Our atmosphere is made up of nearly 80% atmospheric N which is dinitrogen or N₂. It is an important nutrient for making things like proteins, chlorophyl, hormones and DNA in plants.

Nitrogen in soil exists in many forms, but only a few can be taken up by plants. Soil microbes are important in changing nitrogen forms, making it available (otherwise known as mineralising) or unavailable (otherwise known as immobilising) depending on soil conditions (see Figure 1).

This is a dynamic process that will change depending on microbial activity and soil management practices. For instance, cultivation of soil will drive organic matter mineralisation and therefore the release of available nitrogen by converting organic nitrogen into the ammonium form of nitrogen. This form can be taken up by plants, or converted to nitrate N, which can also be taken up by the plant. But nitrate can also be easily leached. Leaching occurs when nitrogen is lost from the root zone. Plants predominantly take up N in two forms, either in ammonium form or nitrate form. Together these forms of N are added and described as mineral-N.

Nitrogen can be added to the soil in several ways, for example fertiliser amendments, animal dung and urine, decomposing plant litter, and atmospheric fixation through legumes, like clover.

Denitrification is the process of soil bacteria converting nitrate N into atmospheric nitrogen gas or N₂. It is important to note that incomplete denitrification will result in the production of Nitrous Oxide, or N₂O, which is a harmful greenhouse gas.

When urea fertiliser is added to the soil, there is risk of volatilisation which is loss of nitrogen into the atmosphere as ammonium gas. This is driven by enzymes in the soil. Warm and windy conditions are drivers of increased volatilisation.