The nitrogen cycle is a biogeochemical process describing how nitrogen moves through the atmosphere, soil, and living organisms. Nitrogen is an indispensable element for life, serving as a fundamental building block for all amino acids, which form proteins, and for the nucleic acids that constitute DNA and RNA. Despite making up about 78% of the Earth’s atmosphere as dinitrogen gas (\(\text{N}_2\)), this form is chemically inert and unusable by most life forms. This means that the vast atmospheric reservoir must be converted into biologically available compounds like ammonia, nitrites, or nitrates before it can support life, a transformation driven almost entirely by specialized microorganisms. The cycle essentially functions as a planetary recycling system, continually transforming nitrogen from its inert atmospheric state into usable forms and eventually back again.
Nitrogen Fixation: Making Atmospheric Nitrogen Usable
The initial conversion of atmospheric dinitrogen gas (\(\text{N}_2\)) into ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)) is called nitrogen fixation, and it is the most energy-intensive step of the entire cycle. The \(\text{N}_2\) molecule is held together by a strong triple bond, requiring a large input of energy to break it apart. Only a select group of prokaryotes, collectively known as diazotrophs, possess the enzyme system called nitrogenase, which can catalyze this conversion at ambient temperatures and pressures.
Biological fixation is performed by both free-living bacteria in the soil, such as Azotobacter and cyanobacteria, and by symbiotic bacteria. The most well-known symbiotic fixers are Rhizobium species, which form a mutualistic relationship within the root nodules of legumes. The nitrogenase enzyme itself is highly sensitive to oxygen, requiring these bacteria to employ specific mechanisms to keep oxygen levels low enough for the enzyme to function.
A small fraction of nitrogen fixation occurs through abiotic processes, primarily via lightning, which provides the immense energy needed to break the \(\text{N}_2\) triple bond. The electrical discharge causes nitrogen and oxygen in the air to react, forming nitrogen oxides (\(\text{NO}_x\)), which then dissolve in rainwater and fall to the earth as nitrates. However, the vast majority of fixed nitrogen available to ecosystems originates from the biological activity of nitrogen-fixing microorganisms.
Nitrification: Converting Ammonia to Nitrates
Once atmospheric nitrogen has been fixed into ammonia, it enters the soil pool, where it is rapidly converted through a two-step process known as nitrification. This process is performed by distinct groups of chemoautotrophic bacteria and is an aerobic reaction, meaning it requires the presence of oxygen. The first stage involves the oxidation of ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)) into nitrite (\(\text{NO}_2^-\)).
Bacteria from the genus Nitrosomonas are the primary drivers of this first step, gaining energy for their own metabolism. Since nitrite (\(\text{NO}_2^-\)) is toxic to most plants, the second stage of nitrification must quickly follow. This second step is the oxidation of nitrite into nitrate (\(\text{NO}_3^-\)), which is the most easily absorbed and preferred form of nitrogen for many non-leguminous plants.
Bacteria such as Nitrobacter and Nitrococcus carry out this final conversion. The overall nitrification process transforms potentially toxic ammonium into the highly mobile and readily usable nitrate form, making it available for plant uptake in the soil.
Assimilation and Ammonification: Incorporating and Recycling Nitrogen
The newly formed inorganic nitrogen compounds, primarily nitrate and ammonium, are then taken up by plants in a process called assimilation. Plants absorb these nutrients through their roots and incorporate the nitrogen atoms into complex organic molecules, such as amino acids, proteins, and DNA. This is the stage where inorganic nitrogen enters the living food web, as animals acquire their nitrogen by consuming plants or other animals.
Nitrogen remains in this organic form as long as the organism is alive. Once organisms die, or when animals excrete waste, the organic nitrogen is returned to the soil through ammonification, also known as mineralization. This recycling process is carried out by decomposers, including various bacteria and fungi, which break down the complex organic matter.
The breakdown of this organic material releases the nitrogen back into the soil in the form of ammonium (\(\text{NH}_4^+\)). This ammonium then re-enters the cycle, becoming available for plant assimilation or for the nitrifying bacteria to begin the nitrification process again. Ammonification prevents nitrogen from being locked away in dead biomass, ensuring a continuous supply of this nutrient for the ecosystem.
Denitrification: Returning Nitrogen to the Atmosphere
The final step in the complete nitrogen cycle is denitrification, which closes the loop by returning fixed nitrogen back to the atmosphere as dinitrogen gas (\(\text{N}_2\)). This process is carried out by various facultative anaerobic bacteria, such as Pseudomonas and Thiobacillus, which thrive in environments with low or no oxygen. Under these anaerobic conditions, these microorganisms use nitrate (\(\text{NO}_3^-\)) as an alternative electron acceptor instead of oxygen for their respiration.
Denitrification is a stepwise reduction process where nitrate is sequentially converted to:
- Nitrite (\(\text{NO}_2^-\))
- Nitric oxide (\(\text{NO}\))
- Nitrous oxide (\(\text{N}_2\text{O}\))
- Dinitrogen gas (\(\text{N}_2\))
The conditions required for this process, such as waterlogged soils, wetlands, or deep ocean sediments, are those where oxygen consumption exceeds the rate of oxygen supply. The resulting \(\text{N}_2\) gas is released back into the atmosphere, completing the cycle.
This step is an important counter-balance to nitrogen fixation, preventing the excessive buildup of fixed nitrogen compounds in ecosystems. Denitrification can also inadvertently release nitrous oxide (\(\text{N}_2\text{O}\)), which is a potent greenhouse gas and an ozone-depleting substance.