Effects of nitrogen deposition on bryophyte species composition of calcareous Grasslands

Abstract

Regular additions of NH4NO3 (35-140 kg N ha -1 yr-1) and (NH4)2SO4 (140 kg N ha-1 yr-1) to a calcareous grassland in northern England over a period of 12 years have resulted in a decline in the frequency of the indigenous bryophyte species and the establishment of non-indigenous calcifuge species, with implications for the structure and composition of this calcareous bryophyte community. The lowest NH4NO3 additions of 35 kg N ha-1 yr-1 produced significant declines in frequency of Hypnum cupressiforme, Campylium chrysophyllum, and Calliergon cuspidatum. Significant reductions in frequency at higher NH 4NO3 application rates were recorded for Pseudoscleropodium purum, Ctenidum molluscum, and Dicranum scoparium. The highest NH4NO3 and (NH4)2SO 4 additions provided conditions conducive for the establishment of two typical calcifuges-Polytrichum spp. and Campylopus introflexus, respectively. Substrate-surface pH measurements showed a dose-related reduction in pH with increasing NH4NO3 deposition rates of 1.6 pH units between the control and highest deposition rate, and a further significant fall in pH, of >1 pH unit, between the NH4NO3 and (NH4)2SO4 treatments. These results suggest that indigenous bryophyte composition may be at risk from nitrogen deposition rates of 35 kg N ha-1 yr-1 or less. These effects are of particular concern for rare or endangered species of low frequency. © 2007 Springer Science+Business Media B.V.