Nitrogen deposition homogenizes moss-microbiomes and associated nitrogen fixation but with host-specific responses

Moss microbiomes contribute significantly to terrestrial nitrogen (N) cycling through N fixation, especially in nutrient-poor environments. Yet, the relative roles of environment (abiotic factors) and host species (biotic factors) in shaping these microbiomes remain understudied even though this could reveal how host-specific moss-associated microbiomes are. To assess whether moss-associated microbiomes are shaped predominantly by the moss host or by the environment, we compared three common moss species ( Hylocomium splendens , Pleurozium schreberi , and Sphagnum sp.) from ecosystems that differ in atmospheric N deposition rates: a temperate forest with >15 kg N ha −1 yr −1 and a boreal forest with 2 kg N ha −1 yr −1 of deposition. We further tested if moss-associated microbiomes and N fixation rates become more similar between the boreal and temperate mosses upon N additions to the boreal mosses. Nitrogen fixation, measured as acetylene reduction, was overall higher in the boreal mosses than in the temperate mosses. Microbial community composition, assessed with 16S rRNA gene sequencing, showed that both environment and host species influence the moss microbiome, but that the environment exerts the stronger influence. Nitrogen additions significantly reduced N fixation rates and changed the composition of moss microbiomes substantially, causing the microbiomes of two of the three investigated mosses to become more similar to each other. Our findings show that the environment (i.e., forest type with different N deposition rates) exerts a strong influence on the moss microbiome and that N deposition favors certain moss-associated microorganisms but lowers N input via moss-associated N fixation considerably. IMPORTANCE Mosses host diverse microbiomes that contribute to terrestrial N cycling through processes such as nitrogen (N) fixation. Thus, these microbial communities play important roles in nutrient-poor environments where mosses often dominate. Here, N fixation is a key source of N input, making it important to understand what drives microbiome composition and associated N fixation activity. We ascertained the roles of the environment and moss-host species in shaping the moss-microbiomes and associated N fixation rates in two forest types with varying N deposition rates. We showed that forest type shapes moss-associated microbiomes and N fixation activity, while aligning N availability also aligned N fixation activity and changed the moss microbiomes with implications for the ecosystem functions they provide.