Biological Nitrogen Fixation: Mechanism, Microorganisms and Its Role in Sustainable Agriculture

Nitrogen is one of the most essential elements required for the growth, development and survival of living organisms. It plays an important role in the formation of amino acids, proteins, nucleic acids, chlorophyll, enzymes and several other biologically important compounds. Although atmospheric nitrogen is abundant in the form of molecular nitrogen (N₂), most plants are unable to use it directly because the two nitrogen atoms are strongly linked by a stable triple covalent bond. Nitrogen -fixation is therefore an important natural and biological process that converts atmospheric nitrogen into forms that can enter biological systems and support plant growth. Free-living organisms such as Azotobacter, Rhizobium and Clostridium etc. can contribute to nitrogen availability without forming a direct symbiotic relationship with plants. Associative nitrogen fixers occur in close proximity to plant roots and benefit from compounds released by the plant. The interaction between Rhizobium and legumes involves several stages, including recognition of plant signals, production of Nod factors, root-hair responses, formation of infection threads, development of root nodules and differentiation of bacteria into nitrogen-fixing bacteroid. This highly coordinated interaction enables the exchange of nutrients between the plant and microorganism, in which the plant supplies carbon compounds and the bacterial partner contributes fixed nitrogen. Overall, biological nitrogen fixation represents a significant connection between atmospheric nitrogen, microorganisms, plants and soil fertility. A better understanding of nitrogen-fixing organisms, their mechanisms, isolation methods, culture conditions and agricultural applications can support the development of sustainable strategies for improving plant nutrition and maintaining soil health.