“Hot spots” in high-latitude moss-associated N fixation: What drives locally high fixation rates?

Moss-associated nitrogen (N) fixation provides a substantial but heterogeneous input of new N to nutrient limited ecosystems at high latitudes. The presence of “hot spots”, defined as a rate of N fixation greater than three standard errors over the mean rate, can further increase the difficulty of scaling N inputs to plant communities or ecosystems. We used ¹⁵N₂ incubations to quantify the fixation rates associated with 34 moss species from 24 sites ranging from 60 to 68 degrees N in Alaska, USA. The total moss-associated fixation rates ranged from 0.08 to 4.4 kg N ha^-1yr^-1, with an average of 1.1 kg N ha^-1yr^-1, based on abundance-weighted averages of all mosses summed for each site. Five of the 24 sampled sites were hot spots of N fixation. We hypothesized that host moss diversity would be correlated with higher N fixation rates, since different mosses often have distinct microbial assemblages and higher microbial diversity has been linked with higher N fixation rates in other ecosystems. However, we found no significant correlation between either moss taxonomic richness or Simpson’s D and N fixation rates (p=0.102, R²=0.01 and p=0.522, R²=0.02, respectively). What we found instead was that certain high-fixing species, most importantly Tomentypnum nitens, were present in almost all hot spots. The relevance of moss taxonomic identity in driving N fixation rates was repeatedly observed in our survey, where both machine learning and mixed model approaches found that moss family was a significant predictor of associated fixation rates across ecosystems in Alaska. Taken together, these results indicate the importance of moss identity in driving hot spots and illustrate that host taxonomy may be a useful tool in generating more accurate large-scale assessments of associated N inputs in these vulnerable and valuable ecosystems.