Symbiotic Nitrogen Fixation by Legumes in Alpine Ecosystems: a Vegetation Experiment

Abstract

The natural nitrogen-15 abundance method does not always make it possible to calculate the rate of symbiotic nitrogen fixation by legumes and needs to be improved. Five legume species typical for the alpine belt of the Teberda Nature Reserve (Anthyllis vulneraria, Astragalus levieri, Hedysarum caucasicum, Oxytropis kubanensis, and Trifolium polyphyllum) have been grown from seeds under conditions of laboratory vegetation experiment. The results show that nodules on the roots of these plants are formed at early stages of their development; Trifolium polyphyllum does not form nodules either under high-mountain conditions or during growth in the laboratory. The natural 15N abundance in the leaves of legume plants in alpine ecosystems makes it possible to calculate the contribution of atmospheric N2 to nitrogen nutrition as early as the first year of their development, while the isotopic nitrogen composition of the roots does not allow this parameter to be determined. The calculation of atmospheric nitrogen fixation rate should take into account isotope fractionation between symbiotic bacteria (nodules) and the host plant; otherwise, the proportion of fixed nitrogen in plant nutrition may be underestimated.