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BIOLOGICAL SCIENCES / ENVIRONMENTAL SCIENCES
Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2
aDepartment of Biology, Boston University, Boston, MA 02215; cEnvironmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831; dDepartment of Forest Environment and Resources, University of Tuscia, I-01100 Viterbo, Italy; eResearch Group of Plant and Vegetation Ecology, Department of Biology, University of Antwerp, B-2610 Wilrijk, Belgium; fSchool of Natural Resources and Environment and Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109; gDepartment of Environmental Sciences, Wageningen University, 6700AA-47 Wageningen, The Netherlands; hDepartment of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996; iSchool of the Environment and Earth Sciences, Duke University, Durham, NC 27708; jNorth Central Research Station, U.S. Department of Agriculture Forest Service, Rhinelander, WI 54501; kInstitute for Forest Botany, University of Göttingen, 37077 Göttingen, Germany; and lDepartment of Biology, College of Charleston, Charleston, SC 29424
Contributed by William H. Schlesinger, July 11, 2007 (received for review March 12, 2007)
Forest ecosystems are important sinks for rising concentrations of atmospheric CO2. In previous research, we showed that net primary production (NPP) increased by 23 ± 2% when four experimental forests were grown under atmospheric concentrations of CO2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO2. Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO2.
global change | net primary production
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0706518104/DC1.
bTo whom correspondence may be addressed. E-mail: afinzi{at}bu.edu or schlesingerw{at}ecostudies.org
© 2007 by The National Academy of Sciences of the USA
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