Impacts of increased nitrogen supply on high Arctic heath: the importance of bryophytes and phosphorus availability.

• This study investigates effects of nitrogen and phosphorus on high Arctic heath vegetation, particularly bryophytes. • Heath communities received factorial combinations of nitrogen (0, 10 and 50 kg ha  yr ) and phosphorus (0 and 5 kg ha yr ) in five applications per growing season, for 8 yr. • Nitrogen decreased lichen cover but did not affect cover of any other functional type. However, just 10 kg ha yr increased the proportion of physiologically active bryophte shoots, and decreased their nitrate assimilation capacity. Phosphorus had greater effects, and the combination of both nutrients altered species composition. Individual bryophyte species displayed contrasting responses to fertilization, suggesting that they should not be grouped as a single functional type. • The 'critical load' of nitrogen for Arctic heath lies below 10 kg ha  yr . Nitrogen and phosphorus are colimiting in this sytem, so the critical load of nitrogen will be lower where phosphorus availability is greater. Responses of vegetation to any increase in net mineralisation due to soil warming will depend on the ratio in which nitrogen and phosphorus availabilities increase. The effects of nutrient enhancement are very persistent.