The relationship between nitrogen deposition, species composition and foliar nitrogen concentrations in woodland flora in the vicinity of livestock farms

Measurements of atmospheric ammonia concentration along a gradient of decreasing concentration, species composition and tissue nitrogen content of a range of plant species were made in woodland in the vicinity of four intensive animal units in Scotland. Ammonia concentrations were large at woodland edges close to the livestock buildings (annual means 20–60 μg m−3) and exceed critical levels for NH3 (8 μg m−3 annual mean). Surveys of species composition of ground flora along an 0.5 km transect from livestock buildings show marked changes within 300 m downwind of the buildings. Species such as Deschampsia flexuosa, Holcus lanatus, Rubus idaeus and Urtica dioica were abundant close to livestock units and their percentage cover decreased rapidly with distance from source, while the more N-sensitive species such as Oxalis acetosella, Galium odoratum, mosses and ferns which are found upwind and outside the influence of the NH3 source, were scarce at all sites receiving>25 kg ha−1 N year−1. Visible injury to pine and spruce needles was observed immediately downwind of the buildings. Foliar nitrogen concentration of a number of species was large close to the buildings and declined with distance. Total nitrogen deposition at the woodland boundaries is estimated to range from 40 to 80 kg N ha−1 year−1 at the 4 sites and exceeds critical loads for acidic coniferous forest, i.e. 15–20 kg N ha−1 year−1 to protect ground flora, and is also often in excess of that (11–50 kg N ha−1 year−1) proposed to protect tree health. Foliar nitrogen content of mosses, (LN, % dry weight) is related to nitrogen deposition (FN, kg N ha−1 year−1) according to LN = 3.81(1-e−0.04FN).