Nitrogen is a fundamental element that underpins all life forms on Earth. While it constitutes approximately 78% of the Earth’s atmosphere, this vast reservoir of nitrogen exists primarily as dinitrogen gas (N₂), a form that is largely inert and unusable by most organisms directly. The transformation of atmospheric nitrogen into reactive forms, such as ammonia and nitrates, is accomplished through a series of natural processes collectively known as the nitrogen cycle. This continuous movement of nitrogen through the atmosphere, soil, water, and living organisms makes it available for biological processes.
Nitrogen’s Role in Human Biology
Nitrogen is an integral component of the human body, ranking as the fourth most abundant element after carbon, hydrogen, and oxygen. It is a building block for amino acids, which form proteins. Proteins perform functions such as building and repairing tissues, facilitating metabolic processes, and supporting immune responses.
Nitrogen is also a constituent of nucleic acids, DNA and RNA. These molecules carry genetic information, essential for cell division, growth, and protein synthesis. Nitrogen is also found in compounds like adenosine triphosphate (ATP), the primary energy currency of cells. Humans acquire nitrogen by consuming plants or animals that have assimilated nitrogen-rich compounds.
Nitrogen and Food Production
The nitrogen cycle plays a significant role in global food security, as nitrogen is a nutrient for plant growth. Plants require nitrogen to synthesize chlorophyll, the pigment responsible for photosynthesis, and to develop healthy tissues, leaves, and fruits. Before the 20th century, agricultural practices relied on natural sources of nitrogen, such as manure and nitrogen-fixing crops like legumes, to enrich soil fertility. However, these natural methods were insufficient to meet the demands of a growing global population.
The Haber-Bosch process, developed in the early 1900s, revolutionized food production. This industrial method converts atmospheric nitrogen into ammonia, which is manufactured into synthetic fertilizers. Widespread application of these fertilizers substantially increased crop yields, enabling a sevenfold rise in global food supply during the 20th century. The Haber-Bosch process currently supports 40% to 50% of the world’s population, as billions would lack food without it.
Environmental Consequences of Nitrogen Imbalance
Human activities have altered the natural nitrogen cycle, leading to environmental imbalances that affect ecosystems and human well-being. One significant consequence is eutrophication, which occurs when excess nitrogen, often from agricultural runoff containing fertilizers and animal waste, enters water bodies. This surplus of nutrients stimulates the rapid growth of algae, leading to dense algal blooms on the water’s surface.
Algal blooms block sunlight, disrupting aquatic ecosystems. When these algal masses die, bacteria consume dissolved oxygen, creating “dead zones” with very low oxygen levels. This leads to widespread mortality of fish and other aquatic life, impacting fisheries and water quality. Some harmful algal blooms also produce toxins detrimental to humans, wildlife, and fish.
Air pollution from nitrogen oxides (NOx) is another concern, formed during high-temperature combustion in vehicles, power plants, and industrial processes. These gases contribute to smog, a brownish haze that reduces visibility and causes respiratory issues. Nitrogen oxides also react with water vapor to form nitric acid, a component of acid rain that harms ecosystems and infrastructure.
The disruption of the nitrogen cycle also contributes to climate change through the release of nitrous oxide (N₂O). Nitrous oxide is a potent greenhouse gas, 265 to 300 times more effective at trapping heat than carbon dioxide over a 100-year period. The primary source of human-caused N₂O emissions is agricultural soil management, including synthetic fertilizers and manure. Nitrous oxide remains in the atmosphere for an average of 121 years and contributes to ozone layer depletion.