impacts of atmospheric N inputs on throughfall, soil and stream water interactions for different aged forest and moorland catchments in Wales

Inorganic nitrogen concentrations in streams, soil waters, throughfall and rainfall were determined over 1 year in 5 moorland and 20 Sitka spruce plantation catchments in upland Wales. Forest ages were between 10 and 55 years. Inorganic nitrogen concentrations in stream waters, B and O horizon soil waters and throughfall were positively correlated with forest stand age. Inorganic nitrogen in streams and B horizon waters occurred entirely as nitrate whereas in O horizons one third was as ammonia. Inorganic nitrogen fluxes in throughfall were also positively correlated with stand age. Inorganic nitrogen concentrations were negligible in moorland and in crops younger than 30 years. Throughfall flux of inorganic nitrogen in the oldest stand was 25.1 kg nitrogen per ha.year. Older forest stands appeared unable to utilize available nitrogen. Nitrification was active in soils of older stands and resulted in soil acidification leading to nitrate leaching. Implications for debate on nitrogen critical loads are discussed. A reduction in inputs to those characteristic of pristine sites was theoretically necessary to ensure that soil acidification was minimized. There are 39 references.