Grassland Study Suggests End Of "Free Ride" On Carbon Dioxide Absorption By Ecosystems

Wednesday, May 15, 2002

DURHAM, N.C. -- According to a new study, the world may soon see the end of the "free ride," in which carbon absorption by natural ecosystems ameliorates the rise in atmospheric CO due to fossil fuel burning and loss of forest.₂₂ emissions, as outlined in the international Kyoto accord on climate change The researchers, led by Duke University ecologist Robert Jackson and USDA Agricultural Research Service researchers Wayne Polley, and Hyrum Johnson, published their findings in the May 16, 2002, Nature. First author of the study is Richard Gill, a former Duke postdoctoral associate, now a faculty member at Washington State University. The research was supported by the Department of Energy and the U.S. Department of Agriculture. "Based on fossil fuel emissions, the carbon dioxide concentration in the atmosphere should be going up twice as fast as it currently is," said Jackson. "However, natural systems such as the regrowing Eastern forests are currently taking up that extra carbon dioxide, so we're really getting a free ride now. "Many of us, myself included, believe that this free ride won't continue to the same extent that it has, because the incremental benefits of the extra CO₂ get smaller and smaller relative to other nutrient constraints," he said. The policy implications of their findings are apparent, said Jackson. "Considering the expected population increase, greater resource use per capita and the inability of natural systems to take up CO₂, we may well be looking at increases per year that are double what they are now, with atmospheric CO₂ concentrations as high as 800 parts per million in this century," he said. "This means that the current lack of interest by the U.S. in participating in the Kyoto accords is especially unfortunate." According to Jackson, the study offered a new approach to studying the ecological effects of increased CO₂. "The study is unique in enabling us to study the effects of CO₂ concentrations ranging from those before the Industrial Revolution to those projected for the next century," said Jackson. "It is also unique in providing a continuous gradient of CO₂ in the field, allowing us to examine nonlinear and threshold responses and limitations of the system. Nitrogen availability appears to be one such limitation on the ability of plants to absorb CO₂." The researchers chose a section of north Texas prairie as the site for their experimental apparatus, which began operation in May 1997. The apparatus consists of two 60-meter-long long plastic-covered chambers -- resembling giant segmented worms -- erected over the grassland. The chambers measured about a meter wide and a meter high. In one chamber, the scientists expose the grasses to a smooth gradient of CO₂ concentrations ranging, from the current 365 parts per million (ppm) level down to the 200 ppm present at the end of the last ice age. The scientists achieve this concentration gradient by blowing ambient air into one end of the chamber, and as the air flows the length of the chamber, CO₂ uptake by the grasses reduces CO₂ concentrations down to 200 ppm. In the other chamber, the scientists pump into one end air enriched to a CO₂ concentration of 550 ppm -- the expected level over the next century -- and the plants' CO₂ absorption reduces this to 350 ppm at the opposite end. The chamber also includes controls to ensure that moisture and temperature levels match those outside. "There have been few experiments, even in growth chambers, that could explore the effects of changes since before the Industrial Revolution, but our design enables us to do just that," said Jackson. "Thus, it gives us insights into what changes occurred in the past and improves our understanding about will happen in the future." Operating the apparatus over multiple growing seasons, the scientists conducted detailed biochemical and biological analyses of the grass plants as well as the soil. They also measured how the species composition of the plant community changed. "We found that many of the plants' physiological processes responded fairly linearly to increases in carbon dioxide, and plant production went up," said Jackson. "However, production and soil carbon storage basically saturated above 400 parts per million, a CO₂ concentration very close to the current one. "For me, this was the most interesting part of the study, because it indicates that we are now right at a threshold where the benefits of extra CO₂ may not be all that great." Particularly important, said Jackson, were the measures of soil nitrogen availability. Soil bacteria metabolize organic matter, mobilizing nitrogen as ammonia and nitrate, which serves as the plants' nitrogen nutrient source "Our measurements showed that soil nitrogen decreased about threefold in a nonlinear way, such that as CO₂ went up, available nitrogen went down," said Jackson. "So that's where the fundamental nutrient limitation of the system occurred. The decrease in nitrogen availability apparently constrains the ability of the plants to use extra CO₂. " According to Jackson, the findings by him and his colleagues agree with tentative findings by the Forest-Atmosphere Carbon Transfer Storage (FACTS-1) facility at Duke. In that facility, sections of open forest are maintained at high CO₂ levels, to study their effects. Data from a prototype FACTS-1 facility indicated that the forest section under study had stopped responding to high CO₂ levels with enhanced growth. The researchers plan future studies using the apparatus to examine another potential limitation, water availability, said Jackson.

Note to editors: Robert Jackson may be reached at (919) 660-7408, e-mail jackson@duke.edu. Contact Dennis Meredith to receive an image of the experimental apparatus.