WP 3: Nitrate leaching

Task 3.1 — Tracer 15N studies at the Afon Gwy to determine short-term pathways for transport of N into surface waters

An isotope addition experiment will be used to trace the pathways and transformation of N through an organo-mineral upland soil under realistic hydrological conditions (these soils are characteristic of 'nitrate leaching zones', as defined in Evans et al, in press). Additions of 15N-labelled NH4+ and NO3- will be made separately to adjacent plots during rain events following a) a dry summer period and b) a wet winter period. Leaching of labelled N will be determined by sampling water from multiple depths within the soil and in overland flow, and retention by subsequent analysis of 15N in bulk soil and vegetation. The chemical form and timing of 15N leaching relative to a conservative tracer will allow hydrological pathway, N transformation and the balance of 'saturation' (pathway 2) versus 'hydrological' (pathway 1) N leaching to be determined. This experiment provides no information on the extent to which biologically retained N may be later released via mineralisation and microbial nitrification (pathway 3).

Work will be carried out on an acid grassland hillslope in the Afon Gwy catchment, Plynlimon. As well as providing a location characteristic of much of the UK upland area impacted by N deposition, this site also benefits from existing infrastructure, e.g. an intensively monitored soil- and ground-water sampling transect established under GANE, the CEH GANE roof experiment, and long-term monitoring data. Measurements of N immobilisation and transformations made at the Gwy during the previous DEFRA freshwaters programmes will also inform the current study.