Por fijación de nitrógeno se entiende la combinación de nitrógeno molecular o dinitrógeno con oxígeno o hidrógeno para dar óxidos o amonio que pueden incorporarse a la biosfera. El nitrógeno molecular, que es el componente mayoritario de la atmósfera, es inerte y no aprovechable directamente por la mayoría de los seres vivos. La fijación de nitrógeno puede ocurrir de manera abiótica (sin intervención de los seres vivos) o por acción de microorganismos (fijación biológica de nitrógeno). La fijación en general supone la incorporación a la biosfera de una importante cantidad de nitrógeno, que a nivel global puede alcanzar unos 250 millones de toneladas al año, de las que 150 corresponden a la fijación biológica.

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Nitrogen fixation is a process in which nitrogen (N2) in the atmosphere is converted into ammonia (NH3). Atmospheric nitrogen or molecular dinitrogen (N2) 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. Nitrogen fixation, natural and synthetic, is essential for all forms of life because nitrogen is required to biosynthesize 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 an important process in the manufacture of explosives (e.g. gunpowder, dynamite, TNT, etc.). Nitrogen fixation occurs naturally in the soil by nitrogen fixing bacteria affiliated with some plants (for example, Azotobacter and legumes). Some nitrogen-fixing bacteria have very close relationships with plants, referred to as symbiotic nitrogen fixation. 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 lightning. All biological nitrogen fixation is done by way of nitrogenase metalloenzymes which contain iron, molybdenum, or vanadium. Microorganisms that can fix nitrogen are prokaryotes (both bacteria and archaea, distributed throughout their respective kingdoms) called diazotrophs. Some higher plants, and some animals (termites), have formed associations (symbiosis) with diazotrophs.

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