Nitrogen Content

Nitrogen may, for example, occur as NH₂ groups attached to carbon atoms. As organic material decomposes in the soil, organically bound nitrogen is mineralised and converted into ammonium (NH₄⁺) or nitrate (NO₃⁻) forms.

Proportions of carbon and nitrogen vary between different materials

The nitrogen content therefore generally follows the variation in carbon content. However, the C:N ratio can vary between different soils and types of organic material. In the humus layer of forest soils, the C/N ratio is typically around 20, but can vary from up to 100 in certain types of fresh litter, such as bark fragments, down to about 10 in well-decomposed organic matter.

The strong competition for nitrogen is the reason why concentrations of ammonium and nitrate nitrogen are very low in soils with high C/N ratios. Under such conditions, mineralised nitrogen is immediately immobilised by other soil organisms. This also means that leaching of nitrogen from forest ecosystems is naturally very low.

Determination of nitrogen content

Since 1993, the nitrogen content of air-dried soil samples has been determined using a CNS analyser, in which the sample is combusted and the combustion gases are separated. The gas quantities are then measured using a thermal conductivity detector (link to Wikipedia). The nitrogen content is expressed as a percentage by weight of the dry matter content.

The dry matter content, in turn, is determined by weighing a subsample of air-dried soil (w₁), drying it at 105 °C for 16–24 hours, and then weighing it again (w₂). The dry matter content is then obtained by dividing w₂ by w₁.