Physiological and growth responses of the montane bryophyte Racomitrium lanuginosum to atmospheric nitrogen deposition.

•  The effects of nitrogen (N) deposition on the moss Racomitrium lanuginosum within montane heath in Scotland were investigated over 5 yr. •  Permanent field plots were sprayed with KNO3 or NH4 Cl solutions, at doses equivalent to 10 and 40 kg N ha-1  yr-1 , in 3-6 applications each summer. •  Racomitrium growth and cover were severely reduced by N addition, whilst the proportion of dead shoots greatly increased. N dose decreased inducibility of shoot nitrate reductase activity (NRA), suggesting that N saturation of Racomitrium occurred, and caused an increase in potassium leakage. At high dosage, effects of NH4 + were more detrimental than NO3 - . •  Physiological responses to N indicate that the habitat's critical load (CL) is exceeded by addition of 10 kg N ha-1  yr-1 . The differential toxicity of the two forms of N suggests that predominant ion type in deposition should be taken into consideration when CLs are set. In contrast to tissue N, NRA correlated well with shoot growth, and may thus be a useful biological indicator of moss condition.