<P> Nitrogen fixation is a process by which nitrogen in the Earth's atmosphere is converted into ammonia (NH) or other molecules available to living organisms . Atmospheric nitrogen or molecular dinitrogen (N) is relatively inert: it does not easily react with other chemicals to form new compounds . The fixation process frees nitrogen atoms from their triply bonded diatomic form, N ≡ N, to be used in other ways . </P> <P> Nitrogen fixation is essential for some forms of life because inorganic nitrogen compounds are required for the biosynthesis of the basic building blocks of plants, animals and other life forms, e.g., nucleotides for DNA and RNA, the coenzyme nicotinamide adenine dinucleotide for its role in metabolism (transferring electrons between molecules), and amino acids for proteins . Therefore, as part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer . It is also, indirectly, relevant to the manufacture of all chemical compounds that contain nitrogen, which includes explosives, most pharmaceuticals, dyes, etc . Nitrogen fixation is carried out naturally in the soil by nitrogen fixing bacteria such as Azotobacter . Some nitrogen - fixing bacteria have symbiotic relationships with some plant groups, especially legumes . Looser relationships between nitrogen - fixing bacteria and plants are often referred to as associative or non-symbiotic, as seen in nitrogen fixation occurring on rice roots . It also occurs naturally in the air by means of NOx production by lightning . </P> <P> All biological nitrogen fixation is done by way of metalloenzymes called nitrogenases . These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium . Microorganisms that can fix nitrogen are prokaryotes (both bacteria and archaea, distributed throughout their respective Domains) called diazotrophs . Some higher plants, and some animals (termites), have formed associations (symbiosis) with diazotrophs . </P>

How nitrogen fixation takes place in the soil