Denitrification – The key component of a groundwater system’s assimilative capacity for nitrate

Abstract

For environmental as well as economic reasons, minimising nitrate losses from the root zone should be given highest priority in agricultural nitrogen management. However, even the best management practices available for a given land use may result in root zone losses that are incompatible with water quality limits, especially in sensitive catchments (e.g. Lake Taupo). It has become evident in recent years that denitrification can significantly reduce the mass of nitrate in some groundwater systems before the contaminated groundwater reaches a water supply well or a surface water body. Taking this natural assimilative capacity into account when making decisions on land use type and intensity can therefore provide a second line of defence with regard to contamination of freshwater resources. Denitrification converts nitrate (NO₃⁻) to gaseous forms of N. In contrast to unsaturated zone denitrification, where incomplete denitrification can result in substantial emissions of the greenhouse gas nitrous oxide (N₂O), complete denitrification to inert dinitrogen (N₂) prevails under the usually more stable redox conditions of reduced groundwater zones. For denitrification to occur, four requirements must be met. Apart from nitrate being present, there need to be oxygen-depleted conditions, a suitable electron donor, and microbes with the metabolic capacity for denitrification. Nitrate and suitable microbes are considered ubiquitous under agricultural land use. Accordingly, the occurrence of denitrification at a particular location is largely determined by the existence of oxygen-depleted conditions (< 2 mg/L dissolved oxygen) and the availability of electron donors. Heterotrophic denitrification is fuelled by organic matter, while reduced inorganic iron and sulphur compounds (e.g. pyrite) can fuel autotrophic denitrification. Through a combination of hydrochemical analyses, isotopic analyses, excess N₂ determinations and lab incubations it has been ascertained that denitrification occurs in the groundwater systems of the Toenepi Stream and Lake Taupo catchments. Particulate organic matter residing in the groundwater zone has been identified as the main electron donor, but additional contributions from reduced inorganic substrates (e.g. pyrite) cannot be excluded. While measurements allow denitrification to be detected locally, quantifying its effect on nitrate fluxes through a sub-catchment or catchment requires modelling based on a sound understanding of the biogeochemical and hydrological conditions.