This process is also an important duty of bacteria in the soil. Nitrogen is present in the air with approximately 78%, and thus represents an inexhaustible source for the plants. Since the plants are not able to receive the nitrogen directly from the air, the microorganisms are the ones that help them in this process. We distinguish symbiotic and non-symbiotic nitrogen fixation.
The symbiotic nitrogen fixationis an important process for supplying the plants with nitrogen from the air, through the symbiotic bacteria that live in the roots of the plant. These bacteria perform root damage in which they settle and with their ability, they fix the essential nitrogen. Most of the nitrogen components created in this way are available for the plants after the end of the vegetation period of the plants where these bacteria become parasitic.
The non-symbiotic nitrogen fixation – is microbiological process, in which basic nitrogen from the air is introduced into the ground through specific bacteria, which do not live in the plant roots, near them. They feed on substances excreted from the roots and during process of their multiplication and dying, they leave behind them large amounts of nitrogen compounds. These compounds become mineralized immediately after the death of bacteria. The advantage of this process is that here it is not necessary for the plant to finish with the vegetation period in order to start using the nourishing compounds, because they are immediately available to the plant. Processes of symbiotic and non-symbiotic nitrogen fixation continue with processes of ammonification, nitrification and denitrification.
Ammonification is a mineralization of organic substances until reaching the level of ammonia. Ammonia remains in bacterial cells for satisfying their needs, until the ratio of C: N become greater than 20: 1. It will be separated when this ratio will drop down. After that, the ammonia will be reduced through the nitrification process.
Nitrification is a process of microbiological oxidation of ammonia into nitrates through nitrites. Since nitrates are quite movable and the oxidation represents a source of energy for the bacteria and possibility for creation of carbonic acid, its role has already been explained. Due to its great mobility, (N) is lost in the soil very quickly.
Denitrification is an utterly reversed process of nitrification. This is a microbiological process, which takes place under the influence of some soil bacteria, where(N) is returned in the air in the same way as it has reached the soil (circulation of matter).