Groundwater nitrate reduction versus dissolved gas production: A tale of two catchments

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 (EF_5g1). 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 N₂ occurred in anaerobic conditions, while at intermediate dissolved oxygen; N₂O was the dominant reaction product. EF_5g1 (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.