- Nitrogen fixation is a vital natural and biochemical process that converts inert atmospheric nitrogen gas (N₂) into biologically usable forms such as ammonia (NH₃), ammonium (NH₄⁺), or nitrate (NO₃⁻). Though nitrogen is abundant in Earth’s atmosphere—making up about 78% of the air—most organisms cannot use it in its gaseous form.
- Nitrogen fixation bridges this gap by making nitrogen available to living organisms, enabling the synthesis of essential biomolecules like amino acids, proteins, nucleic acids, and chlorophyll.
- There are three main types of nitrogen fixation: biological, abiotic (non-biological), and industrial. Among these, biological nitrogen fixation (BNF) is the most ecologically significant and sustainable. It is carried out by certain prokaryotes, including free-living bacteria (like Azotobacter), cyanobacteria (like Anabaena), and especially symbiotic bacteria (notably Rhizobium species) that live in specialized structures called root nodules of leguminous plants. These bacteria contain the enzyme nitrogenase, which breaks the strong triple bond of molecular nitrogen and reduces it to ammonia. The host plant benefits by receiving usable nitrogen, while the bacteria gain carbohydrates and a protected environment from the plant—a classic example of mutualism.
- Abiotic nitrogen fixation occurs naturally through lightning strikes, where the immense energy breaks nitrogen bonds in the atmosphere, allowing nitrogen to combine with oxygen to form nitrogen oxides (NOₓ). These compounds dissolve in rainwater and enter the soil, where they can be absorbed by plants. Although this form of fixation contributes only a small portion of the nitrogen available to ecosystems, it plays a role in areas where biological nitrogen fixers are less prevalent.
- The third form is industrial nitrogen fixation, most notably through the Haber-Bosch process, which was developed in the early 20th century. This method synthesizes ammonia from atmospheric nitrogen and hydrogen gas under high temperatures and pressure, using a metal catalyst. It revolutionized agriculture by enabling the mass production of nitrogen-based fertilizers, significantly boosting global food production. However, excessive fertilizer use has also led to environmental issues such as eutrophication, soil degradation, and increased greenhouse gas emissions.
- Nitrogen fixation is a foundational step in the nitrogen cycle, which circulates nitrogen through the biosphere, atmosphere, and geosphere. After fixation, the usable nitrogen is taken up by plants and then passed through food chains to animals. Eventually, nitrogen is returned to the soil and atmosphere through processes such as ammonification, nitrification, assimilation, and denitrification. This continuous cycling maintains ecosystem productivity and soil fertility.
