| Nitrogen (N) limits the productivity in boreal forests. A major source of “new”
N for these forests is the fixation of atmospheric N2 preformed by cyanobacteria
living in association with mosses and lichens. Mosses are a dominant feature in
boreal forests, accounting for 60-90% of the groundcover in pristine boreal forests
and have been found to be colonized by several N2-fixing cyanobacteria. Given
the ubiquitous nature of mosses in these forests, their association with N2-fixing
cyanobacteria could characterize the N cycle in these ecosystems. For instance, the feather
moss Pleurozium schreberi with its associated cyanobacteria fixes 1-2 kg N ha-1
yr-1, which equals the amount that enters northern boreal forests via atmospheric N
deposition.
Nitrogen fixation in moss-cyanobacteria associations is affected by numerous abiotic
factors that could modulate the N input to the system via the moss-cyanobacteria pathway.
For instance, high N availability and dry conditions inhibit N2 fixation in moss-cyanobacteria
associations while phosphorus availability and moist conditions promote N2 fixation. Further,
N2fixation in moss-cyanobacteria associations is resilient, and can recover from increased N
inputs (12 - 15 kg N ha-1 yr-1) as well as from drought stress (moss < 9% field moisture)
upon removal of these stressors. Nevertheless, the question as to how important the N2
fixing capability of moss-cyanobacteria associations is as a source of “new” N for
the N cycle in boreal forests remains. For instance, mosses can retain acquired N
over long periods of time (> 1 year) and the transfer of N from moss to soil in the
short-term has so far only been shown to occur after disturbances (e.g. drying rewetting
events, fires). I will present results from laboratory as well as field experiments
aimed to elucidate the role moss-cyanobacteria associations play for the N cycle
in boreal forests and how abiotic factors control the fixation of atmospheric N2. |