Nitrogen cycle

The nitrogen (N 2 ) is an extremely important chemical element for living beings, as part of the composition of organic molecules such as amino acids that make up proteins, and nucleic acids. The atmosphere is composed of about 80% of nitrogen in the form of gas, and vegetables can only use it in the form of ammonia (NH 3 ) or nitrate (NO – 3 ), and animals in the form of amino acids. Thus, we can observe that, with the exception of some microorganisms, living beings are unable to use this gas in the form in which it is available in the atmosphere.

The nitrogen available in the atmosphere is mainly fixed by the cyanobacteria Nostoc and Anabaena , and by bacteria that live free in the soil (of the genus Azotobacter ) or are associated with the roots of leguminous plants (genus Rhizobium ). There are also certain fungi that associate with plant roots that are not legumes and form nodules on the roots of these plants. We call this mycorrhiza association, but remember that not all mycorrhiza are formed by fungi.

All of these microorganisms transform the nitrogen available in the atmosphere into ammonia. In the roots of leguminous plants we can find numerous nodules that are formed by millions of bacteria of the genus Rhizobium. These bacteria fix nitrogen, which reacts with hydrogen to produce ammonia. Part of this ammonia is supplied to the plant and the other part is released into the soil. Thus, we can affirm that there is a mutualism relationship between the plant and the bacteria, since the bacteria acts as a live fertilizer when supplying nitrogen to the plant, while the plant provides shelter and food for the bacteria.

The ammonia that is available in the soil comes from nitrogen fixation by fixing microorganisms, but another part comes from the decomposition of proteins, nucleic acids and nitrogenous residues present in excreta and cadavers. When the transformation of nitrogen into ammonia occurs, carried out by bacteria, fungi and decomposers, we call this ammonification process . In this process, bacteria and fungi obtain the energy necessary for their metabolism, with decomposition being the product of the cellular respiration of these microorganisms.

There are plants that can use the ammonia available in the soil, while others can use nitrate (NO – 3 ). The process that forms ammonia into nitrates is called nitrification . The nitrification process is divided into two stages, the first stage being called nitrosation . In nitrosation, part of the ammonia not used by the plants is oxidized to nitrite by nitrifying bacteria of the genus Nitrosomonas, Nitrosococcus and Nitrosolobus. In this process, energy is released and these bacteria use it in the production of organic compounds.

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