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Nitrogen fixation is the process of converting atmospheric nitrogen gas (N2) into ammonia (NH3) or nitrate (NO3-), which can be used by plants and other organisms in the biosphere. There are three main types of nitrogen fixation: biological, atmospheric, and industrial. Biological nitrogen fixation is carried out by certain bacteria and cyanobacteria, which have specialized enzymes called nitrogenase. These organisms, including Rhizobium bacteria that live in the root nodules of legume plants, can convert N2 into ammonia (NH3). Atmospheric nitrogen fixation occurs when high-energy events like lightning cause nitrogen and oxygen molecules in the atmosphere to combine and form nitrogen oxides (NOx). These nitrogen compounds then dissolve in rainwater and convert into nitrate (NO3-) through chemical reactions. Industrial nitrogen fixation primarily refers to the Haber-Bosch process, a synthetic method used to produce ammonia (NH3) from N2 and hydrogen (H2) under high pressure and temperature. The produced ammonia is then used to manufacture fertilizers, which contribute to nitrogen being incorporated into the biosphere.

Assimilation is the process by which plants and other primary producers take up nitrogen compounds, such as ammonia (NH3) and nitrate (NO3-), from the soil and incorporate them into their tissues to build proteins and other organic molecules. Plants primarily assimilate nitrogen in the form of nitrate, which is absorbed by the plant roots and transported to other parts of the plant. Some plants can also directly absorb ammonia. Once inside the plant, nitrate is reduced to nitrite (NO2-) and then to ammonia, which is converted to amino acids and other nitrogen-containing compounds necessary for the plant's growth and development.

Ammonification, also known as mineralization, is the process by which organic nitrogen compounds in dead plants, animals, and other organic materials are broken down by decomposer organisms, such as bacteria and fungi. This process releases ammonia (NH3) and ammonium ions (NH4+) back into the soil as inorganic nitrogen compounds, which can then be used and assimilated by plants or further processed by other nitrogen-transforming microorganisms. These three processes - nitrogen fixation, assimilation, and ammonification - represent the main pathways through which nitrogen gas (N2) becomes incorporated into the biosphere, ensuring the availability of nitrogen compounds for the growth and survival of living organisms.