A study of inorganic-N concentrations in streams, soil waters, throughfall and rainfall was conducted for one year in five moorland and 20 Sitka spruce plantation catchments in upland Wales. The forest ages ranged from 10 to 55 yr. Highly significant positive relationships with forest stand age existed for inorganic-N concentrations in streamwater, B and O horizon soil waters and throughfall. Inorganic-N in streams and B horizon waters was entirely NO3-. Inorganic-N fluxes in throughfall also showed a significant, positive relationship with stand age. Throughfall flux of inorganic-N in the oldest stand was 25.1 kgN ha-1 yr-1, double that in incident rainfall. The older forest stands appear unable to utilise the available N. Nitrification is very active in the soils of these older stands, resulting in significant soil acidification. The processes responsible for the observed NO3- leaching losses, and the implications for the debate on Nitrogen Critical Loads are discussed.
