The effects of nitrogen, potassium and phosphorus addition on the vegetation of a Somerset peat moor under cutting management

1. A range of nitrogen (N), phosphorus (P) and potassium (K) fertilizer treatments was applied for 4 years in a randomized block experiment to a species-rich hay meadow on peat soil, within the Somerset Moors and Levels Environmentally Sensitive Area. 2. The percentage composition of each species present was recorded in May and October each year on plots cut for hay, followed by further cuts of aftermath growth. The effects on species richness, diversity and dominance were analysed, and ordination techniques were used to investigate the effects of fertilizers on plant community composition. Data for mean annual biomass production are also presented. 3. Botanical results were compared with those of a concurrent experiment where aftermath growth was grazed. 4. P was more important than N in determining both biomass production and botanical change, In both cases, the effects were small when substantial amounts of N and K were applied without P, but when high rates of P were included biomass increased very significantly and species diversity was severely reduced, with Holcus lanatus, Rumex acetosa and Lolium perenne dominating. 5. Lolium perenne was not increased by N and modest rates of P in the absence of aftermath grazing, but dominated all fertilized plots when aftermath grazing was maintained. Agrostis canina came to dominate plots receiving 200 kg ha(-1) of N with modest rates of P and K in the absence of aftermath grazing, but was negatively associated with N where the aftermath was grazed. 6. Trifolium pratense became very abundant where P and K were applied with nil or 25 kg ha(-1) of N each year, both with and without aftermath grazing, but all legumes were suppressed at high rates of N, particularly in conjunction with high P. 7. Substantial botanical change occurred on unfertilized plots as a result of the cessation of aftermath grazing. These plots became dominated by Plantago lanceolata, with significant increases in Leontodon hispidus and L. autumnalis.