Nitrogen is an indispensable nutrient for all plant life, acting as a foundational component for the molecules that drive growth and metabolism. It is necessary for creating chlorophyll, the green pigment that powers photosynthesis, and for synthesizing amino acids, which are the building blocks of proteins and enzymes. Although the atmosphere consists of nearly 78% nitrogen gas (\(\text{N}_2\)), this inert molecule is unusable by plants in its gaseous form. For plants to access this abundant resource, atmospheric nitrogen must be “fixed,” or converted into reactive, soluble forms like ammonium (\(\text{NH}_4^+\)) or nitrate (\(\text{NO}_3^-\)) that can be absorbed from the soil. This conversion is achieved through a combination of natural, biological processes and industrial methods, all of which contribute to the total nitrogen content of the soil.
Harnessing Nitrogen from the Air
The largest natural source of nitrogen input into the soil is Biological Nitrogen Fixation (BNF), carried out by specialized microorganisms. These prokaryotic organisms use an enzyme complex called nitrogenase to break the strong triple bond of atmospheric nitrogen gas (\(\text{N}_2\)) and reduce it to ammonia (\(\text{NH}_3\)). This chemical reaction is energetically expensive for the bacteria.
The most recognized form of BNF involves a symbiotic relationship between bacteria, primarily Rhizobia, and plants belonging to the legume family, such as peas, clover, and beans. The bacteria reside within specialized structures on the roots called nodules, where they fix nitrogen in exchange for carbohydrates supplied by the host plant. Once fixed into ammonia, it is rapidly converted to ammonium (\(\text{NH}_4^+\)), a form the legume can readily use.
Nitrogen fixation also occurs independently through free-living bacteria in the soil, such as Azotobacter and certain cyanobacteria. These free-living fixers contribute nitrogen directly to the soil solution, where it becomes available to surrounding plants. This biological process is responsible for the vast majority of the natural conversion of atmospheric nitrogen into a usable form.
The Role of Decaying Materials
Another significant natural path for adding nitrogen to the soil is through the decomposition of organic matter. This input originates from nitrogen that is already bound up in dead plant tissues, animal waste, manure, and the bodies of dead soil microbes. The nitrogen in these materials is in an organic form, meaning it is chemically bonded to carbon compounds and not immediately available for plant uptake.
Soil microorganisms, including various bacteria and fungi, release enzymes that break down these complex organic compounds in a two-step process known as mineralization. First, the organic nitrogen is converted into inorganic ammonium (\(\text{NH}_4^+\)) through ammonification. This release of usable nitrogen is a byproduct of the microbes consuming the organic matter for their own energy and growth.
The rate at which this organic nitrogen is mineralized depends heavily on environmental conditions like soil temperature, moisture levels, and oxygen availability. Warmer, well-aerated, and moist soils typically experience a faster release of ammonium. This continuous cycle ensures that the nitrogen stored in the soil’s organic pool is recycled and made available for new plant growth.
Synthetic Fertilizers and Atmospheric Deposition
Beyond the natural microbial processes, two other major sources contribute nitrogen to the soil, one industrial and one atmospheric. The industrial method, known as the Haber-Bosch process, is the primary means by which humanity now adds nitrogen to agricultural lands globally. This process synthesizes ammonia (\(\text{NH}_3\)) by combining nitrogen gas from the air with hydrogen gas under extremely high heat and pressure.
The resulting ammonia is then used as the base for manufacturing various synthetic nitrogen fertilizers, such as urea and ammonium nitrate. This industrially fixed nitrogen is instantly available to plants, allowing for the massive crop yields that support the modern global population. The Haber-Bosch process, however, is highly energy-intensive.
A smaller, yet constant, source of nitrogen comes from atmospheric deposition, which involves both wet and dry forms. Naturally, lightning provides a minor input by generating intense heat that causes nitrogen and oxygen in the air to react, forming nitrogen oxides that fall to earth with rain. However, human activity now accounts for the majority of atmospheric deposition.
Combustion of fossil fuels in vehicles and power plants releases nitrogen oxides (\(\text{NO}_x\)) into the atmosphere. Additionally, ammonia gas (\(\text{NH}_3\)) can volatilize from agricultural sources like manure and synthetic fertilizers. These reactive nitrogen compounds return to the ground as wet deposition (rain, snow) or dry deposition (dust and gas), contributing a steady source of nitrogen to the soil.