Deacute;termination de l'identiteacute; isotopique de l'azote fixeacute; par leFrankiaassocieacute; au genreAlnus

摘要

To use the15N natural abundance method to evaluate the symbiotic nitrogen fixation by actinorhizal trees, it is necessary to determine the isotopic identity of assimilated nitrogen from two sources: the soil and the air. This study reports an isotopic value of fixed nitrogen by two alder species (Alnus incana(L.) Moench andAlnus glutinosa(L.) Gaertn. growing on nitrogen-free medium in greenhouse experiments. The delta;15N value of the aerial parts was minus;2. This value was stable with time and did not depend on theFrankiastrains used. This value could be used to estimate the nitrogen fixation in the natural ecosystem. Other parameters such as the mobilization of nitrogen reserves and the choice of the reference plant must be investigated to apply this method. The nodules of these two alder species were enriched in15N relative to the rest of the plant but there was no relationship between symbiotic effectiveness ofFrankiastrains and15N enrichment of nodules. On the other hand, for naturally growing trees, an enrichment in15N was found primarily in the vesicles of nodules that are the sites of nitrogen fixation.