New insights into the drivers of moss-associated nitrogen fixation and cyanobacterial biomass in the eastern Canadian boreal forest

Moss-associated cyanobacteria nitrogen (N-2-) fixation can contribute to support moss growth and constitutes a major source of new N in boreal forest ecosystems. The biomass of moss-colonizing cyanobacteria and their N-2-fixation are usually considered linearly correlated. Yet, recent evidence has shown that cyanobacterial biomass and N-2-fixation can be decoupled, suggesting that they are not necessary affected by the same environmental and ecological drivers. Climate and nutrients were reported as affecting moss-associated N-2-fixation, with equivocal results, whereas drivers of moss cyanobacterial biomass remain unclear. In addition, these drivers are often determined through manipulative experiments (e.g. fertilization and incubation) and remain to be validated with complementary observational studies to help us better understand future impacts of global change on the moss-cyanobacteria symbiosis. We hypothesized that moss-associated cyanobacterial biomass is controlled in situ by factors affecting bacterial growth, whereas N-2-fixation is controlled by factors affecting enzymatic reactions. Using random forests, Spearman correlations and linear mixed-effects models, we determined the main drivers of cyanobacterial biomass and N-2-fixation of two feather moss species, which were collected over 3 years along a 1000-km latitudinal transect in the eastern Canadian boreal forest. We found that temperature, precipitation and phosphorus were the main positive drivers of moss cyanobacterial biomass and that temperature and molybdenum were the main positive drivers of N-2-fixation. Vanadium was a negative driver of N-2-fixation, suggesting the use of alternative nitrogenases by cyanobacteria. Both cyanobacterial biomass and N-2-fixation were strongly influenced by the moss species and were negatively correlated with moss C:N stoichiometry, highlighting the role of N-2-fixation in moss N enrichment. Synthesis. We identified for the first time some environmental drivers of moss-associated cyanobacterial biomass and showed that they contrast with the drivers of N-2-fixation, which should be considered in further research and confirmed in other experimental settings. This is an important advance in our knowledge of moss-cyanobacteria associations, which would greatly help in better predicting the impacts of global change on this symbiosis and on nitrogen inputs in boreal forest ecosystems.

Automated summary

Moss-associated cyanobacteria nitrogen fixation can contribute to moss growth, but factors affecting cyanobacterial biomass and nitrogen fixation may not be the same. Climate, nutrients, temperature, and precipitation can impact these processes, but further research is needed.