What processes remove contaminants from the environment?
Not all nitrate lost from the soil will remain as nitrate as chemical processes that occur naturally in the landscape can convert the nitrogen back into nitrogen gas (the main gas that makes up the air we breathe). The type of chemical reaction is known as a redox process (or oxidation-reduction reaction).
Redox processes are a type of chemical process in soil and shallow groundwater that control the concentration of the dissolved forms of nitrate and nitrite, oxygen, manganese, iron, sulphate, and heavy metals. Redox also indirectly controls the leachability and mobility of phosphorus species in soils, aquifers, and subsequently surface waters. Soil zone redox processes, in conjunction with nitrogen load, also determine the magnitude of soil zone greenhouse gas emissions, such as nitrous oxide, methane, and carbon dioxide.
What is redox and reduction potential?
The physical properties of soil and geology can be used to explain biogeochemical processes occurring in the landscape, as they are important for the oxidation-reduction (redox) process. In basic terms, the redox state is characterised as the presence of oxygen (oxic) or absence (anoxic) of oxygen, however it is more accurately described as chemical reactions which involve the transfer of electrons. The chemical species which loses the electron (increase in oxidation state) is oxidised, while the chemical species that gains the electron (decrease in oxidation state) is reduced. Typically, well drained soils are characterised as oxidising, while poorly drained soils are characterised as reducing.
Oxidising soils and geology have a low abundance of electron donors and low reduction potential which means they are less reactive. Nitrate lost through the soil in these areas are more likely to accumulate in the groundwater as there is a low potential for reduction from biological removal (denitrification).
Organic carbon has high abundance of electron donors and therefore a high reduction potential under anoxic conditions (no oxygen). Any nitrate present in organic soils such as peat, will likely be denitrified to nitrogen gas.
As redox processes occur in both the soil zone and underlying aquifer, it is important to know the hydrology to establish if the reduction potential is realised. For example, a highly drained mineral soil will have a lower reduction potential than the same soil unmodified by drainage as the contact and time water spends in the soil under saturation is significantly lower.
What is denitrification?
Denitrification is a redox reaction that deals specifically with the transformation of nitrogen in which oxidised nitrogen (nitrate, NO3-) accepts an electron and is reduced to nitrous oxide (NO or N2O) or nitrogen gas (N2). This reaction can only occur under anoxic conditions where there is no oxygen present. Denitrification is also a biological process as microbes (bacteria) drive the chemical process.
Why is redox important?
Redox is important to a range of environmental concerns including low dissolved oxygen in surface waters, where leached nitrate is likely to be removed by denitrification, where phosphorus is likely to be leached and/or more mobile within soils and aquifers. This information also enables an understanding of where shallow groundwater is likely to contain elevated manganese, iron, and arsenic (in areas with arsenic bearing minerals), limiting its potential as a drinking water source. Soil zone redox processes, in conjunction with nitrogen load, also determine the magnitude of soil zone greenhouse gas emissions, such as nitrous oxide, methane, and carbon dioxide.