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Principper for beregning af nitratreduktion i jordlagene under rodzonen

Summary

Downward leaching water from the surface normally contains nitrate under farmland as well as under natural land. Often the concentrations of nitrate in the water leaching from farmland are greater than in water leaching from natural land where no nitrogen fertilisers are applied. Leaching of nitrate relates to the climatic conditions and the land use and for the farmland also to the type of crops and agricultural practice.

The fate of the nitrate in the downward leaching water depends on the geological, hydrological and geochemical conditions in the sediments present in the recharge area.

Studies of Danish sediments of the Quaternary age have demonstrated a relation between the colour of the sediments and the occurrence of nitrate, as nitrate was measured in the oxidised sediments of yellow, yellow brown, brown and greyish brown colours only.

I most parts of Denmark, the oxidised zone was formed after the Weichselian ice age, about 12 000 years ago. Other parts of Denmark remained unglaciated and here the oxidation processes may have been active for more than 100 000 years. Oxygen, either dissolved in the downward leaching water or transported by diffusion into the sediments, plays a very important role in the development of the oxidised geochemical environment and the increased input of nitrate during the latest about 60 years plays only a minor role (which is equivalent to the potential of 300 years with almost nitrate-free water).

Based on existing data, the thickness of the oxidised zone seems to vary between few meters in the young clayey till areas to more than 50 meters in sand sediments of Saalian age. This makes the deep aquifers much more vulnerable to nitrate in the old glacial deposits of Saalian age than in the clayey till areas of Weichselian age where the redox interface occurs in the aquitard above the deep aquifers.

In the unsaturated zone the denitrification processes takes place in the reduced microenvironments only and may be located around either inherited organic matter or organic matter that has been transported from the surface and deeper down into the subsurface sediments. If nitrate is introduced into a manganese/iron or sulfate-reducing zone it is thermodynamically unstable and may become reduced by either microbial or chemical processes. Low microbial denitrification potentials of most Danish sediments due to low amounts of easily decomposable carbon source draw the attention to other processes that may contribute to the reduction of nitrate. The importance of these processes is controlled by the presence of the different reduced compounds; e.g. organic matter, ferrous iron, pyrite and methane. The character and importance of these processes in the typical geological setting still needs to be studied in details.

Until a better understanding of the processes has been established the nitrate reduction capacity of the sediments will be related to the content of reduced compounds in the sediments. The content of reduced compounds in the reduced sediments makes up the potential source of reduction capacity and the difference between the content of reduced compounds in the reduced and the oxidised sediments makes up the actual reduction capacity. The difference between the potential and the actual reduction capacity makes up a pool that is slowly or very slowly available for the reduction processes.