Nitrogen is a fundamental element for life on Earth, forming the basis of many biological structures. Although the atmosphere is nearly four-fifths nitrogen gas, this form is largely inert and unusable by most organisms. The continuous circulation of nitrogen between the atmosphere, soil, water, and living things is known as the nitrogen cycle. This cycle converts atmospheric nitrogen into reactive forms that support ecosystems. However, when human activities disrupt its natural balance, nitrogen can become a widespread pollutant.
Nitrogen’s Essential Biological Function
Nitrogen is a component of every living cell. Its primary biological function is as a building block for two major classes of biomolecules: amino acids and nucleic acids. Amino acids link together to form proteins, which perform almost all cellular functions, from structural support to catalyzing metabolic reactions as enzymes.
Nitrogen is also a constituent of nucleic acids, specifically DNA and RNA, which are the blueprints for life. These molecules store and transmit genetic information, meaning nitrogen is directly involved in growth, reproduction, and all hereditary processes. Beyond these core functions, nitrogen is found in molecules like adenosine triphosphate (ATP), the primary energy-transfer compound in cells, and in chlorophyll, which is necessary for photosynthesis in plants. Plants must absorb nitrogen from the soil, typically as nitrate or ammonium, to synthesize these compounds, forming the base of the nitrogen food web.
The Natural Stages of the Cycle
The process begins with nitrogen fixation, where inert atmospheric nitrogen gas (\(\text{N}_2\)) is converted into biologically usable ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)). This conversion is largely carried out by nitrogen-fixing bacteria, such as Rhizobium, which often live symbiotically in the root nodules of legume plants. A minor amount of nitrogen is also fixed by non-biological means, such as lightning, which creates nitrogen oxides that dissolve in rain.
The next step is nitrification, a two-part process that makes nitrogen more accessible to most plants. First, certain bacteria oxidize ammonia or ammonium into nitrites (\(\text{NO}_2^-\)), which are toxic to plants. A separate group of nitrifying bacteria then rapidly converts the nitrites into nitrates (\(\text{NO}_3^-\)), a form readily taken up by plant roots. This uptake is called assimilation, where plants incorporate the inorganic nitrogen compounds into their own organic tissues, such as proteins and nucleic acids.
When plants and animals die, or when animals excrete waste, the nitrogen compounds are returned to the soil. Decomposers, including fungi and bacteria, break down this organic matter in a process called ammonification. This decomposition releases the nitrogen back into the soil as ammonia or ammonium, making it available for nitrification or direct uptake by plants.
The cycle is completed by denitrification, which removes reactive nitrogen from the ecosystem and returns it to the atmosphere. Denitrifying bacteria, which thrive in anaerobic (oxygen-poor) conditions like waterlogged soils, use nitrate as a substitute for oxygen in their metabolism. This action reduces nitrate back into gaseous nitrogen (\(\text{N}_2\)), releasing it back into the atmosphere and closing the natural loop.
Anthropogenic Sources of Nitrogen Input
Human activity has significantly accelerated the conversion of inert atmospheric nitrogen into reactive forms. The single largest human contribution comes from the industrial creation of synthetic nitrogen fertilizers through the Haber-Bosch process. This energy-intensive chemical reaction combines nitrogen gas with hydrogen gas under high heat and pressure to produce ammonia (\(\text{NH}_3\)).
The Haber-Bosch process allows for the production of over 90 million metric tons of nitrogen fertilizer annually. This massive input has been credited with supporting the global population increase by dramatically boosting crop yields. However, the process is heavily reliant on fossil fuels, consuming an estimated one to two percent of the world’s energy supply.
Another major source of reactive nitrogen is high-temperature combustion in vehicle engines and power plants. Burning fossil fuels causes atmospheric nitrogen and oxygen to react, forming nitrogen oxides (\(\text{NO}_x\)). These compounds are released into the air, contributing to atmospheric pollution and settling onto land and water bodies through atmospheric deposition.
The release of nitrogen through wastewater and sewage also adds to the environmental load. Wastewater treatment facilities that do not have specific nitrogen removal processes release water containing excess nitrogen compounds into rivers and coastal areas. Furthermore, the over-application of synthetic fertilizers in agriculture often results in nitrogen runoff into waterways, as crops cannot absorb all the reactive nitrogen provided.
Environmental Outcomes of Nitrogen Overload
Excess reactive nitrogen introduced by human activities creates negative environmental effects across land and water systems. In aquatic environments, the influx of nitrogen, primarily from agricultural runoff and wastewater, causes eutrophication. Eutrophication is the over-enrichment of water bodies, which triggers rapid growth of algae and aquatic plants, commonly referred to as algal blooms.
When these blooms die, their decomposition by bacteria consumes vast amounts of dissolved oxygen in the water. This oxygen depletion creates hypoxic zones, often called “dead zones,” where fish and other aquatic life cannot survive. Nitrogen oxides released from combustion are also a major component of air pollution, contributing to the formation of smog and ground-level ozone. Ozone at ground level is a respiratory irritant that impairs breathing and can damage plant tissues.
In terrestrial ecosystems, the atmospheric deposition of nitrogen compounds contributes to soil acidification. This acidification can leach important nutrients from the soil, making them less available to plants. Furthermore, during denitrification, some nitrogen is converted into nitrous oxide (\(\text{N}_2\text{O}\)) instead of harmless atmospheric nitrogen. Nitrous oxide is a long-lived greenhouse gas, possessing a warming potential 300 times greater than carbon dioxide over a 100-year period, contributing significantly to climate change.