Nitrogen, with the chemical symbol N, is the most abundant gas in our atmosphere, making up about 78% of the air we breathe. In the atmosphere, two nitrogen atoms form a diatomic molecule (N2) with a triple covalent bond. This bond is one of the strongest in nature, which makes the N2 molecule largely inert and unreactive.
This stability means that while nitrogen is abundant, it is not readily available for use by most living organisms. The journey of this element from an inaccessible atmospheric gas to a usable component of life is a complex process.
Nitrogen as a Building Block of Life
Nitrogen is a component of all proteins and nucleic acids. Amino acids, the molecules that form complex proteins, each contain a nitrogen-containing amino group. These proteins perform a vast array of functions, from building tissues to acting as enzymes that facilitate biochemical reactions.
Nitrogen is also integral to the structure of nucleic acids, DNA and RNA, which carry the genetic instructions for all known organisms. Each nucleotide, the building block of these molecules, contains a nitrogenous base.
The availability of usable nitrogen in an ecosystem often dictates the amount of life it can support. Because it is often in short supply, nitrogen is referred to as a limiting nutrient. Its scarcity can restrict the growth of plants, which in turn limits the populations of animals that depend on them for food.
The Natural Nitrogen Cycle
The movement of nitrogen from the atmosphere to the earth, through living organisms, and back to the atmosphere is the nitrogen cycle. This process transforms nitrogen into various chemical forms, making it available to living organisms. The cycle consists of several interconnected stages, facilitated by different types of microorganisms.
Nitrogen Fixation
The first step is nitrogen fixation, the process of converting atmospheric nitrogen (N2) into ammonia (NH3), a form that can be used by plants. This conversion is primarily carried out by specialized microorganisms. Some of these nitrogen-fixing bacteria live freely in the soil, while others, like Rhizobium, live in a symbiotic relationship with legume plants, such as peas and beans. Lightning can also fix small amounts of nitrogen by reacting it with oxygen.
Nitrification
Once nitrogen is fixed as ammonia in the soil, nitrification begins. During this two-step process, specific soil bacteria convert ammonia into nitrites (NO2−) and then into nitrates (NO3−). First, bacteria like Nitrosomonas oxidize ammonia to nitrites, and then other bacteria, such as Nitrobacter, oxidize the nitrites to nitrates. Nitrates are the primary form of nitrogen that plants absorb through their roots.
Assimilation
Assimilation is the process by which plants and animals incorporate usable nitrogen into their bodies. Plants absorb nitrates and ammonium from the soil through their roots and use this nitrogen to synthesize amino acids, proteins, and nucleic acids. Animals obtain nitrogen by consuming plants or other animals, incorporating it into their own bodies.
Ammonification
Ammonification occurs when plants and animals die or excrete waste, returning nitrogen in their organic matter to the soil. Decomposers, like bacteria and fungi, break down the organic molecules in dead organisms and waste. During this decomposition, the nitrogen is converted back into ammonium (NH4+), which can be used again by plants or be converted to nitrates through nitrification.
Denitrification
The final stage is denitrification, which returns nitrogen gas to the atmosphere, completing the cycle. This process is carried out by denitrifying bacteria, such as Pseudomonas, which live in anaerobic, or oxygen-poor, environments. These bacteria use nitrates for respiration, converting them back into N2 gas that is released into the atmosphere.
Human Impact on Nitrogen Levels
For most of Earth’s history, the nitrogen cycle was a balanced system. Over the past century, human activities have altered this balance by introducing large quantities of reactive nitrogen into the environment. The amount of nitrogen fixed by human processes now exceeds the amount fixed by all natural terrestrial processes combined.
The primary driver of this change is modern agriculture and the industrial production of synthetic fertilizers. The Haber-Bosch process, developed in the early 20th century, converts atmospheric nitrogen to ammonia on an industrial scale for crops. While this has increased food production, much of the nitrogen applied to fields is not taken up by plants and enters the wider environment.
The combustion of fossil fuels is another source of nitrogen pollution. At high temperatures, atmospheric nitrogen reacts with oxygen to form nitrogen oxides (NOx). These compounds are released into the atmosphere and contribute to environmental problems.
This overload of reactive nitrogen has consequences. In aquatic ecosystems, nitrogen runoff from agricultural lands leads to eutrophication, where excess nitrogen fertilizes algae. This causes algal blooms that block sunlight, and their decomposition consumes oxygen, creating hypoxic “dead zones” where marine life cannot survive.
In the atmosphere, nitrogen oxides contribute to smog and acid rain, which can damage forests and buildings. Nitrous oxide (N2O), a byproduct of these activities, is a greenhouse gas. It is about 300 times more effective at trapping heat than carbon dioxide over a 100-year period, contributing to global climate change.