The nitrogen cycle describes how nitrogen moves between the atmosphere, the biosphere, and the geosphere. Nitrogen is a fundamental element for all life on Earth, yet the vast reservoir of nitrogen gas ($N_2$) in the atmosphere is chemically inert and unusable by most organisms. Nitrogen is a core component of amino acids, the building blocks of all proteins, and the nitrogenous bases that form deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
The Essential Steps Transforming Nitrogen
The cycle begins with nitrogen fixation, which converts inert atmospheric nitrogen ($N_2$) into a reactive form, ammonia ($NH_3$). This conversion requires energy to break the triple bond holding the two nitrogen atoms together. Biological fixation is primarily carried out by specialized microorganisms called diazotrophs, which possess the enzyme nitrogenase. These microbes, including free-living bacteria like Azotobacter and symbiotic bacteria such as Rhizobium found in legume root nodules, are responsible for fixing approximately 90% of the nitrogen that enters the biosphere.
Once ammonia is introduced into the soil, nitrification begins, converting it into forms that plants can more easily absorb. First, ammonia ($NH_3$) or ammonium ($NH_4^+$) is converted into nitrite ($NO_2^-$) by ammonia-oxidizing bacteria, such as Nitrosomonas.
Next, nitrite is oxidized into nitrate ($NO_3^-$) by nitrite-oxidizing bacteria, including Nitrobacter. Nitrate is highly water-soluble and is the most common form of nitrogen taken up by plants during assimilation.
Assimilation occurs when plants absorb these inorganic nitrogen compounds, primarily nitrate and ammonium, and incorporate them into organic molecules like amino acids and nucleic acids. When organisms excrete waste or die, ammonification occurs. Decomposers, including various bacteria and fungi, break down organic nitrogen compounds and return the nitrogen to the soil as ammonium ($NH_4^+$).
Denitrification closes the loop by returning nitrogen gas to the atmosphere. Denitrifying bacteria utilize nitrates as an electron acceptor in an anoxic environment, reducing the nitrate molecules back into atmospheric nitrogen ($N_2$). This biological pathway is the primary mechanism that prevents nitrogen from accumulating in the oceans and soils.
How Nitrogen Fuels Life on Earth
Nitrogen functions as a limiting nutrient, frequently restricting growth in many terrestrial and marine ecosystems. Despite the atmosphere being 78% nitrogen gas, the energy barrier required to break the $N_2$ triple bond means that the rate of biological nitrogen fixation governs the availability of this element for all primary producers.
Nitrogen is required for the synthesis of proteins, including enzymes, hormones, and cell membrane components. Nitrogen is also a constituent of chlorophyll, the molecule that captures light energy during photosynthesis. Without sufficient nitrogen, a plant cannot produce enough chlorophyll, leading to stunted growth and reduced productivity.
In aquatic ecosystems, nitrogen is commonly the limiting nutrient for the growth of phytoplankton, which form the base of the marine food web. When nitrogen is absorbed by primary producers, it moves through the food web as consumers eat plants and other organisms. This transfer of nitrogen sustains higher life forms, cycling the element through the ecosystem before it is returned to the soil or atmosphere.
Human Impact on the Global Nitrogen Balance
Human activity has significantly altered the global nitrogen balance, primarily through industrial processes that create reactive nitrogen faster than natural systems can process it. The most impactful change came with the development of the Haber-Bosch process in the early 20th century. This industrial method synthesizes ammonia ($NH_3$) directly from atmospheric nitrogen and hydrogen gas under high heat and pressure.
This process allowed for the mass production of synthetic nitrogen fertilizers, which dramatically increased crop yields and is estimated to support the food supply for billions of people. However, the Haber-Bosch process is highly energy-intensive, relying heavily on natural gas and other fossil fuels. This reliance contributes to greenhouse gas emissions.
The combustion of fossil fuels in vehicles and power generation generates nitrogen oxides ($NO_x$), which are reactive forms of nitrogen. These $NO_x$ compounds contribute to the formation of ground-level ozone (smog) and react with water vapor to create nitric acid, a component of acid rain. Fertilizer production and the agricultural use of nitrogen also release nitrous oxide ($N_2O$), a potent greenhouse gas.
The overuse of synthetic fertilizers results in excess reactive nitrogen compounds, such as nitrates, leaching from agricultural fields into groundwater and surface waters. This nutrient overload in aquatic ecosystems, known as eutrophication, fuels the rapid growth of algae. The subsequent decomposition of this massive algal bloom consumes dissolved oxygen, creating hypoxic conditions, or “dead zones,” that are incapable of supporting fish and other marine life.