N leaching to groundwater from dairy production involving grazing over the winter on a clay-loam soil

摘要

This study investigated concentrations of various N species in shallow groundwater (<2.2 m below ground level) and N losses from dairy production involving grazing over the winter period on a clay loam soil with a high natural attenuation capacity in southern Ireland (52°51 'N, 08°21 'W) over a 2-year period. A dense network of shallow groundwater piezometers was installed to determine groundwater flow direction and N spatial and temporal variation. Estimated vertical travel times through the unsaturated zone (<0.5 yr, time lag) allowed the correlation of management with groundwater N within a short space of time. There was a two way interaction of the system and sampling date (P<0.05) on concentrations of DON, oxidised N and NO_3~--N. In contrast, concentrations of NH_4~+-N and NO_2~--N were unaffected by the dairy system. Crazing over the winter had no effect on N losses to groundwater. Mean concentrations of DON, NH_4~+-N, NH_2~--N and NO+3~--N were 2.16, 0.35, 0.01 and 0.37 mg L~(-1) respectively. Soil attenuation processes such as denitrification and DNRA resulted in increased NH_4~+-N levels. For this reason, DON and NH_4~+-N represented the highest proportion of N losses from the site. Some of the spatial and temporal variation of N concentrations was explained by correlations with selected chemical and hydro-topographical parameters (NO+3~--N/Cl~- ratio, distance of the sampling point from the closest receptor, watertable depth, depth of sampling piezometer, DOC concentration). A high explanatory power of NO_3~--N/Cl~- ratio and the distance of the sampling point from the closest receptor indicated the influence of point sources and groundwater-surface water interactions.