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Declining nitrogen availability reduces CO2 fertilization effects, researchers find

Tree growth in ambient (black) and elevated (green) carbon dioxide. The percentage difference is shown for each year.
OAK RIDGE, Tenn., Nov. 15, 2010 — Climate models that project future atmospheric carbon dioxide concentrations should include a coupled nitrogen cycle, a team of researchers led by the Department of Energy's Oak Ridge National Laboratory has concluded.

In the past, coupled climate carbon cycle models have not included feedback from the nitrogen cycle, but scientists have long speculated that the inevitable nitrogen limitation of forest production would limit the widely reported carbon dioxide fertilization effect on trees. Now they have data to support their theory and the results are published in the Proceedings of the National Academy of Sciences.

"We now have a longer data record from the ORNL free-air CO2 enrichment experiment, and we report that the increase of forest production to elevated CO2 has declined over time, a significant change from our previous observations," said Rich Norby, lead author of the paper and a member of ORNL's Environmental Sciences Division.

Previous research performed at ORNL and three other forested free-air CO2 enrichment, or FACE, experiments had shown a sustained increase in forest production in response to elevated levels of atmospheric CO2. While those increases matched projections of models used in the last Intergovernmental Panel on Climate Change assessment, this may have been misleading.

"Climate change models need to include coupled carbon and nitrogen cycles if we are to have confidence in their predictions," Norby said.

During the first six years of the FACE experiment at ORNL, researchers observed a 23 percent increase in forest growth under elevated CO2, but the stimulation in growth, or net primary productivity, under elevated CO2 declined to 9 percent in 2008. As a result, global analyses that assume a sustained carbon dioxide fertilization effect are no longer supported by the FACE experiment, according to the PNAS publication.

"Our nitrogen budget analysis supports the premise that nitrogen availability was limiting to tree growth and declining over time, an expected consequence of stand development, and this was exacerbated by elevated CO2," Norby said.

The FACE experiment at ORNL, supported by the DOE Office of Science, consisted of three plots of sweetgum trees that were control sites and two plots of trees that were exposed to levels of carbon dioxide projected in 2050 (550 parts per million). The experiment ran for 12 years and resulted in numerous scientific findings and several dozen publications.

Co-authors of the paper, titled "CO2 Enhancement of Forest Productivity Constrained by Limited Nitrogen Availability," from ORNL are Jeffrey Warren and Colleen Iversen of the Environmental Sciences Division. Other authors are from Macquarie University and University of New South Wales in Australia. Funding for this research was provided by DOE's Office of Science, Biological and Environmental Research.

ORNL is managed by UT-Battelle for the Department of Energy's Office of Science.