Nitrogen fixation in the High Arctic: a source of 'new' nitrogen?

Biological nitrogen (N-2) fixation performed by diazotrophs (N-2 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 N-2 to non-diazotroph associated plants is lacking to date. Here, we present results from an in situ N-15-N-2 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 N-15-N-2 incubations. The N-15-label was followed into the dominant N-2 fixer associations, soil, soil microbial biomass and non-diazotroph associated plants three days and three weeks after labelling. Mosses contributed most to habitat N-2 fixation throughout the measuring campaigns, and N-2 fixation activity was highest at the beginning of the growing season in all plots. Fixed N-15-N-2 became quickly (within 3 days) available to non-diazotroph associated plants in all investigated vegetation types, proving that N-2 fixation is an actual source of available N in pristine ecosystems.