Plants do not directly obtain nitrogen from the air. While Earth’s atmosphere is composed of approximately 78% nitrogen gas (N₂), plants cannot absorb or utilize it in this gaseous form. Nitrogen is an important nutrient for plant life, but it must undergo specific transformations before plants can incorporate it into their tissues.
The Unusable Form of Atmospheric Nitrogen
Atmospheric nitrogen exists as a diatomic molecule (N₂), where two nitrogen atoms are joined by a very strong triple covalent bond. This bond makes N₂ highly stable and chemically unreactive. Plants lack the specialized enzymatic machinery required to break this bond and convert gaseous nitrogen into a usable form.
Unlike carbon dioxide or oxygen, which plants directly absorb, N₂’s stability prevents its direct uptake. Plants do not possess the biological tools, like the nitrogenase enzyme found in certain microorganisms, to split this molecule. Thus, despite its abundance, atmospheric nitrogen remains inaccessible to plants without prior conversion.
How Nitrogen Becomes Usable
Nitrogen becomes usable for plants through nitrogen fixation, which converts atmospheric N₂ into more reactive compounds. The primary natural method is biological nitrogen fixation (BNF), performed by specialized microorganisms. These include symbiotic bacteria like Rhizobium in root nodules of leguminous plants (e.g., peas, beans), converting N₂ into ammonia (NH₃) or ammonium (NH₄⁺).
Other free-living soil bacteria, including Azotobacter and certain cyanobacteria, also contribute to BNF by synthesizing the nitrogenase enzyme. Additionally, atmospheric events like lightning provide a natural, non-biological form of nitrogen fixation. Lightning’s energy breaks nitrogen molecules, allowing them to combine with oxygen to form nitrogen oxides, which dissolve in rainwater and are deposited as nitrates (NO₃⁻) in the soil.
Beyond natural processes, industrial nitrogen fixation, notably the Haber-Bosch process, plays a significant role in making nitrogen available. This method synthesizes ammonia from atmospheric nitrogen and hydrogen under high temperatures and pressures. The ammonia produced is a key component of synthetic fertilizers, contributing to the global supply of usable nitrogen for agriculture.
Ammonia and ammonium can be further transformed in the soil through nitrification, a two-step biological process. Ammonia-oxidizing bacteria (Nitrosomonas) convert ammonium (NH₄⁺) into nitrite (NO₂⁻). Subsequently, nitrite-oxidizing bacteria (Nitrobacter) convert nitrite into nitrate (NO₃⁻). Both ammonium and nitrate are forms plants can readily absorb from the soil.
How Plants Take Up Nitrogen
Plants primarily absorb nitrogen from the soil through their root systems. The two main forms of nitrogen plants can take up are nitrate (NO₃⁻) and ammonium (NH₄⁺). These nitrogen ions dissolve in soil water and are transported into root cells.
Specific transporter proteins in the plasma membranes of root cells facilitate this uptake, often through active transport mechanisms requiring energy. Soil health and microbial activity are important in converting nitrogen into these absorbable forms and making them available near the roots. While root uptake is the primary pathway, plants can also absorb some nitrogen through their leaves, particularly when applied as foliar sprays.
Why Nitrogen is Crucial for Plant Growth
Nitrogen is a macronutrient that plays an important role in plant growth and development. It is a major component of chlorophyll, the green pigment responsible for photosynthesis, the process by which plants convert sunlight into energy. Adequate nitrogen levels are directly linked to higher chlorophyll content and increased photosynthetic activity.
Nitrogen is also a building block for amino acids, which are the fundamental units of proteins. Proteins are important for cell structure, enzyme functions, and various metabolic processes within plants. Nitrogen is also a component of nucleic acids like DNA and RNA, which carry genetic information, and of energy-transfer compounds such as ATP. Without sufficient nitrogen, plants exhibit stunted growth, yellowing of older leaves, and reduced overall productivity.