Nitrogen (N) is the primary growth-limiting nutrient in circumboreal forests and many organisms developed symbiotic interactions with N-fixing bacteria to fulfill their needs. N-fixation associated with moss-dwelling cyanobacteria plays key roles in N and carbon cycling of boreal forests. Mosses cover the forest floor in many boreal forests, their abundance controlled by forest type, age, and the presence or absence of broadleaf leaf litter. However, the influence of these factors on moss microbiomes and N-fixation, and the generality of these patterns is unknown. Here, we 1) examined impacts of forest type and broadleaf litter on moss-associated microbial communities and N-fixation in Alaska and 2) quantified N-fixation associated with seven moss species along a post-fire successional gradient in eastern Canada. We conducted a field-based moss transplant and leaf litter manipulation experiment in 58 years old paper birch (Betula neoalaskana) and black spruce (Picea mariana) forests. We characterized bacterial communities using marker gene sequencing and determined N-fixation using stable isotopes (15N2) for the mosses Hylocomium splendens and Pleurozium schreberi. We measured N-fixation rates of Dicranum polysetum, P. schreberi, H. splendens and four Sphagnum species with the acetylene reduction method in eight black spruce dominated sites (64-724 years old) in eastern Canada.
Results/Conclusions
Alaskan spruce forests supported higher N-fixation rates and distinct moss microbial communities compared to birch forests. High deciduous leaf litter inputs shifted the composition of moss microbial communities and reduced N-fixation rates for H. splendens. N-fixation rates were positively associated with several bacterial phylotypes, including cyanobacteria. In spruce forests of eastern Canada, stand-scale N-fixation rates increased with forest age (from 0.0004 to 0.133 kg N ha-1 year-1), but were generally low compared to 58 years-old Alaskan spruce forests (0.796 kg N ha-1 year-1, note that methods differed). Increased Sphagnum abundance due to paludification and higher N-fixation frequency, rather than higher moss-scale rates, seemed to be driving the observed increase with forest age. In both Alaska and eastern Canada, some of the mechanisms controlling N-fixation differed from those identified in more heavily studied Fennoscandia. In Alaska, predicted shifts from spruce- to deciduous-dominated stands will interact with relative abundances of mosses supporting different microbiomes and N2-fixation rates, which could affect stand-level N inputs. In addition, our work provides one of the first records of moss-associated N2-fixation in eastern Canada and suggests that geographical variation in ecosystem and community assembly processes have the potential to affect moss contributions to N and carbon cycling.