Aspects of the nitrogen cycle in peatland and plantation forest ecosystems in western Ireland

Extensive plantation forests cover large areas of blanket peatland in western Ireland. Sites are characterised by the ombrotrophic nature of the peat and the often extreme maritime conditions prevailing. The study area is located close to two coastlines and in consequence, ions of marine origin are dominant in the bulk precipitation. Mean annual nitrogen deposition is 2.26 kg ha^-1. Forestry development in the region dates from the early 1950s. Deficiency of phosphorus is universally encountered, sometimes accompanied by a shortage of nitrogen. A fertilizer experiment in the study area was maintained for 16 years. The principal response was to applied phosphorus and although nitrogen had a positive influence on growth in the early years, it was of little consequence in the longer term.Over 900 kg N ha^-1 was accumulated in the forest floor. In a mineralisation study of peat collected from plots fertilized 14 years previously, differences in total mineral nitrogen production between treatments were small, but in nitrogen-treated plots a higher proportion of the mineral nitrogen was as nitrate than in those which had not received fertilizer nitrogen. Throughfall measurements in pole-stage crops of Sitka spruce and lodgepole pine which had received no fertilizer nitrogen, showed significantly greater quantities of nitrogen than bulk precipitation.