Introduction
Nitrogen is one of the most essential elements for life. It forms the building blocks of amino acids, proteins, nucleic acids (DNA and RNA), chlorophyll molecules, enzymes, vitamins, and alkaloids. Though nitrogen gas (N₂) is the most abundant atmospheric gas, making up nearly 78% of the Earth’s atmosphere, it is chemically inert due to the strong triple bond between nitrogen atoms. This makes it unavailable for direct uptake by most living organisms.
To be biologically useful, nitrogen must be converted into reactive forms such as ammonia (NH₃), ammonium (NH₄⁺), nitrites (NO₂⁻), or nitrates (NO₃⁻). This transformation and cycling of nitrogen between the atmosphere, biosphere, lithosphere, and hydrosphere is called the Nitrogen Cycle.
The nitrogen cycle not only ensures a continuous supply of nitrogen for plant and animal life but also maintains ecosystem stability and soil fertility.
Steps in the Nitrogen Cycle
1. Nitrogen Fixation
This is the first step of the cycle. Atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related compounds that plants can utilize.
- Biological Fixation: Carried out by diazotrophs such as Rhizobium, Azotobacter, Frankia, and cyanobacteria. The enzyme nitrogenase catalyzes the reaction:
N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi - Abiotic Fixation: Lightning produces NO and NO₂ that dissolve in rainwater forming nitrates. Industrial fixation (Haber-Bosch process) combines N₂ and H₂ to form NH₃ using high temperature, pressure, and catalysts.
2. Ammonification (Mineralization)
Dead organisms and excreta release organic nitrogen which decomposer microbes (e.g., Bacillus, Proteus, Clostridium) convert to ammonia (NH₃) or ammonium ions (NH₄⁺). This process recycles organic nitrogen back into the soil.
3. Nitrification
A two-step aerobic process carried out by chemoautotrophic bacteria:
- Ammonia oxidation: NH₄⁺ → NO₂⁻ (by Nitrosomonas, Nitrosospira)
- Nitrite oxidation: NO₂⁻ → NO₃⁻ (by Nitrobacter, Nitrococcus)
Nitrates formed are highly soluble and readily absorbed by plants.
4. Assimilation
Plants absorb nitrogen mainly as nitrates (NO₃⁻) and sometimes as ammonium (NH₄⁺). Inside plants:
- Nitrate reductase converts NO₃⁻ → NO₂⁻
- Nitrite reductase converts NO₂⁻ → NH₄⁺
Ammonium is then incorporated into amino acids, proteins, nucleic acids, and other biomolecules. Animals obtain nitrogen through food chains by eating plants or other animals.
5. Denitrification
Denitrification is the reduction of nitrates back into gaseous nitrogen under anaerobic conditions, carried out by bacteria like Pseudomonas denitrificans and Paracoccus. The stepwise reduction is:
NO₃⁻ → NO₂⁻ → NO → N₂O → N₂↑
This process returns nitrogen to the atmosphere and completes the cycle.
6. Anammox (Anaerobic Ammonium Oxidation)
In aquatic and oxygen-limited environments, bacteria such as Brocadia and Kuenenia oxidize ammonium with nitrite to form nitrogen gas:
NH₄⁺ + NO₂⁻ → N₂ + 2H₂O
Anammox plays a significant role in nitrogen removal from oceans and wastewater.
Summary of Nitrogen Transformations
- Fixation: N₂ → NH₃/NH₄⁺
- Ammonification: Organic nitrogen → NH₄⁺
- Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻
- Assimilation: NO₃⁻/NH₄⁺ → Organic nitrogen in plants & animals
- Denitrification: NO₃⁻/NO₂⁻ → N₂/N₂O
- Anammox: NH₄⁺ + NO₂⁻ → N₂
Ecological Importance
- Maintains balance of nitrogen in nature
- Ensures soil fertility and crop production
- Recycles nitrogen and prevents nutrient loss
- Controls environmental nitrogen pollution
- Regulates greenhouse gases (e.g., N₂O)
- Supports agriculture through biofertilizers, crop rotation, and green manures
