Future atmospheric levels of the notorious heat-trapping gas, carbon dioxide, remain a controversial topic among environmental scientists. Many researchers believe that increasing amounts of CO2, belched into the atmosphere by human fossil fuel use, will be captured through nature's ability to lock up the carbon in soil organic matter and faster growing trees. But it's not so simple. A new report, published in the November 28 Science, shows that the availability of nitrogen, in forms usable by plants, will probably be too low for large increases in carbon storage.
Ecosystems on land can store carbon, through bigger trees and more organic matter in soils, but shortages of mineral nutrients, especially nitrogen, curb potential future carbon storage. Several approaches to calculating ecosystem carbon storage, including some featured in the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) assume that nitrogen available to plants is abundant, even though current nitrogen limitation is widespread. "Realistic scenarios for future changes in nitrogen availability limit ecosystem carbon storage to the low end of the range presented in the recent IPCC report," says Field.
"In a garden limited by water, a gardener would not expect a big increase in growth from adding potassium. Similarly, plants in natural ecosystems limited by nitrogen may not grow much faster when they are exposed to increased levels of carbon dioxide," explained co-author Jeffrey Dukes. "Plants will need more nitrogen if they're going to lock up more carbon. The models used by the IPCC just didn't acknowledge that to a sufficient extent." Human activities tend to add biologically available nitrogen to ecosystems, but the additions are patchy in space and the added nitrogen can be rapidly lost. According to Field, "Even with generous assumptions about future increases in biologically available nitrogen, we still couldn't find enough nitrogen to support the range of carbon storage discussed in the IPCC report."
These new findings highlight the challenge of limiting global warming. Dukes concludes, "Our study suggests that we've been counting too much on the natural ecosystems to bail us out of our carbon emissions problem. The natural systems can help, but there are limits to their response. We have to make sure these limits are incorporated into our models."
Authors on this study were Bruce Hungate, Merriam-Powell Center for Environment Research, Northern Arizona University, Flagstaff; Jeffrey Dukes, M. Rebecca Shaw and Christopher Field, Department of Global Ecology, Carnegie Institution, Stanford, CA, and Yiqi Luo, Department of Botany and Microbiology, University of Oklahoma, Norman. This study was funded by the National Science Foundation, through the National Center for Ecological Analysis and Synthesis.
The Carnegie Institution of Washington (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments in the U.S.: Plant Biology and Global Ecology in Stanford, CA.; The Observatories in Pasadena, CA, and Chile; Embryology, in Baltimore, MD.; and the Department of Terrestrial Magnetism and the Geophysical Laboratory in Washington, DC.