Nitrogen is a fundamental building block for all life forms, constituting about 78% of Earth’s atmosphere. Despite its abundance, atmospheric nitrogen is largely unusable by most organisms in its gaseous form. This element is a key component of vital biological molecules, such as proteins and nucleic acids like DNA and RNA. Its necessity highlights the intricate processes that convert it into accessible forms.
Nitrogen’s Journey in Nature
Nitrogen continuously cycles through Earth’s atmosphere, land, and water in a process known as the nitrogen cycle. This cycle transforms atmospheric nitrogen gas (N₂) into various chemical forms, making it available to living organisms and then returning it to the atmosphere. The initial step is nitrogen fixation, where N₂ is converted into ammonia (NH₃) or ammonium (NH₄⁺). This conversion is primarily carried out by specialized bacteria, such as Rhizobium in legume root nodules, and free-living bacteria in soil and water. Lightning also contributes a small amount by converting atmospheric nitrogen into nitrogen oxides.
Following nitrogen fixation, other microbial processes transform these nitrogen compounds. Nitrification involves a two-step process where nitrifying bacteria convert ammonium into nitrites (NO₂⁻) and then into nitrates (NO₃⁻). For example, Nitrosomonas bacteria oxidize ammonia to nitrites, and Nitrobacter bacteria convert nitrites to nitrates. This conversion is significant because nitrates are a primary form of nitrogen readily absorbed by plants.
Nitrogen also returns to the atmosphere through denitrification. In this process, denitrifying bacteria convert nitrates back into nitrogen gas (N₂). This typically occurs in anaerobic environments, such as waterlogged soils or sediments, where oxygen is scarce. Additionally, ammonification occurs as decomposer bacteria and fungi break down organic nitrogen from dead organisms and waste products into ammonia and ammonium.
How Plants Acquire Nitrogen
Plants, as primary producers, incorporate nitrogen into the food web. They cannot directly utilize atmospheric nitrogen gas. Instead, plants primarily absorb nitrogen from the soil as inorganic ions: nitrates (NO₃⁻) and ammonium (NH₄⁺). The nitrogen cycle processes, particularly nitrification, are fundamental in providing plants with these usable compounds.
Many plants also benefit from symbiotic relationships with nitrogen-fixing bacteria. A prominent example is the association between legumes (like peas and beans) and Rhizobium bacteria. These bacteria reside within specialized root nodules on the plant’s roots, converting atmospheric nitrogen into ammonia for plant use. This mutualistic relationship allows legumes to thrive in nitrogen-poor soils and enriches the soil with fixed nitrogen. Human agricultural practices also supplement nitrogen for plants through fertilizers, providing readily available nitrate and ammonium to boost crop growth.
How Animals and Humans Obtain Nitrogen
Animals and humans cannot directly use atmospheric nitrogen or fix it into usable forms. They acquire the nitrogen they need by consuming other organisms that have already incorporated this element into organic compounds. This means obtaining nitrogen through their diet, by eating plants or other animals.
The nitrogen in consumed food is predominantly in the form of proteins. During digestion, these proteins are broken down into their constituent amino acids. These amino acids, which contain nitrogen, are then absorbed and used by the body to synthesize its own proteins, enzymes, and nucleic acids. The body also requires specific “essential amino acids” that it cannot produce internally and must obtain directly from the diet.