Nitrogen is a macronutrient for plant life, serving as a structural component in chlorophyll, necessary for photosynthesis, and in the building blocks of DNA and proteins. Despite its importance, the atmosphere is nearly 78% nitrogen, but holds the element in an inert gaseous form (\(text{N}_2\)) that is chemically inaccessible to plants. For plants to absorb and use this nutrient for growth, it must first be converted into reactive compounds that can be dissolved in soil water. This supply mechanism involves a complex series of biological and chemical transformations that make the atmospheric reservoir available to the root systems.
Usable Forms and Immediate Soil Supply
Plants absorb nitrogen primarily through their roots in two inorganic forms: the negatively charged nitrate ion (\(text{NO}_3^-\)) and the positively charged ammonium ion (\(text{NH}_4^+\)). Nitrate is generally the most readily available form because it is highly soluble and moves freely with soil water toward the root zone, where it is taken up by specific transporters. Although ammonium is also absorbed, it binds tightly to the negatively charged clay and organic matter particles in the soil, making it less mobile than nitrate.
Beyond these inorganic ions, nitrogen available to plants comes from the decay of dead organic material, such as plant residues and animal waste. This organic nitrogen is not directly usable, but soil microbes quickly convert it into ammonium through a process called ammonification or mineralization. Up to 99% of the potentially available nitrogen in the soil exists in these organic forms, providing a slow-release reservoir that requires microbial processing. Some plants also possess the ability to directly absorb certain organic compounds, like amino acids, through their roots, supplementing their inorganic uptake.
Converting Atmospheric Nitrogen
The conversion of inert atmospheric nitrogen (\(text{N}_2\)) into chemically reactive forms is known as nitrogen fixation, which occurs through biological and atmospheric mechanisms. Biological nitrogen fixation is responsible for the vast majority of this conversion and is carried out exclusively by certain prokaryotes. The process involves the nitrogenase enzyme complex, which reduces \(text{N}_2\) into ammonia (\(text{NH}_3\)) using a substantial amount of energy.
The most significant biological fixation occurs in a symbiotic relationship between leguminous plants (like peas and clover) and Rhizobium bacteria. These bacteria invade the plant’s root hairs, leading to the formation of specialized structures called root nodules. Inside the nodule, the bacteria become bacteroids that perform fixation, and the plant supplies an iron-linked protein called leghemoglobin to control oxygen levels, which protects the oxygen-sensitive nitrogenase enzyme. Additionally, free-living bacteria, such as Azotobacter, fix nitrogen directly in the soil, but their contribution is less significant than the symbiotic relationship.
A non-biological mechanism, atmospheric fixation, also contributes nitrogen to the soil. The immense energy of lightning converts gaseous nitrogen and oxygen into nitrogen oxides. These compounds dissolve in rainwater and fall to the earth, introducing small quantities of nitrate directly into the soil solution. While this process is minor compared to biological fixation, it provides a steady, natural input of usable nitrogen.
Transformations Within the Soil
Once atmospheric nitrogen has been fixed into ammonia or organic matter has decomposed, a series of major microbial transformations determines the final form and availability of the nutrient. Ammonification, the initial step, is the conversion of organic nitrogen from decaying residues into ammonium (\(text{NH}_4^+\)) by soil microorganisms. This ammonium can either be absorbed by plant roots or undergo the next transformation, nitrification.
Nitrification is a two-step process carried out by different groups of chemoautotrophic bacteria under aerobic conditions. First, Nitrosomonas bacteria oxidize the ammonium (\(text{NH}_4^+\)) into nitrite (\(text{NO}_2^-\)), a compound that is often toxic to plants. Immediately following this, Nitrobacter bacteria quickly convert the nitrite into the highly mobile nitrate ion (\(text{NO}_3^-\)), which is the form most plants prefer for uptake. This conversion is important because nitrate does not bind to soil particles and is easily transported throughout the root zone.
The final major transformation is denitrification, which results in a loss of nitrogen from the soil system. This process occurs when denitrifying bacteria, such as Pseudomonas, use nitrate (\(text{NO}_3^-\)) as an electron acceptor in saturated, oxygen-poor (anaerobic) soil environments. The bacteria reduce the nitrate back into gaseous forms, including nitrous oxide and inert nitrogen gas (\(text{N}_2\)), which then escape into the atmosphere. This mechanism represents a constant, natural counter-balance to nitrogen fixation, regulating the overall nutrient level in the soil.
Supplementing Nitrogen Sources
In agricultural settings, human intervention provides a supplemental source of fixed nitrogen to maximize crop yields. This is accomplished through the industrial production of synthetic fertilizers, a process that chemically fixes atmospheric nitrogen gas and hydrogen to create ammonia (\(text{NH}_3\)). This ammonia is then used to manufacture commercial products like urea, ammonium nitrate, and ammonium sulfate, which are applied directly to fields. These compounds enter the soil and are rapidly converted by microbes into ammonium and nitrate forms, bypassing the slow pace of natural biological fixation.
Plants in specialized ecological niches employ adaptations to acquire supplemental nitrogen. Carnivorous plants, such as the Venus flytrap and pitcher plants, thrive in waterlogged, acidic bogs where soil nitrogen is extremely low. These plants trap and digest insects and small arthropods, which act as a rich, external source of the nutrient. The prey is enzymatically digested, and the plant then absorbs the released nitrogen compounds through specialized leaf cells.