Nitrogen Uptake - Synthesis Research

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New Analysis of Nitrogen Uptake by Forests in Elevated CO2 Challenges Assumptions in Biogeochemical Models

Nitrogen availability commonly limits forest productivity and may constrain increases in net primary production (NPP) that are otherwise expected to result from rising concentrations of atmospheric CO2. To sustain the increased rates of forest NPP in elevated CO2, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary. We analyzed the response of N uptake and N-use efficiency to elevated CO2 (~550 ppm) in four free-air CO2 enrichment (FACE) experiments in forest stands. Uptake of N increased under elevated CO2 at the three of the FACE sites, yet fertilization studies at two of them showed that tree growth and forest NPP were strongly limited by N availability. By contrast, N-use efficiency increased under elevated CO2 at the most productive site, where fertilization studies showed that N was not limiting to tree growth.

Our analysis shows that the larger quantities of C entering the belowground 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.

Three of the experiments providing data for this synthesis are supported by the U.S. Department of Energy Office of Science, Biological and Environmental Research (BER). The synthesis activity was supported by BER, the National Science Foundation Research Coordination Network on Terrestrial Ecosystem Response to Atmospheric and Climatic Change, and Research Foundation–Flanders Scientific Research Network on Impact of Global Change on Terrestrial Ecosystems. ORNL is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725.


  • This research is presented in: Finzi AC, Norby RJ, Calfapietra C, Gallet-Budynek A, Gielen B, Holmes WE, Hoosbeek MR, Iversen CM, Jackson RB, Kubiske ME, Ledford J, Liberloo M, Oren R, Polle A, Pritchard S, Zak DR, Schlesinger WH, Ceulemans R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences 104: 14014-14019.

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