EXPERIMENTAL ASSESSMENT OF THE EFFECTS OF NITROGEN ADDITION UNDER HAY-CUTTING AND AFTERMATH GRAZING ON THE VEGETATION OF MEADOWS ON A SOMERSET PEAT MOOR

1. Grassland on lowland peat has considerable potential for agriculture and conservation. 2. The limited use of fertilizer on the Somerset Moors, designated as an Environmentally Sensitive Area has enabled the impact on vegetation of applying a range of nitrogen levels to be investigated using a randomized block experiment with large plots at Tadham Moor. 3. From 1986 to 1990, the plot measurements each year included the individual percentage cover of some 100 species; these enabled the derivation of a number of measures of plant community structure. 4. The applications of fertilizer encouraged grasses (notably Lolium perenne and Holcus lanatus) which came to dominate the sward at the expense of most other species. All measures of species diversity declined significantly and the sward became taller in plots receiving high levels of nitrogen. Only a very few forbs (e.g. Rumex acetosa) maintained or increased their cover with increased nitrogen. 5. All Carex species, Juncus species and mosses became significantly less common in plots treated with nitrogen, producing an apparently more mesotrophic sward. Short-lived species and low-growing wetland forbs also declined with nitrogen, perhaps due to shading by tall grasses. Legumes were suppressed by high levels of nitrogen, but were common in plots receiving 25 kg ha-1 year-1 N, where replacement levels of phosphorus and potassium were applied after cutting. 6. Except for grasses, many species common in the experimental site in 1986 declined in cover, particularly in plots receiving high levels of nitrogen. However, the data showed no instances of extinction by 1990. The application of 25 kg ha-1 N encouraged the spread of agriculturally productive grasses within 2 years and 50 kg ha-1 year-1 N significantly reduced species richness in 3 years. 7. Certain detailed findings from this large-scale study of nitrogen application in meadows enable contrasts with previous results to be identified.