A conceptual model of spatially heterogeneous nitrogen leaching from a Welsh moorland catchment

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 3 – ), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NO 3 – reaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NO 3 – in 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 NO 3 – derives 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 NO 3 – concentrations.