Nitrogen Inputs by Associative Cyanobacteria across a Low Arctic Tundra Landscape

Abstract

Available soil N is a key factor limiting plant productivity in most low arctic terrestrial ecosystems. Atmospheric N2-fixation by cyanobacteria is often the primary source of newly fixed N in these nutrient-poor environments. We examined temporal and spatial variation in N2-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 N2-fixation over the growing season using a microclimatically driven model. Moisture and temperature were the main environmental factors influencing N2-fixation. In general, N2-fixation rates were largest at the height of the growing season, although each N2-fixing association had distinct seasonal patterns due to ecosystem differences in microclimatic conditions. Ecosystem types differed strongly in N2-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) N2-fixation input from measured components across a carefully mapped landscape study area (26.7 km2) was estimated at 0.68 kg ha−1 yr−1, which is approximately twice the estimated average N input via wet deposition. Although biological N2-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.