Nitrate Retention and Removal in Sub-Tropical Riparian Zones

Abstract

Worldwide, contamination of streams and groundwater with excess nitrate has been linked to agricultural land use and particularly to the application of nitrogen fertilisers to increase agricultural production. Nitrate is an effective contaminant in agricultural areas; it is highly mobile, having a low affinity for soil sorption, and so moves with runoff and sub-surface flows. Excess nitrate can cause ecological impacts on waterways and coastal receiving water through eutrophication and, in some cases, contributes to coastal ‘dead zones’. Nitrate also has toxicological effects on aquatic organisms and those using contaminated water as a drinking source. Riparian zones, those zones where interaction of aquatic and terrestrial environments occurs, are identified as areas of intense biogeochemical cycling and can act as buffers against excess nitrate by reducing the amount of nitrate reaching stream channels. Nitrate retention processes of biotic uptake and transformation to less mobile forms can increase the residence time of nitrate within the riparian zone, while removal processes of denitrification can permanently remove nitrate-nitrogen in gaseous forms.