Declining nitrogen availability reduces CO2 fertilization effects

Climate models that project future atmospheric carbon dioxide concentrations should include a coupled nitrogen cycle, a team of researchers led by Oak Ridge National Laboratory has concluded.

In the past, coupled climate carbon cycle models have ignored feedback from the nitrogen cycle, but scientists have long speculated that the inevitable nitrogen limitation of forest production would reduce 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 response of forest production to elevated CO_{2 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 a coupled nitrogen cycle 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 Department of Energy's FACE experiment at ORNL 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.