“Free nitrogen” refers to the nitrogen gas (N2) that constitutes approximately 78% of Earth’s atmosphere. This abundant gas exists as a diatomic molecule, composed of two nitrogen atoms bonded together. It is termed “free” because it is not chemically combined with other elements or compounds.
Characteristics of Atmospheric Nitrogen
Atmospheric nitrogen (N2) is a diatomic molecule characterized by an exceptionally strong triple covalent bond between its two nitrogen atoms. This triple bond makes the N2 molecule highly stable and largely unreactive under normal conditions. Despite its considerable abundance, forming about 78% of the air we breathe, it remains chemically inert. The term “free” emphasizes that this nitrogen is not bound within compounds but is a standalone gas in the atmosphere.
Why It’s Not Directly Usable
The primary reason free nitrogen is not directly usable by most living organisms stems from the immense strength of the triple bond holding the two nitrogen atoms together. Breaking this bond requires a substantial amount of energy, which most biological systems cannot provide. Organisms lack the specialized enzymatic machinery to split the N2 molecule and incorporate its nitrogen into organic compounds. Consequently, this vital element, a building block for proteins and nucleic acids, remains largely inaccessible in its atmospheric form.
Transforming Free Nitrogen into Usable Forms
Since free nitrogen is unusable by most life forms, it must undergo transformation into reactive compounds through a process known as nitrogen fixation. This essential conversion occurs through several distinct pathways.
Biological Nitrogen Fixation
Biological nitrogen fixation is primarily carried out by specific microorganisms, notably certain bacteria. These include symbiotic bacteria, like Rhizobium, which form a mutually beneficial relationship with leguminous plants such as peas and beans, residing in specialized root nodules.
Free-living nitrogen-fixing bacteria, such as Azotobacter and cyanobacteria, also convert atmospheric nitrogen directly into ammonia (NH3) in the soil or aquatic environments. This biological process accounts for the majority of natural nitrogen fixation, providing a foundational input for ecosystems.
Atmospheric Nitrogen Fixation
Atmospheric nitrogen fixation occurs naturally during lightning strikes. The extreme energy from a lightning bolt is sufficient to break the strong triple bond of nitrogen molecules. The liberated nitrogen atoms then react with oxygen in the air to form nitrogen oxides. These nitrogen oxides dissolve in rainwater and are deposited onto Earth’s surface, making a small but consistent contribution to usable nitrogen in soils.
Industrial Nitrogen Fixation
Industrial nitrogen fixation, primarily through the Haber-Bosch process, represents a significant human intervention in the nitrogen cycle. Developed in the early 20th century, this process combines atmospheric nitrogen with hydrogen under high temperatures (400-650°C) and immense pressures (200-400 atmospheres) using an iron catalyst. The output is ammonia, which is then used on a massive scale to produce synthetic fertilizers, thereby supporting global agricultural production.
Its Role in Earth’s Cycles
Once free nitrogen is converted into usable forms, it enters the global nitrogen cycle, becoming available to living organisms. Plants absorb these fixed nitrogen compounds, primarily as nitrates or ammonium, from the soil. This nitrogen is then incorporated into organic molecules, serving as a fundamental building block for amino acids, proteins, and nucleic acids. Animals acquire nitrogen by consuming plants or other animals, integrating it into their own biological structures.
Nitrogen continuously moves through various forms and reservoirs within this cycle. When plants and animals die, or when animals excrete waste, decomposers such as bacteria and fungi break down organic nitrogen compounds. This process releases nitrogen back into the soil, often as ammonium, which can then be converted into nitrites and nitrates by nitrifying bacteria. The cycle is completed by denitrification, where specific bacteria convert nitrates back into gaseous free nitrogen (N2), releasing it into the atmosphere under anaerobic conditions.