ARCUS | Arctic Research Consortium of the United States

7th Annual ARCUS Award for Arctic Research Excellence


Submitted by   Marissa Weiss
Authors   Marissa Weiss, S. E. Hobbie, and G. M. Gettel
Category   Life Science
Title   Contrasting Responses of Nitrogen Fixation in Arctic Lichens to Experimental and Observed Nitrogen and Phosphorus Availability
Affiliation   Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN, USA

Abstract

Here we investigate the influence of nitrogen (N) and phosphorus (P) on N fixation and abundance of two of the most common N fixing arctic lichens, Peltigera aphthosa and Peltigera polydactyla in two different common moist upland tundra types, acidic and non-acidic tundra, at Toolik Lake, Alaska. Acidic tundra has higher N availability and lower P availability than non-acidic tundra. Acidic and non-acidic tundra also differ in pH, plant community composition, and plant biomass, all factors which may influence lichen N-fixation and/or abundance. We measured the abundance of the lichens in control (no fertilization), N- and P-fertilized plots, and N-fixation using the acetylene reduction assay (ARA) method on lichens from control and P-fertilized plots in both tundra types. Lichens on N-treated plots were too scarce to include in our N fixation estimates. We compared the rates of N-fixation and lichen abundance among the fertilization treatments and between acidic and non-acidic tundra types to assess the influence of nutrient availability on lichen abundance and N-fixation. Lichen abundance was much lower in plots fertilized with N than in control and P-fertilized plots, while per-biomass N-fixation rates were higher in P-fertilized plots than in control plots. Therefore, N-fixation appears to increase with an experimental increase in P availability relative to N availability. We did not find significant differences in per-biomass rates of N-fixation between acidic and non-acidic tundra, but both lichen species are more abundant on acidic tundra than on non-acidic tundra. Thus, despite per-biomass stimulation of N-fixation by experimental P addition, variation in ambient P availability at the acidic and non-acidic tundra sites is not an important factor influencing either N-fixation or abundance of N-fixing lichens, since lichens had similar rates of fixation and lower abundance at the more P-rich non-acidic site. Similarly, despite a reduction in abundance of lichens in response to N fertilization, lichens were more abundant at the more N-rich site suggesting that N availability per se is not the primary control of the abundance of N fixing lichens in moist upland tundra.