Document: TAL-3-34-50

Contrasting response of an N-fixing and a non-fixing forb to elevated CO₂ and nitrogen: Implications for grassland community response to rising CO₂.

LEE, T.D.*, M.G.TJOELKER and P.B.REICH

University of Minnesota, St. Paul, MN, 55108, U.S.A. ¹

Abstract:
It is generally accepted that nutrient availability influences the extent of plant response to elevated CO₂ concentrations. Legumes, with the ability to fix atmospheric N, may be able to alleviate the N-limitations often encountered and thought to limit plant response to elevated CO₂. In a series of field (FACE) and chamber experiments, we investigated the response of the N-fixing forb, Lupinus perennis, and the non-fixing forb, Achillea millefolium, to elevated CO₂ under varying levels of soil N. Plant growth, photosynthesis, and proportion of N derived from fixation were determined to test the following hypotheses: N-fixers show greater and more sustained responses to elevated CO₂, and the presence of N-fixers in plant communities will improve the N status and hence, the response of coexisting non-fixers to elevated CO₂. Both Lupinus and Achillea consistently showed a positive growth response to elevated CO₂. In controlled chambers, growth responses were four times greater for Lupinus plants than for Achillea plants, whereas in the field, the magnitude of their CO₂ responses were similar. Elevated CO₂ decreased tissue N in both species, however the extent of this decrease was greater in the non-fixing Achillea (-28%). ¹⁵N methods revealed that Lupinus derived a greater proportion of its tissue N from symbiotic N₂ fixation in elevated CO₂ (+37%). Lupinus biomass and photosynthesis showed no response to increasing soil N, while Achillea biomass and photosynthesis increased. In both field and chamber studies, only the positive CO₂ response of Achillea increased in magnitude with increasing soil N, suggesting the CO₂ response was N-limited. This was also demonstrated in multi-species field plots where Achillea had higher biomass and photosynthetic rates when grown in combination with N-fixers than when grown without, consistent with the effects of N-fixers on increasing soil N availability. Evidence suggests that the presence of N-fixers may enhance grassland community response to projected rising atmospheric CO₂ levels.

Keywords:

This abstract is being presented at: 4:00 PM in session:
Oral Session #30: Effects of Elevated Carbon Dioxide.