title: Does nitrogen nutrition restrict the CO2response of fertile grassland lacking legumes?
abstract: The extent of the response of plant growth to atmospheric CO2enrichment depends on the availability of resources other than CO2. An important growth-limiting resource under field conditions is nitrogen (N). N may, therefore, influence the CO2response of plants. The effect of elevated CO2(60 Pa) partial pressure (pCO2) on the N nutrition of field-grown Lolium perenne swards, cultivated alone or in association with Trifolium repens, was investigated using free air carbon dioxide enrichment (FACE) technology over 3 years. The established grassland ecosystems were treated with two N fertilization levels and were defoliated at two frequencies. Under elevated pCO2, the above-ground plant material of the L. perenne monoculture showed a consistent and significant decline in N concentration which, in general, led to a lower total annual N yield. Despite the decline in the critical N concentration (minimum N concentration required for non-N-limited biomass production) under elevated pCO2, the index of N nutrition (ratio of actual N concentration and critical N concentration) was lower under elevated pCO2than under ambient pCO2in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO2was evoked indirectly by a reduction of plant-available N. For L. perenne grown in association with T. repens and exposed to elevated pCO2, there was an increase in the contribution of symbiotically fixed N to the total N yield of the grass. This can be explained by an increased apparent transfer of N from the associated N2-fixing legume species to the non-fixing grass. The total annual N yield of the mixed grass/legume swards increased under elevated pCO2. All the additional N yielded was due to symbiotically fixed N. Through the presence of an N2-fixing plant species more symbiotically fixed N was introduced into the system and consequently helped to overcome N limitation under elevated pCO2.
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: S
authors.initials: S
authors.lastname: Zanetti
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: U A
authors.initials: UA
authors.lastname: Hartwig
authors.affiliation: Department of Agronomy and Range Science, University of California, Davis, CA 95616, USA, , , , , , US.
authors.forename: C
authors.initials: C
authors.lastname: van Kessel
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: A
authors.initials: A
authors.lastname: Lüscher
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: T
authors.initials: T
authors.lastname: Hebeisen
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: M
authors.initials: M
authors.lastname: Frehner
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: B U
authors.initials: BU
authors.lastname: Fischer
authors.affiliation: Department of Applied Science, Brookhaven National Laboratory, Upton, Long Island, NY 11973, USA, , , , , , IS.
authors.forename: G R
authors.initials: GR
authors.lastname: Hendrey
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: H
authors.initials: H
authors.lastname: Blum
authors.affiliation: Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: ueli.hartwig@ipw.agrl.ethz.ch, , , , , , CH.
authors.forename: J
authors.initials: J
authors.lastname: Nösberger
keywords: Elevated partial pressure of carbon dioxide; Key words Nitrogen nutrition; Lolium perenne; Transfer of symbiotically fixed nitrogen; Trifolium repens