Nitrogen release to air

Immobilisation is the process by which available inorganic forms of nitrogen are returned to the organic form and in so doing become unavailable again. Immobilised nitrogen is reasonably secure in the soil in this form and can continue to be recycled within the soil from organic-mineral-organic states again via the mineralisation, nitrification immobilisation stages. The organic form provides safe storage for the nitrogen supply but in the mineral form, particularly the nitrate form, it is liable to leaching out of the soil system. Immobilisation of nitrogen is a temporary phase in the soil and the nitrogen will again become subject to mineralisation and become available to plants again.

Denitrification is the mechanism that allows the return of nitrogen to the atmosphere thus completing the cycle: atmosphere-fixation in the soil-mineralisation and nitrification for use by plants-denitrification and return to the atmosphere. Denitrification is the reduction of nitrate to gaseous nitrogen, enabling it to pass from the soil to the atmosphere again. The process is performed by various microscopic bacteria, fungi and other organisms. It is a significant process in soils because it is a loss mechanism for one of the soils most precious and needed nutrients and clearly relies on the incoming parts of the cycle to recharge the soil and compensate for what has been lost.

In the past 200 years humans have had an important influence on the nitrogen cycle. For example, changing the system of agriculture to a more intensive one, coupled with the manufacture of large amounts of fertilizer has had a major effect. More land being used to grow crops and deforestation have led to less naturally occurring nitrogen in the soil. In the case of agricultural cropping, for example, crop needs for nitrogen can quickly exceed the nitrogen supplied into the system by natural fixing. This has led to a huge increase in the use of nitrogen fertiliser to supplement the amounts derived from natural processes.The amount of nitrogen fertilizer applied to land, particularly since the 1950s, has tripled in many countries. In many cases this has exceeded crop needs and the surplus has been leached from the soil into the underlying groundwater from where it can enter streams and rivers, leading to eutrophication and potential damage to human health. Further, this increased application has required the manufacture of large amounts of artificial nitrogen and the factories involved have released large amounts of nitrogen to the atmosphere. Humans are thus now affecting the nitrogen cycle on a global scale.