trumbore is a University of California, Irvine scientist with expertise in the study of so-called "bomb carbon."
The scientists' measurements depended on the fact that open-air nuclear testing in the 1950s and 60s nearly doubled amounts of Carbon-14 in the Earth's atmosphere before the1963 test ban treaty. This pulse of Carbon-14 allows that radioactive form of carbon to serve as a tracer for various studies of how natural processes use CO2.
During the life of the Calhoun Experimental Forest, the authors' measurements show that by the 1990s its 40-year-old trees had taken up nearly 70 percent of all new CO2-derived carbon in above-ground woods and leaves, while another 30 percent was accumulated below ground in the forest litter, tree roots and soil.
Less than 1 percent of carbon had been retained by organically active soil matter underlying the litter, even though measurements in the 1960s showed high initial inputs of radioactive Carbon-14 there.
Paradoxically, the scientists found that the lowermost 13 to 23 inches of soil depth actually lost carbon during the 40 years of the study, while soil at the 3- to 3-inch depth had elevated levels.
The loss of carbon in lower soil depths was because soil microorganisms in South Carolina's warm, damp climate actively decomposed organic matter soils, releasing carbon, Richter explained.
"Since pine forests are so vigorous in taking up carbon, we expected there would be ample opportunity for the topsoil carbon to reaccumulate over the four decades," he said. "What we underestimated was the vigor of the soil in being an engine of organic carbon's decomposition."
Also, said Richter, the Calhoun forest's soil is of a type that does not bind well to carbon to protect it from microbial attack.
Richter suspects that carbon storage has now slowed within the Calhoun's trees and underlying litter, but that its soils will continue to reaccumulate carbon as the forest ages and its pine trees are succeeded by hardwood stands. The presence of hardwoods is important, he said, because earthworms, beetles and other animals that prefer to live under hardwoods are better able to help soil store carbon.
The Calhoun's sandy and coarse-textured soil is characteristic of about half of the interior Piedmont region of the Southeastern United States, stretching between Virginia and Alabama, he said. "Our longer-term interest is, in fact, to study the South as a window to a huge fraction of the Earth's surface, especially in the tropics, that has similar, very weathered and acidic soils."
In a related policy commentary in the "Compass" section of the June 25 issue of the research journal Science, Duke ecologist and soil scientist William Schlesinger argued against suggestions that intensive agricultural practices could sequester significant amounts of atmospheric carbon dioxide in soils. These practices include using large amounts of fertilizers as well as irrigating crops on marginal, semi-arid land.
Other soil scientists, wrote Schlesinger, have suggested that such agricultural practices could significantly help the United States meet the guidelines of the international Kyoto Protocol to allay global warming. These scientists have predicted that such practices could take up all carbon dioxide released by conventional farming, as well as sequestering a significant portion of CO2 emitted during the burning of fossil fuels.
But Schlesinger, a professor with Duke's botany department and Nicholas School of the Environment, disagreed. Additional applications of nitrogen fertilizer that accompany such practices could actually result in the release of more CO2 into the atmosphere, he predicted, because manufacturing, transporting and applying fertilizers themselves emit so much of the greenhouse gas.
Also negating any additional storage of carbon dioxide would be the need to power pumps to supply irrigation water for dry-land farming, combined with chemical reactions between arid soils and calcium in that water.
"A number of senators and representatives see a credit given to farmers in big agricultural states as a vote-getting mechanism," Schlesinger said in an interview. "In other words, a farmer might get paid to sequester carbon in a field in South Dakota. What I think they need to do is realize that might come at a cost of carbon emissions at a fertilizer plant in Louisiana.
"If you really want to sequester carbon you don't manage agricultural land more intensely," he added. "You abandon it totally and let it grow back to forest. Then you've got the storage of carbon in vegetation and the storage of carbon in soil. And, as Dan Richter's study has shown, most of it is stored in vegetation."
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