title: Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment.
abstract: 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 (CO(2)). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N(2 )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 N(2) fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO(2 )than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO(2) at current ambient and elevated (560 micromol mol(-1)) concentrations using free-air CO(2) 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 below ground 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 CO(2) 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 CO(2). Non-fixer leaf N concentrations declined similarly in response to elevated CO(2) 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, CO(2) enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.
authors.affiliation: Department of Forest Resources, University of Minnesota, 115 Green Hall, 1530 Cleveland Ave. N., St. Paul, MN 55108, USA. leetd@uwec.edu
authors.forename: Tali D
authors.initials: TD
authors.lastname: Lee
authors.forename: Peter B
authors.initials: PB
authors.lastname: Reich
authors.forename: Mark G
authors.initials: MG
authors.lastname: Tjoelker
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D017753:Ecosystem; D006801:Humans; D031297:Lupinus / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D010788:Photosynthesis; D018515:Plant Leaves / Q000378:metabolism
chemical_list: D002245:Carbon Dioxide; D009584:Nitrogen