摘要：Available soil N is a key factor limiting plant productivity in most low arctic terrestrial ecosystems. Atmospheric N 2 -fixation by cyanobacteria is often the primary source of newly fixed N in these nutrient-poor environments. We examined temporal and spatial variation in N 2 -fixation by the principal cyanobacterial associations (biological soil crusts, Sphagnum spp. associations, and Stereocaulon paschale ) in a wide range of ecosystems within a Canadian low arctic tundra landscape, and estimated N input via N 2 -fixation over the growing season using a microclimatically driven model. Moisture and temperature were the main environmental factors influencing N 2 -fixation. In general, N 2 -fixation rates were largest at the height of the growing season, although each N 2 -fixing association had distinct seasonal patterns due to ecosystem differences in microclimatic conditions. Ecosystem types differed strongly in N 2 -fixation rates with the highest N input (10.89 kg ha −1 yr −1 ) occurring in low-lying Wet Sedge Meadow and the lowest N input (0.73 kg ha −1 yr −1 ) in Xerophytic Herb Tundra on upper esker slopes. Total growing season (3 June–13 September) N 2 -fixation input from measured components across a carefully mapped landscape study area (26.7 km 2 ) was estimated at 0.68 kg ha −1 yr −1 , which is approximately twice the estimated average N input via wet deposition. Although biological N 2 -fixation input rates were small compared to internal soil N cycling rates, our data suggest that cyanobacterial associations may play an important role in determining patterns of plant productivity across low arctic tundra landscapes.
