The effect of long-term nitrogen additions on the bryophyte cover of upland acidic grasslands

Regular applications of NH4NO3 (3.5-14 g N m(-2) yr(-1)) and (NH4)(2)SO4 (14 g N m(-2) yr(-1)) to areas of acidic grassland in the Derbyshire Peak District over a period of six years have resulted in marked changes in the abundance of the bryophyte species present on the site. A dose-related reduction in bryophyte cover, significant at all levels of nitrogen addition, was obtained after only one year of applications and this effect has been maintained over a further five years of treatment. (NH4)(2)SO4 additions at the same rate as the highest NH4NO3 input (14 g N m(-2) yr(-1)) produced greater reductions in cover with losses of 80-95% compared with 45-55% for the 14 g NH4NO3 treatment. Soil pH measurements taken from cores collected in December 1994 showed a small reduction in the pH of the (NH4)(2)SO4 treated plots (4.03 compared with 4.41 for the controls), whereas the NH4NO3 treatments were unaffected. The mean stem density of the dominant bryophyte species Rhytidiadelphus squarrosus was significantly reduced at the lowest level of nitrogen addition and this was associated with marked increases in the total stem nitrogen content of this species. Pleurozium schreberi, present at much lower cover values, however showed no significant change except at the highest level of application, suggesting differential effects of the treatments on these two species. These results are indicative of significant losses in the bryophyte cover of acidic grasslands at atmospheric input rates of 5 g N m(-2) yr(-1) or less, and suggest that these species may be particularly at risk from the high atmospheric nitrogen deposition rates that are becoming a feature of many upland areas.