Nitrogen fixation converts inert atmospheric nitrogen into compounds usable by living organisms. This essential process enables nitrogen to become part of biological systems. Without this conversion, atmospheric nitrogen would remain inaccessible to most life forms on Earth. The process underpins the productivity of ecosystems.
The Essential Role of Nitrogen
Nitrogen constitutes approximately 78% of Earth’s atmosphere. Despite its abundance, atmospheric nitrogen (N₂) has a very strong triple bond, making it unreactive and unusable by most plants and animals in its gaseous form. Organisms cannot directly break this bond to incorporate nitrogen into their structures. Nitrogen is a component of essential biological molecules, including amino acids (the building blocks of proteins) and nucleic acids like DNA and RNA, which carry genetic information. It is also found in adenosine triphosphate (ATP), a molecule that stores and transfers energy within cells, and in chlorophyll, which plants use for photosynthesis. Without a mechanism to convert atmospheric nitrogen into a usable form, the construction of these molecules would be impossible.
Natural and Industrial Processes
Nitrogen fixation occurs through both natural and industrial processes. Natural atmospheric fixation happens when lightning provides the energy to break nitrogen’s triple bond. This converts nitrogen gas into nitrogen oxides, which dissolve in rainwater and enter the soil as nitrates. This process accounts for a small portion of annual nitrogen fixation.
Biological nitrogen fixation, carried out by microorganisms, is the most significant natural pathway. These microbes convert atmospheric nitrogen into ammonia, making it available for plants and the food web. These natural processes cycle nitrogen through ecosystems.
The Haber-Bosch process is a major industrial intervention. Developed in the early 20th century, it synthesizes ammonia (NH₃) from atmospheric nitrogen and hydrogen gas. It requires high temperatures (400-450°C) and high pressures (150-200 atmospheres) with an iron-based catalyst. The ammonia produced primarily manufactures synthetic fertilizers, which revolutionized global agricultural productivity.
Microbial Architects of Nitrogen Fixation
Microorganisms are central to biological nitrogen fixation, converting atmospheric nitrogen into forms usable by plants. These microbes use the enzyme nitrogenase, which breaks the triple bond of nitrogen gas and combines it with hydrogen to form ammonia. This process requires significant energy and is sensitive to oxygen, needing specific environments.
Symbiotic Fixation
Symbiotic nitrogen fixation involves a mutually beneficial relationship between certain plants and bacteria. Leguminous plants, such as beans, peas, clover, and alfalfa, form root nodules where Rhizobium bacteria reside. The plant provides the bacteria with a protected, low-oxygen environment and carbohydrates. In return, the bacteria convert atmospheric nitrogen into ammonia, which the plant utilizes for growth. This relationship allows legumes to thrive in nitrogen-poor soils and enriches the soil.
Free-Living Fixation
Free-living nitrogen-fixing bacteria also contribute to the global nitrogen supply. These bacteria do not require a host plant and fix nitrogen independently in soil or aquatic environments. Examples include cyanobacteria (Anabaena, Nostoc) and bacterial genera such as Azotobacter and Clostridium.
Global Significance and Human Influence
Fixed nitrogen is essential for ecosystem health, entering food webs and supporting plant growth and other life forms. It cycles through soil, water, and organisms, contributing to soil fertility and the nutrient balance of terrestrial and aquatic systems.
Human activities have significantly altered the global nitrogen cycle, particularly through the widespread use of the Haber-Bosch process. This industrial method produces hundreds of millions of tons of ammonia annually for synthetic fertilizers. These fertilizers have increased agricultural yields, enabling food production for billions worldwide.
However, extensive use of synthetic nitrogen fertilizers has environmental consequences. Excess nitrogen can leach into groundwater and run off into rivers, lakes, and coastal waters. This influx of nutrients can lead to eutrophication, where excessive plant and algal growth depletes oxygen and harms aquatic life. Human activities have effectively doubled the amount of reactive nitrogen cycling through the environment, leading to both benefits in food production and environmental challenges.