Symbiotic N2 fixation of various legume species along an altitudinal gradient in the Swiss Alps

Symbiotic N2 fixation may be an important source of N for legumes in alpine ecosystems, though, this has hardly been investigated. Symbiotic N2 fixation in nine legume species in permanent grassland over an altitudinal gradient (from 900 up to 2600 m a.s.l.) was investigated in the Swiss Alps on strictly siliceous soils. To assess symbiotic N2 fixation, an enriched 15N isotope dilution method was established for low N input, permanent grasslands and was evaluated with the 15N natural abundance method. The non-N2-fixing reference species used in both methods differed significantly in their 15N atom%-excess. However, when several reference species were combined, the enriched 15N isotope dilution method was reliable and led to the conclusion that up to their altitudinal limit, legumes may acquire from 59% to more than 90% of their N through symbiotic N2 fixation depending on the species. These findings were confirmed by the 15N natural abundance method. Even at the legumes’ altitudinal limit all plants investigated showed apparently active nodules. Moreover, a clear host-microsymbiont specificity between plant and rhizobia was evident at high altitudes. This suggests that symbiotic N2 fixation is well adapted to the climatic and acidic soil conditions in the Alps and contributes, up to the altitudinal limit, a significant amount of N to the N nutrition of legumes.