Background

Nitrogen is an essential plant nutrient and is applied to fields as animal manure and fertilizer. In the root zone organic nitrogen, which cannot be used directly by the plants, is mineralized to the plant available inorganic N-forms nitrate and ammonium. Ammonium (NH4+) is a cation and is therefore bound relatively strongly in the soil. Nitrate (NO3-) on the other hand is an anion and is therefore not bound in the soil. Consequently, nitrate that has not been used by the plants will therefore be leached from the soil. The excess nitrate is transported out of the root zone with recharging water to the groundwater zone, from where it is transported further on to surface and coastal waters.

However; not all excess nitrate is transported all the way to the sea. Nitrate can be naturally transformed to N2 by reduction. Nitrate reduction occurs under anaerobic conditions and in the presence of a reduced compound (organic carbon, pyrite or Fe⁺²). Nitrate reduction occur several places within a catchment; in the root zone, in groundwater, in wetlands and riparian lowland and in lakes and stream bed sediments.

In the groundwater zone is nitrate reduced at the redox interface, which define the transition from aerobic to anaerobic conditions. The nitrate must therefore be transported with the following groundwater below this interface for reduction to occur. The amount of nitrate reduction within a catchment depends therefore both on the depth of the redox interface but also on the groundwater flow patterns. Most of Denmark is covered by young glacial sediments and the redox interface is often found close to the surface. The high redox interface together with a groundwater dominated hydrology results in a high degree of nitrate reduction in groundwater in Denmark.

The subsurface geology is often heterogeneous resulting in spatial variations in depth of the redox interface and the groundwater flow patterns on local scale. The amount of nitrate reduction in groundwater can therefore vary a lot within a catchment. The effectiveness of wetlands, riparian lowland, lakes and streams to remove nitrate can also vary significantly. It is these spatial variations in the natural reduction of nitrate that a spatially differentiated N-regulation will take into account and focus on decreasing the N-leaching from areas that have a low natural N reduction on the flow path from below the root zone and to the coastal waters. However; in order to point out these focus areas assessment of the spatial variation in nitrate reduction on a sufficient small scale is required.

Assessment of the spatial variation in nitrate reduction is at present impeded by critical knowledge gaps specifically related to the location and importance of drainage systems, degradation in riparian lowlands and the redox conditions in the subsurface. TReNDS will advance the scientific basis on these important issues.