**1. Introduction**

Fixation of molecular nitrogen is one of the processes determining the biological productivi‐ ty of our planet, which is why its study is one of the primary tasks of modern biology. The nitrogen cycle in nature is one of the key components of biogeochemical cycles of the Earth. The atmosphere consists of almost 80% (by volume) of the chemical element and is its main source. Nitrogen makes up a part of proteins and other molecules that form the basis of the structural organization of all levels of life. Humans and animals need it in the form of ani‐ mal and vegetable protein. Plants need it in the form of salts of nitric acid and ammonium ions. [1-3].

The economic and environmental crisis, the decline in the quality of crop production, and the deterioration of the natural fertility of soil - are all reasons for increased attention to bio‐ logical farming, which focuses on using the potential of natural ecosystems, in particular, ni‐ trogen-fixing microorganisms.

Microorganisms which assimilate molecular nitrogen in the atmosphere are diazotrophs and they have a similar biochemical mechanism of nitrogen fixation. There are two main groups of nitrogen-fixing microorganisms, those that enter into a symbiosis with plants and those living freely in the soil. The second group includes associative nitrogen fixers and microor‐ ganisms, more adapted to a free existence in the soil. The division of nitrogen fixers into freeliving and associative is conditional, since the ability to live freely in the soil is typical to all nitrogen-fixing bacteria, whilst only symbiotic nitrogen fixers are able to assimilate mo‐ lecular nitrogen very closely with plants. Links of cyanobacteria with other organisms can be quite diverse: there are phycobionts in lichens which live in the air-cells of mosses, in the leaves of water ferns, and in other places. It should be noted that the potential of symbiotic nitrogen fixers is significantly higher than of those that are free living [4-7].

© 2013 Sytnikov; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Sytnikov; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The symbiotic and associative systems of plants and diazotrophs are an example of the evo‐ lution of the interaction of living organisms. Their study is particularly relevant with the im‐ plementation of highly productive and environmentally friendly farming. The biological fixation of molecular nitrogen from the air is a process of fixation and assimilation of nitro‐ gen by microorganisms. It is of great practical importance, since the industrial production of chemical nitrogen fertilizer requires significant use of costly energy resources, which by themselves can be harmful for the environment. A comprehensive study of this problem is necessary due to the need to create new and effective biological preparations. The creation and use of biological agents on the basis of nitrogen-fixing microorganisms is the most justi‐ fiable method of increasing the productivity of plants and the quality of their harvest, which allows maintenance of the natural fertility of soils and the ecological balance of the environ‐ ment. Their use makes it possible to regulate the number and activity of beneficial micro‐ flora in the rhizosphere of crops, and to provide plants with nitrogen fixed from the atmosphere. For example, in addressing the shortage of high-grade protein the key role be‐ longs to the soybean. However, the soils on which the crop is grown for the first time [8] usually do not have nodule bacteria compatible with soybean or bacteria number is small (up to 20 per gram of soil).
