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A field-based system used to quantify the response of acid grassland to reduced atmospheric nitrogen and sulphur deposition, and to investigate the effects of elevated soil temperature on acid grassland development is described. The system is based on 12 retractable roofs, covering undisturbed experimental plots of acid grassland and three controls. Nine roofs are used to exclude natural precipitation and three roofs used to retain emitted IR radiation at night. An irrigation system has been developed to simulate natural precipitation, allowing for the application of specific treatment regimes of ambient, reduced nitrogen and reduced nitrogen/sulphur deposition beneath the nine rain exclusion plots. Plant, soil parameters, leachate chemistry and gaseous fluxes are being monitored and initial results on soil water chemistry are described. Warming appeared to enhance nitrate concentrations in soil water but this was not sustained beyond the first year of treatment. In contrast, the deposition reduction treatments decreased soil water nitrate concentrations within a few weeks of reducing deposition. This was not observed for other solutes such as sulphate or ammonium suggesting a more direct link between deposition of nitrate and leaching losses. [ABSTRACT FROM AUTHOR]

Soil- and stream-water data from the Plynlimon research area, mid-Wales, have been used to develop a conceptual model of spatial variations in nitrogen (N) leaching within moorland catchments. Extensive peats, in both hilltop and valley locations, are considered near-complete sinks for inorganic N, but leach the most dissolved organic nitrogen (DON). Peaty mineral soils on hillslopes also retain inorganic N within upper organic horizons, but a proportion percolates into mineral horizons as nitrate (NO), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NOreaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NOin hillslope waters discharging into larger valley wetlands will be removed before reaching the stream. A concept of catchment ‘nitrate leaching zones’ is proposed, whereby most stream NOderives from localised areas of mineral soil hillslope draining directly to the stream; the extent of these zones within a catchment may thus determine its overall susceptibility to elevated surface water NOconcentrations. [ABSTRACT FROM AUTHOR]