Responses to ammonium and nitrate additions by boreal plants and their natural enemies.

Separate effects of ammonium (NH4+) and nitrate (NO3-) on boreal forest understorey vegetation were investigated in an experiment where 12.5 and 50.0 kg nitrogen (N) ha(-1) year(-1) was added to 2 m2 sized plots during 4 years. The dwarf-shrubs dominating the plant community, Vaccinium myrtillus and V. vitis-idaea, took up little of the added N independent of the chemical form, and their growth did not respond to the N treatments. The grass Deschampsia flexuosa increased from the N additions and most so in response to NO3-. Bryophytes took up predominately NH4+ and there was a negative correlation between moss N concentration and abundance. Plant pathogenic fungi increased from the N additions, but showed no differences in response to the two N forms. Because the relative contribution of NH4+ and NO3- to the total N deposition on a regional scale can vary substantially, the N load a habitat can sustain without substantial changes in the biota should be set considering specific vegetation responses to the predominant N form in deposition.