Denitrification in the shallow groundwater system of a lowland catchment: a laboratory study

Abstract

Evidence for the occurrence of denitrification in shallow groundwater systems in New Zealand (NZ) is poorly documented; however, an observational study of the Toenepi dairying catchment in the Waikato region of NZ revealed a prevalence of reduced groundwater with very low nitrate (NO₃⁻) concentrations (predominantly < 0.5 mg L⁻¹ N). Denitrification in the shallow groundwater system could play an important role in catchments with high NO₃⁻ leaching losses, by reducing the impact of land use and management practices on freshwater bodies. International studies have shown that denitrification below the root zone is often limited by the low availability of carbon (C) or other electron donors. In this laboratory incubation study we investigated three different profiles (to a maximum depth of 4.7 m below ground surface) in the Toenepi catchment (15 km²). Denitrification capacity was measured following the addition of ¹⁵N-enriched NO₃⁻ , while glucose was used as a readily available C source when ascertaining the denitrification potentials. The largest total ¹⁵N fluxes (without C amendment) were observed in samples from the Kereone (1.3–2.1 nmol N g⁻¹ h⁻¹) and Topehaehae sites (2.8 nmol N g⁻¹ h⁻¹); however, all sites had samples with fluxes of a similar magnitude (3.3–4.7 nmol N g⁻¹ h⁻¹) with no significant difference between sites (p > 0.05) when glucose was added. The profiles were generally C-limited, as indicated by more than 80% of samples showing an increase (p < 0.01) in total ¹⁵N gas (¹⁵N₂ + ¹⁵N₂O) production after C addition. The composition of the total ¹⁵N gas flux varied with depth but ¹⁵N₂ was ≥ 69% of the total ¹⁵N flux and typically > 92%. Extrapolation of denitrification capacity rates to field temperatures (14 °C) indicates that much of the material found at depth, particularly at the Kereone and Topehaehae sites could contribute substantially towards attenuating the estimated NO₃⁻ leaching losses (29–42 kg N ha⁻ 1 year⁻1) from the root zone and is likely the cause of the very low NO₃⁻ concentrations prevalent throughout the catchment.