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Ecosystem response to elevated CO2 levels limited by nitrogen-induced plant species shift


Terrestrial ecosystems gain carbon through photosynthesis and lose it mostly in the form of carbon dioxide (CO2). The extent to which the biosphere can act as a buffer against rising atmospheric CO2 concentration in global climate change projections remains uncertain at the present stage1,2,3,4. Biogeochemical theory predicts that soil nitrogen (N) scarcity may limit natural ecosystem response to elevated CO2 concentration, diminishing the CO2-fertilization effect on terrestrial plant productivity in unmanaged ecosystems3,4,5,6,7. Recent models have incorporated such carbon–nitrogen interactions and suggest that anthropogenic N sources could help sustain the future CO2-fertilization effect8,9. However, conclusive demonstration that added N enhances plant productivity in response to CO2-fertilization in natural ecosystems remains elusive. Here we manipulated atmospheric CO2 concentration and soil N availability in a herbaceous brackish wetland where plant community composition is dominated by a C3 sedge and C4 grasses, and is capable of responding rapidly to environmental change10. We found that N addition enhanced the CO2-stimulation of plant productivity in the first year of a multi-year experiment, indicating N-limitation of the CO2 response. But we also found that N addition strongly promotes the encroachment of C4 plant species that respond less strongly to elevated CO2 concentrations. Overall, we found that the observed shift in the plant community composition ultimately suppresses the CO2-stimulation of plant productivity by the third and fourth years. Although extensive research has shown that global change factors such as elevated CO2 concentrations and N pollution affect plant species differently11,12,13 and that they may drive plant community changes14,15,16,17, we demonstrate that plant community shifts can act as a feedback effect that alters the whole ecosystem response to elevated CO2 concentrations. Moreover, we suggest that trade-offs between the abilities of plant taxa to respond positively to different perturbations may constrain natural ecosystem response to global change.

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Figure 1: The magnitude of the CO 2 effect.
Figure 2: Porewater ammonium concentrations over four growing seasons averaged over three depths.
Figure 3: The trajectories of plant biomass according to contributions from C 3 and C 4 plants in each treatment group.

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We acknowledge the support of D. Cahoon, our primary US Geological Survey collaborator, who co-developed the experimental design of this study. We thank J. Duls, J. Keller, M. Sigrist, G. Peresta, B. Drake, E. Sage, A. Martin, D. McKinley, N. Mudd and K. White for the construction and maintenance of the field site at the Smithsonian Climate Change Facility. We appreciate comments from S. Chapman, A. Classen, J. Hines, B. Hungate, T. Mozdzer, A. Sutton-Grier and D. Whigham. The field study was supported by the USGS Global Change Research Program (cooperative agreement 06ERAG0011), the US Department of Energy (grant DE-FG02-97ER62458), the US Department of Energy’s Office of Science (BER) through the Coastal Center of the National Institute of Climate Change Research at Tulane University, and the Smithsonian Institution.

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Both J.A.L. and J.P.M. designed the experiment, interpreted the data and wrote the paper.

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Correspondence to J. Adam Langley.

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The authors declare no competing financial interests.

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Langley, J., Megonigal, J. Ecosystem response to elevated CO2 levels limited by nitrogen-induced plant species shift. Nature 466, 96–99 (2010).

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