Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems

Abstract

The biological fixation of atmospheric nitrogen (N) is a major pathway for available N entering ecosystems. In N-limited boreal forests, a significant amount of N2 is fixed by cyanobacteria living in association with mosses, contributing up to 50% to the total N input. In this review, we synthesize reports on the drivers of N2 fixation in feather moss-cyanobacteria associations to gain a deeper understanding of their role for ecosystem-N-cycling. Nitrogen fixation in moss-cyanobacteria associations is inhibited by N inputs and therefore, significant fixation occurs only in low N-deposition areas. While it has been shown that artificial N additions in the laboratory as well as in the field inhibit N2 fixation in moss-cyanobacteria associations, the type, as well as the amounts of N that enters the system, affect N2 fixation differently. Another major driver of N2 fixation is the moisture status of the cyanobacteria-hosting moss, wherein moist conditions promote N2 fixation. Mosses experience large fluctuations in their hydrological status, undergoing significant natural drying and rewetting cycles over the course of only a few hours, especially in summer, which likely compromises the N input to the system via N2 fixation. Perhaps the most central question, however, that remains unanswered is the fate of the fixed N2 in mosses. The cyanobacteria are likely to leak N, but whether this N is transferred to the soil and if so, at which rates and timescales, is unknown. Despite our increasing understanding of the drivers of N2 fixation, the role moss-cyanobacteria associations play in ecosystem-N-cycling remains unresolved. Further, the relationship mosses and cyanobacteria share is unknown to date and warrants further investigation.

Effects of nitrogen fertilization on diazotrophic activity of microorganisms associated with Sphagnum magellanicum

“…It is likely that N 2 fixation can match the increased N-demand of Sphagnum at higher P availability, as has been discussed for P addition experiments with feather mosses occurring in boreal forests (Rousk et al 2013a). P addition experiments generally did show a stimulating effect on Sphagnum growth Fritz et al 2012) and in P. schreberi on N 2 fixation (Rousk et al 2013a).…”

“…P addition experiments generally did show a stimulating effect on Sphagnum growth Fritz et al 2012) and in P. schreberi on N 2 fixation (Rousk et al 2013a). In accordance with these studies, the P-fertilized mosses in this study demonstrated an increase of both the C and N content upon N fertilization, which indicated that these mosses, in contrast to all others, could make use of the additional N-load for growth (Fritz et al 2012).…”

Symbiosis revisited: phosphorus and acid buffering stimulate N<sub>2</sub> fixation but not <i>Sphagnum</i> growth

“…A highly diverse microbial community, including Proteobacteria, Verrucomicrobia and Cyanobacteria, has been found to colonize peat mosses (Bragina et al, 2014), and many of these microorganisms have the capacity to fix N 2 (Bragina et al, 2013;Kox et al, 2016). Also, in other bryophytes, like Hylocomiaceae (feather mosses), such a symbiotic relationship can be found with N 2 fixing cyanobacteria, supplying up to 50 % of the total N input in boreal forests (Rousk et al, 2013). These phototrophic diazotrophs provide N to their host in exchange for C compounds (Bay et al, 2013;Leppänen et al, 2013), a process that we refer to as a direct mutualism, with reference to the direct transfer of chemicals between host and symbiont (Ho and Bodelier, 2015).…”

Can mosses serve as model organisms for forest research?

“…The role of mosses for nitrogen fixation in boreal forests is a research area of high interest, as a substantial amount of nitrogen fixation can rely on feather moss-cyanobacteria associations (DeLuca et al 2002;Rousk et al 2013).…”