title: Groundwater nitrate reduction versus dissolved gas production: A tale of two catchments.
abstract: At the catchment scale, a complex mosaic of environmental, hydrogeological and physicochemical characteristics combine to regulate the distribution of groundwater and stream nitrate (NO3-). The efficiency of NO3-removal (via denitrification) versus the ratio of accumulated reaction products, dinitrogen (excess N2) & nitrous oxide (N2O), remains poorly understood. Groundwater was investigated in two well drained agricultural catchments (10km2) in Ireland with contrasting subsurface lithologies (sandstone vs. slate) and landuse. Denitrification capacity was assessed by measuring concentration and distribution patterns of nitrogen (N) species, aquifer hydrogeochemistry, stable isotope signatures and aquifer hydraulic properties. A hierarchy of scale whereby physical factors including agronomy, water table elevation and permeability determined the hydrogeochemical signature of the aquifers was observed. This hydrogeochemical signature acted as the dominant control on denitrification reaction progress. High permeability, aerobic conditions and a lack of bacterial energy sources in the slate catchment resulted in low denitrification reaction progress (0-32%), high NO3-and comparatively low N2O emission factors (EF5g1). In the sandstone catchment denitrification progress ranged from 4 to 94% and was highly dependent on permeability, water table elevation, dissolved oxygen concentration solid phase bacterial energy sources. Denitrification of NO3- to N2occurred in anaerobic conditions, while at intermediate dissolved oxygen; N2O was the dominant reaction product. EF5g1 (mean: 0.0018) in the denitrifying sandstone catchment was 32% less than the IPCC default. The denitrification observations across catchments were supported by stable isotope signatures. Stream NO3-occurrence was 32% lower in the sandstone catchment even though N loading was substantially higher than the slate catchment.
authors.affiliation: Geology Department/Trinity Centre for the Environment, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland; Agricultural Catchments Programme, Teagasc, Environment Research Centre, Johnstown Castle, Wexford, Ireland; Crops, Environment and Land Use Programme, Teagasc Environment Research Centre, Johnstown Castle, Wexford, Ireland. Electronic address: e.mcaleer1@gmail.com.
authors.forename: E B
authors.initials: EB
authors.lastname: McAleer
authors.affiliation: Geology Department/Trinity Centre for the Environment, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland.
authors.forename: C E
authors.initials: CE
authors.lastname: Coxon
authors.affiliation: Crops, Environment and Land Use Programme, Teagasc Environment Research Centre, Johnstown Castle, Wexford, Ireland. Electronic address: Karl.Richards@teagasc.ie.
authors.forename: K G
authors.initials: KG
authors.lastname: Richards
authors.affiliation: Crops, Environment and Land Use Programme, Teagasc Environment Research Centre, Johnstown Castle, Wexford, Ireland.
authors.forename: M M R
authors.initials: MMR
authors.lastname: Jahangir
authors.affiliation: Teagasc, Ashtown Food Research Centre, Dublin, Ireland.
authors.forename: J
authors.initials: J
authors.lastname: Grant
authors.affiliation: Agricultural Catchments Programme, Teagasc, Environment Research Centre, Johnstown Castle, Wexford, Ireland.
authors.forename: Per E
authors.initials: PE
authors.lastname: Mellander
keywords: Agriculture; Denitrification; Dissolved oxygen; Excess N(2); N(2)O; Pollutant swapping