LAUSR

Biological nitrogen (N2) fixation performed by diazotrophs (N2 fixing bacteria) is thought to be one of the main sources of plant available N in pristine ecosystems like arctic tundra. However, direct evidence of a transfer of fixed N2 to non-diazotroph associated plants is lacking to date. Here, we present results from an in situ 15N–N2 labelling study in the High Arctic. Three dominant vegetation types (organic crust composed of free-living cyanobacteria, mosses, cotton grass) were subjected to acetylene reduction assays (ARA) performed regularly throughout the growing season, as well as 15N–N2 incubations. The 15N-label was followed into the dominant N2 fixer associations, soil, soil microbial biomass and non-diazotroph associated plants three days and three weeks after labelling. Mosses contributed most to habitat N2 fixation throughout the measuring campaigns, and N2 fixation activity was highest at the beginning of the growing season in all plots. Fixed 15N–N2 became quickly (within 3 days) available to non-diazotroph associated plants in all investigated vegetation types, proving that N2 fixation is an actual source of available N in pristine ecosystems.