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) fertilize r treatments was applied for 4 years in a randomized block experiment to a species-rich hay meadow on peat soil, within the Somerset Moors a nd Levels Environmentally Sensitive Area. 2. The percentage compositio n 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, an d ordination techniques were used to investigate the effects of fertil izers on plant community composition. Data for mean annual biomass pro duction are also presented. 3. Botanical results were compared with th ose 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 substanti al amounts of N and K were applied without P, but when high rates of P were included biomass increased very significantly and species divers ity was severely reduced, with Holcus lanatus, Rumex acetosa and Loliu m perenne dominating. 5. Lolium perenne was not increased by N and mod est rates of P in the absence of aftermath grazing, but dominated all fertilized plots when aftermath grazing was maintained. Agrostis canin a came to dominate plots receiving 200 kg ha(-1) of N with modest rate s of P and K in the absence of aftermath grazing, but was negatively a ssociated 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 l egumes 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.