Nitrogen (N), atomic number 7, is the most abundant gas in Earth’s atmosphere, constituting approximately 78% of the air. This ubiquitous element plays a foundational role across various natural processes and human endeavors.
Essential Role in Biological Systems
Nitrogen is a building block for all life, indispensable within biological molecules. It is a core component of amino acids, which form proteins. Proteins are essential for cellular structure, function as enzymes, and participate in many cellular processes.
Nitrogen is also present in the nitrogenous bases—adenine, guanine, cytosine, and thymine (or uracil in RNA)—that form DNA and RNA. These nucleic acids carry genetic information, directing protein synthesis and trait inheritance. Nitrogen also contributes to adenosine triphosphate (ATP), the primary energy currency for cellular activities.
Nitrogen is a component of chlorophyll, the green pigment in plants and algae. Chlorophyll enables photosynthesis, converting light energy into chemical energy, which forms the basis of most food webs. Without nitrogen, these processes cannot occur.
The Global Nitrogen Cycle: An Ecological Function
The global nitrogen cycle describes the movement and transformation of nitrogen through Earth’s atmosphere, land, and water. This biogeochemical cycle makes nitrogen available to living organisms and recycles it within ecosystems.
One primary process is nitrogen fixation, where atmospheric nitrogen gas (N2) is converted into usable forms like ammonia or ammonium. This is primarily carried out by specialized bacteria, such as Rhizobium in legume root nodules, and free-living soil bacteria. Lightning strikes also contribute by converting atmospheric nitrogen into nitrogen oxides that dissolve in rain and enter the soil.
Nitrification then occurs, where soil bacteria convert ammonium into nitrites and nitrates. Plants absorb these nitrates and ammonium through assimilation. When organisms die or excrete waste, decomposers break down organic nitrogen. This process, called ammonification, releases ammonium back into the soil for nitrification or plant uptake.
The cycle concludes with denitrification, where denitrifying bacteria convert nitrates back into atmospheric nitrogen gas. This returns nitrogen to the atmosphere, completing the natural loop and ensuring a steady supply for life.
Diverse Functions in Industry
Humans harness nitrogen for its properties in a wide range of industrial and technological applications. In agriculture, nitrogen is a nutrient in fertilizers, primarily as ammonia and nitrates. This boosts agricultural productivity and crop yields worldwide.
Nitrogen serves as an inert gas, displacing oxygen to prevent oxidation. This makes it valuable in food packaging to preserve freshness, in electronics manufacturing to protect components, and in chemical processing for non-reactive environments. Its lack of reactivity helps maintain product integrity.
Liquid nitrogen is a coolant due to its extremely low boiling point. This cryogenic property is used for rapid freezing, medical cryopreservation, and maintaining low temperatures in scientific research.
Nitrogen also plays a role in the production of explosives like nitroglycerin and TNT, where its chemical properties enable rapid energy release. It is used in car airbags and contributes to the creation of alloys, including stainless steel.
Nitrogen’s Influence on Environmental Systems
Nitrogen compounds influence environmental systems, especially when human activities alter their natural balance. In the atmosphere, nitrogen is the primary diluent of oxygen, moderating its reactivity. However, nitrogen oxides (NOx) from combustion processes, like vehicle emissions and industrial activities, contribute to air quality issues.
These nitrogen oxides are precursors to ground-level ozone, a harmful component of smog affecting respiratory health. They also contribute to acid rain by forming nitric acid, which can damage forests and aquatic ecosystems. Nitrous oxide (N2O), released from agricultural soils and fossil fuel combustion, is a greenhouse gas, contributing to climate change.
In aquatic environments, excess nitrogen, often from agricultural runoff, sewage, and industrial discharge, is a nutrient pollutant. This excess stimulates rapid algal growth, leading to algal blooms. When these algal mats die and decompose, they consume dissolved oxygen, creating “dead zones” where aquatic life cannot survive due to oxygen depletion.