Impact Type: Deposition of pollutant
Not all species will be similarly sensitive. Mosses growing in disturbed, eutrophied habitats such as Eurynchium praelongum may benefit from low N doses. However it is negatively affected by higher levels (100-150 kg ha-1) of nitrogen deposition (Virtanen et al. 2000). Polytrichum commune and Plagiothecium also apparently quite shade tolerant, seem to be very tolerant when the N is supplied little and often, but up to 50 kg N ha-1 yr-1 as NH4NO3. (Sheppard unpublished). Negative impacts often reflect increased litterfall which often accompanies eutrophication, both factors restricting light penetration to the understorey vegetation. Eutrophication will initially stimulate growth and there is a rapid increase in N content especially in amino acids. Effects of bog mosses have been summarised for Sphagnum balticum.
Understorey mosses tend to see a 'modified' N climate from those growing in the open as overstorey vegetation will modify the N content of rain by providing a surface/filter for dry deposition and exchange of soluble ions. Tree and moorland foliage can substantially reduce the N input (via foliar uptake and exchange of NO3+ and NH4+ ions) to understorey mosses (Sheppard unpublished). The accompanying ion, may also impact on mosses (e.g. SO42 could be the driving ion and not NH4+, Sheppard unpublished). Some of the negative effects attributed to N have come from studies where SO42- has been the counter ion or where the treatment has been applied as a solid.