CO2 CIRCLES At the FACE project, towers encircle mature loblolly pines in Duke Forest. Carbon dioxide is pumped into the air within half of the circles, raising CO2 concentrations to about 560 ppm, 200 ppm higher than normal. PHOTO BY WILL OWENS

After an initial surge, the growth of pine trees living in an atmosphere enriched in CO₂ slowed down unless given a fertilizer kick [Nature, 411, 469 (2001)]. And although the amount of pine needles and other carbon-containing litter on the forest floor increased, so did their rate of breakdown, with little carbon accumulating in the deeper mineral soil layers [Nature, 411, 466 (2001)]. Expectations that increases in carbon sequestration can compensate for increasing CO₂ in the atmosphere are "unduly optimistic," the researchers who carried out the studies at Duke University's Duke Forest conclude.

"We expected greater rates of plant growth and greater rates of soil carbon storage during fumigation of the site with high CO₂," says William H. Schlesinger, Duke professor of biogeochemistry and principal investigator for the Free-Air CO₂ Enrichment (FACE) project. "What is surprising is that limited supplies of soil nutrients eliminated that response so quickly--and completely," he adds. "And it is also surprising that the additional inputs of carbon to the soil during the first three years of the experiment did not lead to greater carbon storage in the soil. The decomposers responded well and decomposed most of it."

Ram Oren, associate professor at Duke's Nicholas School of the Environment, tells C&EN that the researchers have not yet been able to account for all the carbon that the plants take up--a significant loose end. And Schlesinger says he would like to see experiments with other variables: "It would be interesting to see what the carbon storage is when a forest is grown at high CO₂ and high temperature, or high CO₂ and high nitrogen."

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