Miljøprojekt, 1023 – Nitratreduktion i den umætttede zone

Nitratreduktion i den umætttede zone

Summary and conclusions

The unsaturated zone (also termed the vadose zone, the zone of aeration, and the soil zone) makes the interface between the soil surface and the deeper aquifer. The thickness of the unsaturated zone varies between a few cm to many meters and so does the importance of this zone for the movement of water and contaminants (e.g., nitrate) to the deeper aquifers.

The unsaturated zone is made up by a 3-phase system (solid particles, gas, and liquid) andd differs from the saturated zone that is made up by a 2-phase system (solid particles and liquid). Oxygen may enter the unsaturated zone either by diffusion as a gas or dissolved in the downward percolating water and create an overall oxygen-rich environment that does not promote the nitrate reduction processes to occur.

In some micro-environments in the unsaturated zone, e.g., created by high biological activity around fragments of bioavailable organic matter or in areas with temporary water saturated conditions and reduced access of oxygen, an oxygen-free environment may develop. In this environement, in the presence of nitrate and bioavailable organic matter, bacteria may by denitrification transform nitrate to molecular nitrogen (N₂). Other reduced compounds like ferrous iron or reduced forms of manganese may also be formed, and may play an important role in the transformation of nitrate. Pyrite will not form in the unsaturated zone and may eventual only be present in an inherent form in the capillary zone.

The review of nitrate reduction under unsaturated condition shows that the denitrification processes based on organic matter may be common occurring in the root zone where a pool of organic matter is continuously regenerated (including detritus, roots, and dead soil fauna and flora). Down to a depth of about 3-meter, the pool of bioavailable organic matter is reported to be renewable. This pool of organic matter may be used in the denitrification process directly or in the formation of other reduced compounds, like ferrous iron, that may later oxidize and promote the formation of an oxygen-free environment or directly be responsible for nitrate reduction. The potential for nitrate reduction in deeper parts of the unsaturated zone relays entirely on the inherent pools of reduced compounds which in some cases may have been preserved in capillary zone.

None of the reported results from laboratory studies on denitrifikation in sediments from the unsaturated zone can be used in the quantification of the amount of nitrate that is denitrified under field conditions. However, the results strongly point out that the level of oxygen, the amount of bioavailable organic matter, the soil temperature, and the level of water saturation as some of the important parameters related to the potential of denitrification in soil and sediments. Studies done under field conditions shows maximum rates of denitrification in springtime, just after spreading of N-fertilizers and when an increased amount of bioavailable organic matter has been formed by frost-thaw processes during the winter season. An increase in the amount of precipitation and water saturation in the unsaturated zone promotes the creating of oxygen-free microenvironments and the potential of denitrification and may be the reason of clayey soils having a higher potential of denitrification than most sandy soils.

The absence of adequate information on the quantitative aspect concerning nitrate reduction under unsaturated conditions, two follow up activities will be initiated. The first activity concerns a lab study on available pools of carbon and nitrogen in sediment samples collected below arable land in areas with either clayey or sandy sediments. The second activity concerns monitoring in the field of properties related to the soil/sediment (pH, Eh, and water table) and soil water quality (nitrate, dissolved organic carbon and nitrogen, ferrous iron, and chloride) in an area of sandy and of clayey deposits.