Intensification and expansion of pastoral farming in New Zealand has resulted in increased nitrate (NO₃⁻) leaching. Nitrate leached from the root zone into the underlying groundwater may travel to surface waters and result in eutrophication. Denitrification in the groundwater system may help reduce the impact of intensification on the receiving waters by converting reactive NO₃⁻ to inert dinitrogen (N₂) gas. Little information exists about the occurrence of denitrification in groundwaters within New Zealand. This project investigates the extent of and limitations on denitrification in two contrasting small catchments within the Waikato region of New Zealand. The lowland Toenepi catchment is under high intensity dairying and features fine textured, alluvial and old volcanic ash deposits. The Waihora well field is an upland catchment under low intensity pastoral agriculture and is formed from coarse, young volcanic deposits.
Isotopic analysis of NO₃⁻ in reduced groundwater samples from the Toenepi catchment showed temporal and spatial variation in the degree of δ¹⁵N and δ¹⁸O enrichments. Insufficient knowledge on groundwater flow paths at the study site as well as low NO₃⁻ concentrations made interpretation of the isotopic results difficult. Thus push-pull tests were performed but these, except one, were unable to demonstrate denitrification under in situ conditions, even when a carbon (C) substrate was added. However, laboratory incubations of aquifer material taken from the vicinity of the well screens demonstrated denitrification capacity was present. Addition of a C substrate (glucose) generally resulted in an increase in total N gas production. This response to C addition indicates that C availability limits denitrification in situ. However, even the low rates of denitrification measured could have a significant impact on the NO₃⁻ leaching into the shallow groundwater, provided flow paths, and therefore reaction time, are long enough.
The isotopic analysis of NO₃⁻ in ground water samples proved ineffective at determining whether denitrification occurred at the Waihora field site, as most reduced groundwater samples had NO₃⁻ concentrations too low for analysis. However, laboratory incubations demonstrated that denitrification could occur below the root zone and was related to the presence of relict organic matter, in the form of buried soils and vegetation. Further experiments determined the denitrification potential of samples from below the A horizon, and indicated that much of the profile was C-limited as addition of glucose or hot-water extractable C resulted in an increase in total N gas fluxes. Despite the lower denitrification capacity at the Waihora well field (compared to Toenepi), the reduced portion of the profile has the potential to make a significant reduction in the NO₃⁻ concentrations in the shallow groundwater as N inputs from the land are much lower.
This research has demonstrated that denitrification does occur in shallow groundwater systems in New Zealand, and that despite being limited by C-availability, significant reduction of NO₃⁻ leached from the root zone could occur as a result of relict organic matter located well below the soil zone in the groundwater system
