Nitrogen is an element required for life, serving as an atomic component for fundamental biological molecules. Its presence is necessary for the construction of all proteins (built from amino acids) and all nucleic acids (DNA and RNA). The movement of nitrogen through the environment, known as the nitrogen cycle, involves a continuous transformation between various chemical forms. These transformations are predominantly mediated by living organisms, particularly specialized microbes, making the cycle a fundamentally biotic process. This article focuses on the forms of nitrogen handled, utilized, or produced by these biological components.
Organic Nitrogen: The Building Blocks of Life
Organic nitrogen represents the final, assimilated form of the element once it has been incorporated into the tissues of a living organism. This form is characterized by nitrogen atoms bound to carbon frameworks, creating complex organic compounds. The nitrogen found within amino acids, the subunits of proteins, and the nucleotide bases of genetic material falls into this category. Nitrogen is also an integral part of the chlorophyll molecule, which is essential for photosynthesis in plants.
Plants and microbes synthesize this organic nitrogen from simpler inorganic forms acquired from the environment. Consumers, such as animals, cannot perform this synthesis and must obtain organic nitrogen by eating other organisms. When any organism dies, or when waste products like urea are excreted, this complex organic nitrogen re-enters the soil. Decomposer organisms, including various bacteria and fungi, break down these complex molecules, making the nitrogen available for recycling.
Assimilable Inorganic Forms (Ammonium and Nitrate)
The primary forms of nitrogen that plants can readily absorb and use for growth are the simple inorganic ions, ammonium (\(\text{NH}_4^+\)) and nitrate (\(\text{NO}_3^-\)). Assimilation is the process where plants take these ions from the soil and convert them into the organic nitrogen required for their structures. Both forms are absorbed from the soil through specialized transporter proteins located in the plant’s roots.
Nitrate (\(\text{NO}_3^-\)) is often the predominant form available in well-aerated, aerobic soils. Once inside the plant cell, nitrate must undergo a two-step reduction process: first to nitrite (\(\text{NO}_2^-\)) by nitrate reductase, and then to ammonia (\(\text{NH}_3\)) by nitrite reductase. This reduction requires energy and is a tightly regulated metabolic pathway, often occurring in the leaves.
Ammonium (\(\text{NH}_4^+\)) is a readily available form that requires less energy for incorporation into organic compounds. However, high concentrations of free ammonium can be toxic to plant cells, meaning it must be rapidly incorporated into amino acids upon uptake. This swift incorporation is achieved through the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway. In anaerobic or waterlogged soils, where oxygen is scarce, ammonium tends to be the more abundant form of nitrogen for plant uptake.
Intermediary Forms and Their Microbial Creators
The transformation of nitrogen between its organic and assimilable inorganic states involves several transient forms created by specialized microbial groups. Ammonia (\(\text{NH}_3\)) is the first stable product generated during the two fundamental biotic processes: nitrogen fixation and ammonification. Nitrogen fixation is the energy-intensive conversion of atmospheric dinitrogen gas (\(\text{N}_2\)) into ammonia by diazotrophs, which include free-living bacteria like Azotobacter and symbiotic bacteria like Rhizobium.
Symbiotic Rhizobium bacteria reside within specialized root nodules of leguminous plants, converting \(\text{N}_2\) into ammonia using the nitrogenase enzyme complex. The ammonia produced during fixation is immediately protonated in the soil or water to form the ammonium ion (\(\text{NH}_4^+\)). Ammonification is the second process that yields ammonia, occurring when decomposer bacteria and fungi break down the organic nitrogen in dead matter and waste products.
Another intermediary form is nitrite (\(\text{NO}_2^-\)), which is a temporary and often toxic compound. Nitrite is produced during nitrification, a two-step process performed by chemotrophic bacteria that gain energy from the chemical conversions. The first step, called nitritation, is the oxidation of ammonium to nitrite, primarily carried out by bacteria such as Nitrosomonas.
The second step of nitrification involves bacteria, such as Nitrobacter, which quickly oxidizes the resulting nitrite (\(\text{NO}_2^-\)) into the stable nitrate (\(\text{NO}_3^-\)). Finally, certain bacteria, including species of Pseudomonas, complete the cycle through denitrification. This process utilizes nitrate or nitrite as an electron acceptor in anaerobic conditions, returning nitrogen to the atmosphere as inert dinitrogen gas (\(\text{N}_2\)) or the greenhouse gas nitrous oxide (\(\text{N}_2\text{O}\)).