Plants cannot directly use the nitrogen gas (N₂) that makes up about 78% of the Earth's atmosphere. Instead, they rely on nitrogen being converted into usable forms through a process called nitrogen fixation.
Why Plants Can't Directly Use Atmospheric Nitrogen
Atmospheric nitrogen (N₂) is a very stable molecule due to the strong triple bond between the two nitrogen atoms. Plants lack the enzymatic machinery needed to break this bond.
How Plants Obtain Nitrogen
Plants obtain nitrogen primarily from the soil in the form of:
- Nitrate (NO₃⁻): A readily available form of nitrogen.
- Ammonium (NH₄⁺): Another form that plants can absorb.
The Process of Nitrogen Fixation
Nitrogen fixation is the process that converts atmospheric nitrogen (N₂) into ammonia (NH₃), which is then converted to ammonium (NH₄⁺). This process is carried out by certain microorganisms:
- Nitrogen-fixing bacteria: Some bacteria, like Rhizobium species, live in a symbiotic relationship with the roots of legumes (e.g., beans, peas, clover). They convert nitrogen gas into ammonia within the root nodules of these plants.
- Free-living nitrogen-fixing bacteria: Other bacteria, such as Azotobacter and Cyanobacteria, live freely in the soil and can also fix nitrogen.
- Industrial Nitrogen Fixation: The Haber-Bosch process is an industrial method for converting atmospheric nitrogen into ammonia. This ammonia is then used to produce fertilizers, which are applied to soil to provide plants with nitrogen.
The Nitrogen Cycle
The nitrogen cycle describes the various transformations of nitrogen in the environment. Key processes include:
- Nitrogen Fixation: Conversion of atmospheric nitrogen into ammonia.
- Nitrification: Conversion of ammonia (NH₃) to nitrite (NO₂⁻) and then to nitrate (NO₃⁻) by nitrifying bacteria.
- Assimilation: Uptake of nitrate or ammonium by plants and incorporation into organic molecules (e.g., amino acids, proteins).
- Ammonification: Decomposition of organic matter by microorganisms, releasing ammonia (NH₃).
- Denitrification: Conversion of nitrate (NO₃⁻) back into nitrogen gas (N₂) by denitrifying bacteria, which occurs under anaerobic conditions.
In Summary
Plants are unable to use nitrogen directly from the air. They rely on nitrogen-fixing microorganisms to convert atmospheric nitrogen into forms they can absorb through their roots, such as nitrates and ammonium. The nitrogen cycle describes the continuous transformation of nitrogen in the environment, ensuring a supply of this essential nutrient for plants.
