The impact of simulated chronic nitrogen deposition on the biomass and N 2 -fixation activity of two boreal feather moss–cyanobacteria associations

Bryophytes achieve substantial biomass and play several key functional roles in boreal forests that can influence how carbon (C) and nitrogen (N) cycling respond to atmospheric deposition of reactive nitrogen (N r ). They associate with cyanobacteria that fix atmospheric N 2 , and downregulation of this process may offset anthropogenic N r inputs to boreal systems. Bryophytes also promote soil C accumulation by thermally insulating soils, and changes in their biomass influence soil C dynamics. Using a unique large-scale (0.1 ha forested plots), long-term experiment (16 years) in northern Sweden where we simulated anthropogenic N r deposition, we measured the biomass and N 2 -fixation response of two bryophyte species, the feather mosses Hylocomium splendens and Pleurozium schreberi . Our data show that the biomass declined for both species; however, N 2 -fixation rates per unit mass and per unit area declined only for H. splendens . The low and high treatments resulted in a 29% and 54% reduction in total feather moss biomass, and a 58% and 97% reduction in total N 2 -fixation rate per unit area, respectively. These results help to quantify the sensitivity of feather moss biomass and N 2 fixation to chronic N r deposition, which is relevant for modelling ecosystem C and N balances in boreal ecosystems.