Nitrogen is an essential element for all life forms. It is built into the structure of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), and is a part of every amino acid that links together to form proteins. Despite its abundance, making up about 78% of the Earth’s atmosphere, atmospheric nitrogen (N₂) exists as an inert gas that most organisms cannot directly utilize. The nitrogen cycle is the continuous mechanism by which this element is chemically transformed and repurposed through the biosphere, converting the unusable atmospheric form into reactive compounds that sustain ecological systems.
Bringing Atmospheric Nitrogen into the System
The process begins with nitrogen fixation, which is the conversion of inert atmospheric N₂ gas into biologically usable compounds, primarily ammonia (NH₃). This chemical transformation is driven by specialized nitrogen-fixing bacteria, which possess the enzyme nitrogenase capable of breaking the strong triple bond in the N₂ molecule.
A highly effective form of fixation occurs through a mutualistic relationship between Rhizobium bacteria and leguminous plants, such as peas, beans, and clover. The bacteria reside within specialized structures on the plant roots called nodules. Here, they convert atmospheric N₂ into ammonium (NH₄⁺), which is transferred directly to the plant. This symbiotic exchange benefits the bacteria by providing carbohydrates, while the plant receives a steady supply of fixed nitrogen.
Once nitrogen has been fixed, plants absorb these inorganic forms from the soil or water, a process called assimilation. The absorbed nitrogen is incorporated into organic compounds, such as amino acids and proteins, becoming part of the plant’s biomass. When animals consume these plants, the nitrogen compounds are transferred up the food chain, integrating the element into their own tissues.
Converting Organic Matter and Ammonia
After organisms excrete waste or die, organic nitrogen is returned to an inorganic state through ammonification in soil and aquatic environments. Bacteria and fungi, acting as decomposers, break down nitrogen-containing organic matter, such as proteins and nucleic acids. This process releases ammonia (NH₃) or ammonium ions (NH₄⁺) into the environment.
The resulting ammonia or ammonium becomes the starting material for nitrification, an aerobic, two-step microbial process. The first step is carried out by chemoautotrophic bacteria, such as Nitrosomonas, which oxidize ammonia or ammonium into nitrite (NO₂⁻). Since nitrite is highly reactive and can be toxic to plants, it is quickly acted upon by a different group of microorganisms.
In the second step of nitrification, bacteria like Nitrobacter oxidize the nitrite (NO₂⁻) further, converting it into nitrate (NO₃⁻). Nitrate is the most readily absorbed and utilized form of nitrogen for the majority of plants. This sequential action ensures that the recycled nitrogen is transformed into the most bioavailable form, ready to be assimilated back into the food web.
Returning Nitrogen to the Atmosphere
The final stage is denitrification, which returns gaseous nitrogen to the atmosphere, completing the cycle. This process is carried out by various facultative anaerobic bacteria, such as Pseudomonas. These bacteria utilize nitrogen compounds as an alternative electron acceptor for respiration when oxygen is scarce, primarily in waterlogged soils and deep sediments.
These denitrifying bacteria convert nitrates (NO₃⁻) back into gaseous forms, first into intermediate compounds like nitrous oxide (N₂O), and finally into inert atmospheric nitrogen (N₂). This step balances the nitrogen budget, preventing the accumulation of fixed nitrogen in the soil and water. By releasing N₂ gas, the element is returned to the atmospheric reservoir.
How Human Activity Alters the Cycle
Modern human activity has disrupted the natural nitrogen cycle, primarily through industrial processes and combustion. The Haber-Bosch process, developed in the early 20th century, allows for the large-scale industrial synthesis of ammonia from atmospheric nitrogen for use in synthetic fertilizers. This process generates over 90 million metric tons of reactive nitrogen annually, significantly increasing the amount of fixed nitrogen entering the environment.
A second disruption comes from the burning of fossil fuels, which releases various nitrogen oxides (NOx) into the atmosphere. These compounds contribute to acid rain and smog, altering the atmospheric nitrogen balance. The widespread use of synthetic fertilizers leads to environmental consequences when excess nitrogen runs off into waterways.
Nitrogen runoff acts as a nutrient pollutant, triggering eutrophication in lakes, rivers, and coastal waters. The influx of nitrogen fuels the growth of algae, known as algal blooms, which then die and are decomposed by bacteria. This decomposition consumes dissolved oxygen, leading to hypoxic conditions, or “dead zones,” that cannot support most aquatic life. Furthermore, intermediates of the cycle, such as nitrous oxide (N₂O), are greenhouse gases contributing to global climate change.