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Differential effects of nitrate and ammonium on three fen bryophyte species in relation to pollutant nitrogen input

Summary 
• In Dutch fens, subjected to high nitrogen (N) deposition, Scorpidium and other brown mosses have declined markedly. A concurrent strong increase of Sphagnum and Polytrichum has promoted acidification. We measured nitrate (NO3−) and ammonium (NH4+) availability in Dutch fens. We also tested preference for either N form of Scorpidium scorpioides, Sphagnum squarrosum and Polytrichum commune. 
• Ion exchange membranes were installed in the field. In a hydroponic experiment, plants were grown on 100 µm N (reflecting concentrations in Dutch precipitation since 1980), provided as NO3−, NH4NO3, or NH4+. 
• NH4+ availability in Sphagnum and Polytrichum stands and NH4+ : NO3− ratio in Sphagnum stands were higher than in brown moss stands. In the experiment, Scorpidium performed best on NO3−. NH4NO3 tended to decrease its growth, whereas NH4+ was very toxic. N treatment did not significantly affect growth of Sphagnum and Polytrichum. Tissue pH and nutrient concentrations confirmed the growth patterns and indicated that Scorpidium was most sensitive to NH4+ stress. 
• We conclude that high NH4+ inputs pose a serious threat to the brown moss flora of rich fens.