Experimental assessment of tree canopy and leaf litter controls on the microbiome and nitrogen fixation rates of two boreal mosses

Nitrogen (N₂)-fixing moss microbial communities play key roles in nitrogen cycling of boreal forests. Forest type and leaf litter inputs regulate moss abundance, but how they control moss microbiomes and N₂-fixation remains understudied. We examined the impacts of forest type and broadleaf litter on microbial community composition and N₂-fixation rates of Hylocomium splendens and Pleurozium schreberi. We conducted a moss transplant and leaf litter manipulation experiment at three sites with paired paper birch (Betula neoalaskana) and black spruce (Picea mariana) stands in Alaska. We characterized bacterial communities using marker gene sequencing, determined N₂-fixation rates using stable isotopes (¹⁵N₂) and measured environmental covariates. Mosses native to and transplanted into spruce stands supported generally higher N₂-fixation and distinct microbial communities compared to similar treatments in birch stands. High leaf litter inputs shifted microbial community composition for both moss species and reduced N₂-fixation rates for H. splendens, which had the highest rates. N₂-fixation was positively associated with several bacterial taxa, including cyanobacteria. The moss microbiome and environmental conditions controlled N₂-fixation at the stand and transplant scales. Predicted shifts from spruce- to deciduous-dominated stands will interact with the relative abundances of mosses supporting different microbiomes and N₂-fixation rates, which could affect stand-level N inputs.