Nitrogen (N) is a fundamental building block for all life on Earth, forming the structural backbone of molecules like deoxyribonucleic acid (DNA) and the amino acids that create proteins. It is also an essential component of chlorophyll, the pigment plants use to capture sunlight for energy. Although the atmosphere is approximately 78% nitrogen gas (\(\text{N}_{2}\)), this form is inert and cannot be directly utilized by most organisms. The strong triple bond holding the two nitrogen atoms together makes the molecule highly stable and chemically inaccessible. Therefore, atmospheric nitrogen must be “fixed” or converted into reactive compounds like ammonia (\(\text{NH}_{3}\)) or nitrates (\(\text{NO}_{3}^{-}\)) that can be readily absorbed.
The Role of Biological Nitrogen Fixation
Nitrogen fixation is the process of converting inert atmospheric nitrogen (\(\text{N}_{2}\)) into a biologically usable form, primarily ammonia. This transformation is performed almost exclusively by prokaryotic microorganisms called diazotrophs. A specialized enzyme complex named nitrogenase, which contains iron and often molybdenum, catalyzes this reaction. Because nitrogenase is sensitive to oxygen, these bacteria require unique mechanisms to protect the enzyme during fixation.
The most significant form is symbiotic nitrogen fixation, involving Rhizobium bacteria. These bacteria establish a mutually beneficial relationship with plants of the legume family, including peas, beans, clover, and alfalfa. The bacteria reside within specialized root nodules, which the plant forms to provide a low-oxygen environment and a steady supply of carbohydrates.
In exchange for this habitat and energy source, Rhizobium converts atmospheric \(\text{N}_{2}\) into ammonia, releasing the fixed nitrogen directly to the host plant. This relationship is foundational to natural ecosystems and agriculture, allowing legumes to thrive in nitrogen-poor soils and enriching the soil for subsequent crops. Other free-living bacteria, such as Azotobacter and cyanobacteria like Nostoc and Anabaena, also contribute to fixation independently in soil and aquatic environments.
Industrial Production of Nitrogen Fertilizers
While biological fixation is slow, human population growth necessitated a faster, high-volume method for nitrogen acquisition. This challenge was solved by the development of the Haber-Bosch process in the early 20th century. This industrial technique synthesizes ammonia (\(\text{NH}_{3}\)) from atmospheric nitrogen and hydrogen gas (\(\text{H}_{2}\)) under extreme conditions.
The process requires high temperatures (between 400 and 500 degrees Celsius) and immense pressure (150 to 300 atmospheres). An iron-based catalyst is used to facilitate the reaction, overcoming the stability of the \(\text{N}_{2}\) molecule. The resultant ammonia is then used to manufacture synthetic nitrogen fertilizers, such as urea and ammonium nitrate.
This synthetic nitrogen acquisition method enabled the massive increase in crop yields known as the Green Revolution. Scientists estimate that the food produced using Haber-Bosch fertilizers currently supports approximately half of the world’s population. However, this energy-intensive process relies heavily on natural gas to produce the hydrogen component, linking global food production directly to fossil fuel consumption.
Natural Methods for Soil Enrichment
Gardeners and small-scale farmers use several practical, non-industrial methods to increase usable nitrogen in the soil. One effective strategy utilizes biological nitrogen fixation by planting cover crops, which are grown specifically to benefit the soil. Species like clover, hairy vetch, and fava beans are legumes that host Rhizobium bacteria in their roots.
When these cover crops are cut and incorporated into the soil before maturity (a practice known as green manuring), the nitrogen fixed in their root nodules and foliage is released as the plant matter decomposes. This provides a slow, steady supply of organic nitrogen for the next cash crop. Crop rotation is another technique where legumes are cycled with non-leguminous plants, allowing the subsequent crop to benefit from the residual fixed nitrogen.
Adding organic materials like compost and well-rotted animal manure is a further method of enrichment. These materials contain nitrogen incorporated into plant and animal proteins, which is released slowly as microorganisms break down the organic matter through ammonification. Specific organic products, such as blood meal or feather meal, are high-nitrogen amendments that can provide a quicker boost. These inputs sustain the soil’s microbial ecosystem, which manages the cycling and availability of nitrogen.
Nitrogen Acquisition Through Diet
For humans and other mammals, nitrogen acquisition is a matter of consumption, not fixation or soil enrichment. Humans obtain all necessary nitrogen by eating food, primarily protein. When protein-rich foods (such as meat, dairy, legumes, or grains) are consumed, the digestive system breaks these large molecules down into their constituent amino acids.
These absorbed amino acids are transported throughout the body, where the nitrogen atoms are used to synthesize new proteins, enzymes, hormones, and nucleic acids like DNA. The body cannot store excess amino acids, so any surplus is processed by the liver via deamination. This process removes the nitrogen-containing amino group, converting it into a less toxic compound called urea.
Urea is filtered by the kidneys and excreted in urine, maintaining the body’s nitrogen balance. Therefore, eating protein is synonymous with nitrogen intake for mammals, and the quality and quantity of dietary protein directly determine the availability of this fundamental element for growth, repair, and metabolism.