The precise ecosystem study of the reaction of a Texas grassland to a range of carbon dioxide levels has shown that soil nitrogen availability may limit the capacity of ecosystems to absorb expected increases in atmospheric CO2. The researchers said their study emphasizes the urgency with which the U.S. and other nations should adopt stringent limitations on CO2 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 (http://www.env.duke.edu/forest/FACTSI.htm). 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.