title: Stimulation of Symbiotic N2 Fixation in Trifolium repens L. under Elevated Atmospheric pCO2 in a Grassland Ecosystem.
abstract: Symbiotic N2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO2). The effect of elevated pCO2 (60 Pa) on symbiotic N2 fixation (15N-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in T. repens under elevated pCO2. All additionally assimilated N was derived from symbiotic N2 fixation, not from the soil. In the mixtures exposed to elevated pCO2, an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m-2 a-1 in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland.
authors.forename: S.
authors.initials: S
authors.lastname: Zanetti
authors.forename: U. A.
authors.initials: UA
authors.lastname: Hartwig
authors.forename: A.
authors.initials: A
authors.lastname: Luscher
authors.forename: T.
authors.initials: T
authors.lastname: Hebeisen
authors.forename: M.
authors.initials: M
authors.lastname: Frehner
authors.forename: B. U.
authors.initials: BU
authors.lastname: Fischer
authors.forename: G. R.
authors.initials: GR
authors.lastname: Hendrey
authors.forename: H.
authors.initials: H
authors.lastname: Blum
authors.forename: J.
authors.initials: J
authors.lastname: Nosberger