Legumes, with the ability to fix atmospheric nitrogen (N), may help alleviate the N limitations thought to constrain plant community response to elevated concentrations of atmospheric carbon dioxide (CO2). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N2 fixer, Lupinus perennis, on biomass accumulation and plant N concentrations of nine-species plots of differing plant composition; (2) leaf-level physiology of co-occurring non-fixing species (Achillea millefolium, Agropyron repens, Koeleria cristata) in these assemblages with and without Lupinus; (3) the effects of elevated CO 2 on Lupinus growth and symbiotic N2 fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO2 than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO2 at current ambient and elevated (560 μmol mol-1) concentrations using free-air CO2 enrichment. Nine-species plots with Lupinus had 32% higher whole plot plant N concentrations and 26% higher total plant N pools than those without Lupinus, based on both above and belowground measurements. Co-occurring non-fixer leaf N concentrations increased 22% and mass-based net photosynthetic rates increased 41% in plots containing Lupinus compared to those without. With CO2 enrichment, Lupinus monocultures accumulated 32% more biomass and increased the proportion of N derived from fixation from 44% to 57%. In nine-species assemblages, Lupinus N derived from fixation increased similarly from 43% to 54%. Although Lupinus presence enhanced photosynthetic rates and leaf N concentrations of co-occurring non-fixers, and increased overall plant N pools, Lupinus presence did not facilitate stronger photosynthetic responses of non-fixing species or larger growth responses of overall plant communities to elevated CO2. Non-fixer leaf N concentrations declined similarly in response to elevated CO2 with and without Lupinus present and the relationship between net photosynthesis and leaf N was not affected by Lupinus presence. Regardless of the presence or absence of Lupinus, CO2 enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.
Bibliographical noteFunding Information:
Acknowledgements We thank M.P. Russelle for assistance with 15N calculations and interpretation and A. Karpinski, J. Goth, W. Bengston, D. Hoover, and numerous interns for assistance with gas exchange measurements and sample preparation. We thank G. Hendrey, K. Lewin, K. Wrage, and D. Bahauddin for designing, constructing and operating the FACE system. Two anonymous reviewers provided very helpful comments. We acknowledge research support funding primarily from the US Department of Energy and additionally from the NSF-supported Cedar Creek Natural History Area Long-Term Ecological Research site. NSF Pre-doctoral and University of Minnesota Graduate School Doctoral Dissertation Fellowship Awards to T.D.L. also supported this study.
- Elevated CO
- N fixation
- Photosynthetic acclimation
- Species interactions