The nitrogen cycle is the natural process by which nitrogen moves between the atmosphere, the soil, and living organisms. Nitrogen is a building block for all life, serving as a component of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and proteins. Although the atmosphere is approximately 78% nitrogen gas (\(text{N}_{2}\)), this dinitrogen form is chemically inert and cannot be used directly by most biological systems. The cycle involves microbial transformations that convert this unusable atmospheric gas into forms like ammonia, nitrites, and nitrates, which are biologically available for plants and other life forms. This continuous circulation is fundamental.
Nitrogen Fixation
Nitrogen fixation is the first step in the cycle, converting atmospheric nitrogen (\(text{N}_{2}\)) into a reactive form, primarily ammonia (\(text{NH}_{3}\)) or ammonium (\(text{NH}_{4}^{+}\)). This transformation is necessary because the \(text{N}_{2}\) molecule is held together by a strong triple covalent bond, requiring substantial energy to break. The most significant amount of fixation, known as biological fixation, is carried out by specialized microorganisms called diazotrophs.
These bacteria, including free-living species and symbiotic bacteria like Rhizobium found in legume root nodules, possess an enzyme complex called nitrogenase. Nitrogenase is the only known enzyme capable of breaking the \(text{N}equivtext{N}\) bond to yield ammonia. Because the enzyme is highly sensitive to oxygen, the bacteria must maintain a low-oxygen environment to protect the nitrogenase complex. A smaller amount of fixation also occurs through high-energy abiotic processes, such as lightning, which converts atmospheric nitrogen into nitrogen oxides that dissolve in rain.
Nitrification
Once ammonia or ammonium is present in the soil, the two-step process of nitrification begins, making the nitrogen more accessible to plants. This process is carried out by soil-dwelling bacteria under aerobic, oxygen-rich conditions. The first step, called nitritation, involves the oxidation of ammonium (\(text{NH}_{4}^{+}\)) to nitrite (\(text{NO}_{2}^{-}\)) by bacteria such as Nitrosomonas.
Since nitrite is generally toxic to plants, the second step, known as nitratation, quickly follows. Bacteria such as Nitrobacter oxidize the nitrite (\(text{NO}_{2}^{-}\)) into nitrate (\(text{NO}_{3}^{-}\)). Nitrates are highly water-soluble and represent the most readily absorbed and utilized form of nitrogen for most plant life. These nitrifying bacteria derive the energy for their metabolic processes from these chemical oxidation reactions.
Assimilation and Ammonification
Nitrogen compounds generated by fixation and nitrification are incorporated into living systems through assimilation. Plants absorb nitrate (\(text{NO}_{3}^{-}\)) and ammonium (\(text{NH}_{4}^{+}\)) through their root systems, integrating the nitrogen into complex organic molecules like amino acids, proteins, and nucleic acids. Animals acquire nitrogen by consuming plants or other animals, incorporating the organic nitrogen into their own tissues.
The return of nitrogen to the soil occurs through ammonification, also known as mineralization. When plants and animals die, or when animals excrete waste, decomposers like bacteria and fungi break down the complex organic nitrogen compounds. Enzymes released by these microorganisms convert the organic nitrogen in tissues and waste back into inorganic ammonium (\(text{NH}_{4}^{+}\)). This replenishes the soil’s pool of ammonium, making it available for the subsequent nitrification step.
Denitrification
Denitrification is the final stage in the cycle, providing the mechanism for returning nitrogen to the atmosphere. This process is the reverse of nitrification, converting nitrate back into gaseous forms of nitrogen. Denitrification is performed by facultative anaerobic bacteria, such as Pseudomonas, that thrive in environments where oxygen is scarce or completely absent.
In low-oxygen conditions, typically found in waterlogged soils or deep sediments, these bacteria use nitrate (\(text{NO}_{3}^{-}\)) instead of oxygen as a terminal electron acceptor for their respiration. The bacteria sequentially reduce the nitrate, producing intermediate gaseous compounds like nitric oxide and nitrous oxide, before ultimately forming dinitrogen gas (\(text{N}_{2}\)). This gaseous \(text{N}_{2}\) then escapes back into the atmosphere, representing a net loss of bioavailable nitrogen from the soil, which helps maintain the overall balance of nitrogen in the environment.