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

Abstract

Regular applications of NH₄NO₃ (3.5–14 g N m⁻² yr^−l) and (NH₄)₂SO₄ (14 g N m⁻² yr^−l) 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.<br/><br/>(NH₄)₂SO₄ 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)₂SO₄ treated plots (4.03 compared with 4.41 for the controls), whereas the NH₄NO₃ treatments were unaffected.<br/><br/>The mean stem density of the dominant bryophyte species <i>Rhytidiadelphus squarrosus</i> 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.<i> Pleurozium schreberi</i>, 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.<br/><br/>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.