Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?
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The extent of the response of plant growth to atmospheric CO2 enrichment 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 CO2 response 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 pCO2 than under ambient pCO2 in frequently defoliated L. perenne monocultures. Thus, we suggest that reduced N yield under elevated pCO2 was 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.
- Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?
Volume 112, Issue 1 , pp 17-25
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- Key words Nitrogen nutrition
- Elevated partial pressure of carbon dioxide
- Lolium perenne
- Trifolium repens
- Transfer of symbiotically fixed nitrogen
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- Author Affiliations
- A1. Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland fax: ++41-1-632-1153; e-mail: email@example.com, CH
- A2. Department of Agronomy and Range Science, University of California, Davis, CA 95616, USA, US
- A3. Department of Applied Science, Brookhaven National Laboratory, Upton, Long Island, NY 11973, USA, IS