The role of feather mosses in N cycling in boreal forest ecosystems

Background/Question/Methods

Boreal forests are generally considered to be nitrogen (N) limited and exhibit little accumulation of inorganic N. Dominant feather mosses account for much of the annual productivity in the boreal biome, and importantly, are colonized by N₂-fixing cyanobacteria which provide much of the annual N input to pristine landscapes of northern Fennoscandia. Little is currently known about the effects of anthropogenic N inputs on the reduction of atmospheric N₂ in mosses and the succeeding effects on soil nutrient concentrations and soil microbial community composition. Here, we aimed to assess the role of feather mosses in ecosystem N cycling in boreal forests by analysing the interactive effects of N inputs; soil microbial community structure and N₂ fixation in carpets of the ubiquitous feather moss Pleurozium schreberi (Brid.). To accomplish this, we collected moss and soil samples along a road corridor gradient (various distances to busy and remote roads) in northern Sweden where background deposition is low (1 - 2 kg N ha^-1 y^-1) and therefore, N₂ fixation rates high. Furthermore, resin lysimeters were installed along the roads to collect throughfall N over the period of one year.

Results/Conclusions

N₂ fixation rates in feather moss carpets were highly affected by road derived N inputs: N₂ fixation was lower along busy roads than along remote roads and increased with increasing distance to busy roads, reaching N₂ fixation rates similar to remote roads at 100 m distance to the busy roads. Microbial communities (estimated with PLFAs) and resin sorbed nutrients (TDC, TDN, NH₄⁺, NO₃^-) followed a weak pattern concerning road type and distance, whereas soil nutrients (e.g. TC, DON, DIN) showed no difference between road types and distance to the roads, leaving N₂ fixation in feather mosses as a sensitive indicator for increased N inputs. Inhibition of N₂ fixation by heavy metals along the busy roads could be ruled out, as N is more N₂ fixation inhibiting than heavy metals. Our results suggest that the reduction of atmospheric N₂ by cyanobacteria living epiphytically on feather mosses represents a major source of N in boreal forests and is highly sensitive to nutrient inputs which could lead to significant alterations in the N cycle in these ecosystems. Furthermore, studies conducted close to urban centres or roads should consider the effects of nutrients on plants, especially mosses, and the possible consequences for nutrient cycling.