Nitrogen Fixation in Surface Soils and Vegetation in an Arctic Tundra Watershed: A Key Source of Atmospheric Nitrogen

Atmospheric nitrogen (N) fixation is a key N input to arctic ecosystems, but relatively few estimates of annual N-fixation rates are available. We measured N-fixation of plant-soil cores by the acetylene reduction technique at different topographic positions in an upland tundra watershed, Imnavait Creek, through two growing seasons in order to evaluate spatial and temporal variation in N-fixation. We also examined the effects of light and temperature on N-fixation to estimate annual N-fixation rates of surface soil in this watershed using field meteorological data. Surface soil at Imnavait Creek had significant acetylene reduction potential throughout the watershed (generally 6 to 10 μmol C2H4 m−2 h−1), indicating that N-fixing organisms were present everywhere. Although acetylene reduction potential was roughly constant through the growing season, moisture, temperature and light intensity strongly affected the measured acetylene reduction rates in laboratory incubations. In addition, the relatively few samples that included the lichen, Peltigera apthosa, had significantly greater acetylene reduction potential, although the overall influence of Peltigera on N-fixation in this watershed seems to be small. The N input via N-fixation at Imnavait Creek was estimated at 80 to 131 mg N m−2 yr−1, indicating that N-fixation contributed 85 to 90% of total watershed N inputs.