Nitrogen is a fundamental element for all life on Earth, forming key components of various molecules like proteins, DNA, and chlorophyll. These are essential for plant growth, photosynthesis, and cellular function. Despite making up about 78% of the air, plants cannot directly use atmospheric nitrogen. This creates a paradox: a plentiful nutrient exists, but plants cannot access it without external help.
The Atmospheric Nitrogen Barrier
The primary reason plants cannot directly absorb atmospheric nitrogen (N₂) lies in its chemical structure. Atmospheric nitrogen exists as a diatomic molecule, with two nitrogen atoms linked by a strong triple covalent bond. This bond is one of the strongest chemical bonds found in nature, requiring a significant amount of energy, about 941 kilojoules per mole, to break.
Plants lack the specialized enzymatic machinery to break this triple bond. Consequently, N₂ gas is inert for most biological processes, meaning it does not readily react with other substances. This chemical stability ensures nitrogen’s prevalence but makes it inaccessible to plants in its gaseous form.
Microbial Nitrogen Conversion
The natural solution to this atmospheric nitrogen barrier comes from specialized microorganisms through a process called nitrogen fixation. Certain bacteria, known as diazotrophs, possess a unique enzyme complex called nitrogenase. This enzyme breaks the triple bond in atmospheric nitrogen, converting it into ammonia (NH₃), a form usable by plants.
Nitrogen-fixing bacteria can be free-living in the soil, like Azotobacter and cyanobacteria, or form symbiotic relationships with plants. A key example involves Rhizobium bacteria and leguminous plants, such as peas and beans. These bacteria reside within specialized structures on plant roots called nodules, where they fix nitrogen in exchange for carbohydrates from the plant. The nitrogenase enzyme, found within these microbes, is sensitive to oxygen, necessitating anaerobic conditions within the nodules for efficient nitrogen fixation.
How Plants Absorb Nitrogen
Once atmospheric nitrogen has been converted into usable forms, primarily ammonia, plants can absorb it from the soil. Plants mainly take up nitrogen in two inorganic forms: nitrate (NO₃⁻) and ammonium (NH₄⁺) ions. These ions are dissolved in soil water and are absorbed by plant roots through specific transporter proteins located in their cell membranes.
The process of nitrification, carried out by other soil bacteria, transforms ammonium into nitrate. For instance, Nitrosomonas bacteria convert ammonia to nitrite, and then Nitrobacter bacteria convert nitrite to nitrate. Plants can utilize both forms, with nitrate often being the most readily available and absorbed in well-aerated soils.
Nitrogen in Agriculture and Ecosystems
The availability of usable nitrogen impacts both natural ecosystems and agricultural productivity. Nitrogen continuously cycles through the atmosphere, soil, and living organisms in the nitrogen cycle. Human intervention has significantly altered this cycle, particularly through the Haber-Bosch process.
This industrial process, developed in the early 20th century, synthesizes ammonia from atmospheric nitrogen and hydrogen under high pressure and temperature using a catalyst. The Haber-Bosch process allowed for the mass production of synthetic nitrogen fertilizers, which has been instrumental in increasing crop yields and supporting the global population growth over the last century. While this has boosted food production, it also highlights the dependence on fixed nitrogen, whether from natural microbial processes or industrial synthesis, for sustaining life on Earth.