Differential effects of ammonium and nitrate deposition on fen phanerogams and bryophytes

Question: High atmospheric nitrogen (N) deposition has been shown to affect productivity and species composition of terrestrial ecosystems. This study focused on the differential effects of the two inorganic N forms in atmospheric deposition (i.e. ammonium and nitrate). Methods and location: Nutrient addition experiments were carried out during 4 years in a mesotrophic fen in a low‐deposition area in Ireland. In a factorial design, plots were fertilized with ammonium and/or nitrate, in two doses comparable with 35 and 70 kg N ha^?1 y^?1 and compared with an unfertilized control. Results: Vascular plant biomass as well as bryophyte biomass were not affected by N dose but showed significantly different responses to the N form. In the ammonium‐fertilized plots, vascular plant biomass was higher and moss biomass was lower than the control, while nitrate additions had no effect. Vascular plant species density was high (16 species per 0.49 m²) and was not affected by any of the treatments; bryophyte species density was also high (seven species per 0.04 m²) but showed a significant decrease upon ammonium fertilization. Conclusion: The vulnerability of the mesotrophic vegetation to enhanced atmospheric N deposition depends strongly on the N form. If N would be mainly deposited as NO_x, no detrimental effects on the vegetation will occur. If, however, the deposition is mainly in the form of NH_y, the bryophyte vegetation will be seriously damaged, while the vascular plant vegetation will show an increased biomass production with possible shifts in dominance from Carex and herb species to grasses and shrubs.