Evaluation of the stable isotope signatures of nitrate to detect denitrification in a shallow groundwater system in New Zealand

• Stable isotopes of nitrate detected denitrification in shallow, young, groundwater.
• Denitrification was shown to be seasonal.
• Understanding the age of groundwater is crucial for isotope interpretation.

Denitrification in the groundwater systems of agricultural catchments can provide a substantial ‘ecosystem service’ by attenuating leached nitrate (NO3−) before it reaches surface water bodies. Samples along a groundwater flow path with low dissolved oxygen and declining NO3− concentrations can indicate the occurrence of denitrification. Isotopic analysis of this NO3− can potentially identify and quantify denitrification activity. In this study, shallow groundwater samples (maximum 5 m below ground surface) were taken from three locations within a small agricultural catchment in the Waikato region of New Zealand. The δ15N and δ18O values of NO3− were analysed to try to determine where denitrification was occurring and at what rate. Results indicated that denitrification rates varied spatially, but interpretation was confounded by insufficiently understood flow paths and extremely low concentrations of NO3− in reduced groundwater. Seasonal denitrification was observed at a Gley soil site where the soil profile was periodically saturated to near the ground surface and δ15N-NO3− values reached +28.5‰ and δ18O-NO3− values up to +19.6‰. In contrast to expectations, NO3− in well-oxidised groundwater samples showed substantial variability in its δ15N and δ18O isotopic signature. This indicated that the NO3− originated from multiple sources, which restricted the quantification of denitrification.