Contrasting responses of nitrogen-fixation in arctic lichens to experimental and ambient nitrogen and phosphorus availability

We investigated the influence of nitrogen (N) and phosphorus (P) on N ₂-fixation and abundance of two of the most common N ₂-fixing arctic lichens, Peltigera aphthosa and P. polydactyla, in two common moist upland tundra types, acidic and non-acidic tundra, at Toolik Lake, Alaska. Acidic tundra has higher N and lower P availability than non-acidic tundra. We measured the abundance of the lichens in control (no fertilization), N- and P-fertilized plots, and N₂-fixation using the acetylene reduction assay method on lichens from control and P-fertilized plots from both tundra types. Lichens on N-treated plots were too scarce to include in our N₂-fixation estimates. Lichen abundance was lower in plots fertilized with N than in control and P-fertilized plots, while per-biomass N₂-fixation rates were higher in P-fertilized plots than in control plots. Per-biomass rates of N₂-fixation did not differ between acidic and non-acidic tundra, but both lichen species are more abundant on acidic tundra. Thus, despite per-biomass stimulation of N₂-fixation by experimental P addition and reduction in lichen abundance with N fertilization, Peltigera contributes more N to the acidic tundra site, indicating that soil N and P availability are not the primary controls of N₂-fixation and abundance of these lichens.