The Earth’s atmosphere contains nitrogen gas (N₂), making up approximately 78% of the air we breathe. This colorless, odorless gas has a strong triple bond between its two nitrogen atoms. This bond makes atmospheric nitrogen highly stable and unreactive under most natural conditions.
Despite its abundance, this inert form of nitrogen is largely unusable by living organisms. For nitrogen to support life, it must undergo a conversion process, known as nitrogen fixation, into more reactive forms such as ammonia or nitrates. This transformation makes this element available for biological processes.
Nitrogen Removal by Microorganisms
Biological nitrogen fixation, carried out by certain microorganisms, accounts for the most significant natural pathway of nitrogen removal from the atmosphere. These bacteria convert atmospheric nitrogen gas (N₂) directly into ammonia (NH₃). This conversion is facilitated by an enzyme complex called nitrogenase.
Nitrogen-fixing bacteria exist in two categories: free-living and symbiotic. Free-living bacteria, such as Azotobacter and certain cyanobacteria like Nostoc and Anabaena, reside independently in soil or water. Symbiotic bacteria, exemplified by Rhizobium, form mutually beneficial relationships with plants, particularly legumes (e.g., peas, beans, and clover). These bacteria inhabit specialized structures called root nodules on legume plants, where nitrogen fixation occurs.
The nitrogenase enzyme, which catalyzes this reaction, is sensitive to oxygen and requires an anaerobic environment to function effectively. In legume root nodules, a protein called leg-hemoglobin helps maintain these low-oxygen conditions. The conversion of nitrogen to ammonia is an energy-intensive process, demanding energy, for instance, 16 molecules of ATP for each molecule of nitrogen converted. Biological nitrogen fixation contributes approximately 65% of the nitrogen input into terrestrial ecosystems.
Nitrogen Removal by Lightning
Lightning represents another natural process that removes nitrogen from the atmosphere, albeit to a lesser extent than biological fixation. The energy discharged during a lightning strike is sufficient to break the triple bond of atmospheric nitrogen molecules. This high-energy environment allows nitrogen atoms to combine with oxygen, forming various nitrogen oxides (NOx), primarily nitric oxide (NO) and nitrogen dioxide (NO₂).
These nitrogen oxides dissolve in atmospheric moisture, leading to the formation of nitrates (NO₃⁻) and nitrites (NO₂⁻). These nitrogen compounds are carried down to the Earth’s surface through precipitation, enriching the soil and water with usable nitrogen. While contributing less to overall nitrogen removal than microbial processes, lightning plays a role in the global nitrogen cycle and can influence atmospheric chemistry.
Human-Engineered Nitrogen Removal
Human ingenuity has developed a significant process for removing nitrogen from the atmosphere, known as the Haber-Bosch process. This industrial method synthesizes ammonia (NH₃) directly from atmospheric nitrogen and hydrogen gas. German chemists Fritz Haber and Carl Bosch pioneered this process in the early 20th century, with Haber developing the laboratory method around 1909 and Bosch scaling it up for industrial production by 1913.
The Haber-Bosch process involves reacting nitrogen (N₂) and hydrogen (H₂) in a 1:3 ratio under specific conditions: high temperatures, ranging from 400 to 500 degrees Celsius, and high pressures, between 150 and 300 atmospheres. An iron-based catalyst facilitates this reaction, converting these gases into ammonia. This process revolutionized agriculture by enabling the large-scale production of synthetic nitrogen fertilizers.
The widespread availability of these fertilizers has impacted global food production. It is estimated that perhaps half of the world’s population relies on crops grown with the aid of these synthetic fertilizers. The Haber-Bosch process adds approximately 165 million tonnes annually of reactive nitrogen to agricultural soils, supplementing natural nitrogen inputs.
Why Removing Nitrogen Matters
Once atmospheric nitrogen is removed and converted into forms like ammonia or nitrates, it transforms from an inert gas into a usable nutrient. This converted nitrogen is a building block for all life on Earth. It forms a core component of amino acids, the fundamental units that construct proteins. Proteins are involved in biological functions, including growth, tissue repair, and metabolic processes that sustain organisms.
Nitrogen is also a significant constituent of nucleic acids, specifically DNA and RNA, which carry the genetic information necessary for life. It is also required for the synthesis of chlorophyll in plants, the green pigment essential for photosynthesis. Photosynthesis is the process by which plants convert sunlight into energy, forming the base of most food webs.
The availability of usable nitrogen directly supports plant growth, which in turn underpins nearly all ecosystems. Without these natural and human-engineered processes to convert atmospheric nitrogen into bioavailable forms, plants would struggle to thrive, leading to significant limitations in food production and, consequently, the sustenance of life on the planet.