Das Projekt "Die Bedeutung von Huelsenfruechten bzw. symbiotischer Stickstofffixierung fuer Graslandoekosysteme entlang eines Hoehengefaelles in den Alpen" wird vom Umweltbundesamt gefördert und von Eidgenössische Technische Hochschule Zürich, Institut für Pflanzenwissenschaften durchgeführt. Legumes are abundant in most grassland communities in temperate climates, even at high altitudes. The endosymbiosis with specific soil bacteria from the family Rhizobiaceae enables legumes to be independent from the often growth limiting mineral nitrogen sources. Trough N-transfer, legumes contribute indirectly to the whole ecosystem nitrogen economy. Nevertheless, the relative ecological significance of legume species for grassland communities at various altitudes in the Alps is unknown; it is also not clear whether legumes at high altitudes fix nitrogen. Performance of symbiotic nitrogen fixation is influenced by the eco-physiological requirements of legumes, rhizobia and those of the symbiotic nitrogen fixation. That the significance of legumes is not constant in grassland ecosystems is evident from results of our free air CO2-enrichment experiment (Zanetti et al., 1995; Hartwig et al., 1996) and from experiments with low temperatures (Nesheim and Boller, 1991). Specific climatic factors of high altitudes will certainly affect both plant growth rate and mineral nitrogen availability. Comparisons of plant nitrogen concentrations along altitudinal gradients showed an increased nitrogen concentration with increased altitude (Koerner, 1989). This would suggest that at high altitudes mineral nitrogen availability is sufficient to support plant growth. However, this fact does not answer the question about the origin of this nitrogen and thus about the significance of legumes and symbiotic nitrogen fixation for nitrogen nutrition of grassland ecosystems at high altitudes. Thus, the aim of this project is to 1) qualify and quantify the ecological significance of legumes and the symbiotic nitrogen fixation for grassland ecosystems along an altitudinal gradient in the Alps and to 2) deepen the understanding of mechanisms that influence the relationships between legume plants and the rest of grassland ecosystems. To answer these questions, a detailed field study is planned in the Swiss Alps on the east edge of the 'Aarmassiv' and thus on strictly siliceous substrate. The objectives will be to measure symbiotic nitrogen fixation (15N-isotope dilution technique) and competitive behaviour of plant species along an attitudinal gradient. Selected legume species (red clover, Trifolfum pratense s.l. L. or birdsfoot trefoil, Lotus corniculatus s.l. L.) will be investigated primarily. Trough this strategy we expect a contribution to the understanding of the ecological significance of legumes and the symbiotic N2 fixation, respectively for plant communities, in particular at various environmental conditions along an attitudinal gradient in the Alps. Such data will help to explain biodiversity in grassland ecosystems. Leading Questions: What is the relative significancPflanzenphysiol..symbiotic nitrogen fixation for grassland ecosystems along on altitudinal gradient in the Alps?