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

Abstract

Moss‐associated cyanobacteria nitrogen (N2‐) 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 N2‐fixation are usually considered linearly correlated. Yet, recent evidence has shown that cyanobacterial biomass and N2‐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 N2‐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 N2‐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 N2‐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 N2‐fixation. Vanadium was a negative driver of N2‐fixation, suggesting the use of alternative nitrogenases by cyanobacteria. Both cyanobacterial biomass and N2‐fixation were strongly influenced by the moss species and were negatively correlated with moss C:N stoichiometry, highlighting the role of N2‐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 N2‐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.

Nitrogen fixation and nifH gene diversity in cyanobacteria living on feather mosses in a subalpine forest of Mt. Fuji

“…splendens and P . schreberi , consistent with findings in previous studies (DeLuca et al 2007 ; Stuart et al 2021a ; Liu and Rousk 2022 ; Renaudin et al 2022 ). While positive correlations between nitrogen fixation and relative abundances of bacteria other than cyanobacteria have been reported for some moss species (Holland-Moritz et al 2021 ), the strong correlation between the acetylene reduction rate and the rate of cyanobacterial colonization in this study suggests that cyanobacteria contribute to nitrogen fixation on moss carpets in subalpine forest of Mt.…”

“…Fallen logs, as coarse woody debris, can be a source of nutrients such as phosphorus as they decompose (Laiho and Prescott 2004 ; Brais et al 2006 ). Cases of phosphorus promoting cyanobacterial biomass (Renaudin et al 2022 ) and acetylene reduction (Zackrisson et al 2004 ) have been reported. This result may be due to the greater P supply from decayed fallen logs than from the ground surface.…”

“…Factors that cause variation in nitrogen fixation by cyanobacteria associated with feather mosses have been investigated in manipulative experiments. In addition, assessing the role of environmental factors in moss-associated nitrogen fixation by observational studies using natural gradients of nutrients or seasonal variations can validate the results of manipulation experiments in a real environment (Renaudin et al 2022 ). However, few observational studies have addressed the environmental factors controlling spatial variation in nitrogen fixation within a single forest stand.…”

Boreal moss-microbe interactions are revealed through metagenome assembly of novel bacterial species

“…The green section is thought to be photosynthetically active and is often in focus when studying bacterial communities associated with mosses. The green section is also associated with nitrogen fixing cyanobacteria (Renaudin et al, 2022a). Meanwhile, microbial communities associated with brown sections are much less studied, even though they are hypothesized to be an important source of nutrients for certain bacterial taxa (Shen et al, 2018).…”

Revealing the transfer pathways of cyanobacterial-fixed N into the boreal forest through the feather-moss microbiome

“…Cyanobacterial N 2 fixation within feather-moss communities is a primary source of new nitrogen (N) in boreal forest ecosystems (DeLuca et al, 2002a, DeLuca et al, 2002bZackrisson et al, 2004;DeLuca et al, 2008;Renaudin et al, 2022a, Renaudin et al, 2022b; however, understanding of how this N source influences key ecosystem processes (e.g. productivity, biodiversity and carbon [C] cycling) remains limited with the precise cellular mechanisms yet to be defined.…”