Nitrogen is a fundamental component of all life, serving as the backbone of amino acids (the building blocks of proteins) and nucleic acids (like DNA and RNA). Although nitrogen gas (\(\text{N}_2\)) makes up approximately 78% of the Earth’s atmosphere, this form is chemically inert and unusable by animals. The two nitrogen atoms are held together by an extremely stable triple covalent bond that animals lack the biological machinery to break. Consequently, animals must acquire nitrogen in a pre-packaged, reactive form by consuming other living or once-living material.
Consuming Nitrogen: The Food Web Pathway
Animals obtain their necessary nitrogen by eating other organisms whose bodies have already incorporated the element into complex molecules. Nitrogen is thus passed up the food web, transferred from one organism to the next. The nitrogen an animal ingests is primarily in the form of proteins and amino acids integrated into the tissues of its meal.
Herbivores, such as deer and cattle, acquire biologically fixed nitrogen by consuming plants, which are the primary producers. When they digest plant material, they break down plant proteins and nucleic acids and reassemble the nitrogen atoms into their own animal proteins and DNA. Carnivores, like wolves and lions, then obtain their nitrogen by consuming these herbivores or other meat-eaters.
Omnivores, including humans, acquire nitrogen through a mixed diet, utilizing protein from both plant and animal sources. During digestion, consumed proteins are broken down into their constituent amino acids, which contain nitrogen. These amino acids are then absorbed and used by the animal’s cells to synthesize new proteins, enzymes, and other compounds required for growth and metabolism.
The Crucial Step: Nitrogen Fixation and Plant Uptake
The animal food web depends on a preliminary process that converts inert atmospheric nitrogen into a biologically available form. This transformation, called nitrogen fixation, is carried out by certain types of bacteria and archaea, not by plants or animals. These microorganisms possess a specialized enzyme complex called nitrogenase, which breaks the strong triple bond of the \(\text{N}_2\) molecule.
Biological nitrogen fixation converts atmospheric nitrogen gas into ammonia (\(\text{NH}_3\)) or ammonium (\(\text{NH}_4^+\)), the first usable forms. Many nitrogen-fixing bacteria, such as Rhizobium, live symbiotically within specialized nodules on the roots of leguminous plants like peas, beans, and clover. The plants provide the bacteria with energy, and the bacteria supply the plant with fixed nitrogen directly.
The fixed nitrogen can then undergo a secondary process known as nitrification, carried out by different groups of soil bacteria. This process involves the oxidation of ammonium first into nitrite (\(\text{NO}_2^-\)) and then into nitrate (\(\text{NO}_3^-\)). Nitrate is a highly soluble and easily absorbed form of nitrogen that most non-leguminous plants prefer to take up through their root systems.
Once plants absorb the ammonium or nitrate from the soil, they assimilate the inorganic nitrogen into organic compounds like amino acids and proteins. Plants are therefore the initial reservoir of biologically available nitrogen, acting as the bridge between the atmospheric supply and the entire consumer food web.
Returning Nitrogen to the Ecosystem
The nitrogen acquired by animals eventually returns to the ecosystem pool, completing the cycle and making the element available for reuse by plants. This exit occurs through two primary mechanisms: the excretion of waste products and the decomposition of the animal’s body after death. Metabolism of nitrogen-containing compounds, particularly amino acids, results in nitrogenous waste that must be expelled.
Depending on the animal species, this waste is excreted as ammonia, urea, or uric acid. For example, most mammals convert toxic ammonia into the less toxic compound urea for elimination in urine, while birds and reptiles excrete uric acid. These nitrogen-rich waste products are deposited back into the soil or water, where they become available for microbial action.
The second major pathway is decomposition, which begins when an animal dies. Decomposers, primarily bacteria and fungi, break down the complex organic macromolecules—proteins and nucleic acids—in the animal’s remains. This process, known as ammonification, releases the nitrogen content back into the environment in the form of ammonium. This ammonium can then be absorbed by plants or further converted by nitrifying bacteria, ensuring the continuous cycling of this life-sustaining element through the ecosystem.