- Nitrogen gas (N2) constitutes approximately 78% of the Earth's atmosphere. However, atmospheric nitrogen is relatively inert and cannot be directly used by most plants and animals.
- Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia (NH3). This process can occur in the soil, where symbiotic bacteria form nodules on the roots of leguminous plants (e.g., beans, peas, and lentils) and convert atmospheric nitrogen into ammonia that the plants can use, or it can occur through free-living bacteria in the soil or in aquatic environments.
Step 2: Nitrification:
- Nitrifying bacteria convert ammonia into nitrite (NO2-) and nitrate (NO3-).
- Nitrosomonas bacteria convert ammonia into nitrite through a process called nitritation.
- Nitrobacter bacteria further oxidize nitrite into nitrate through nitratation.
Step 3: Assimilation:
- Plants absorb the available nitrates and nitrites through their roots.
- Inside plant tissues, nitrates and nitrites are reduced and incorporated into amino acids, proteins, nucleic acids, and other nitrogenous compounds essential for plant growth.
Step 4: Ammonification:
- When plants and animals die, their nitrogenous organic matter (e.g., proteins, nucleic acids) is broken down by decomposer organisms (mainly bacteria and fungi) in the soil.
- During ammonification, microorganisms decompose the organic nitrogen compounds, releasing ammonia gas (NH3).
Step 5: Nitrification (again):
- The released ammonia is then re-converted into nitrite and nitrate through the nitrification process described in Step 2.
Step 6: Denitrification:
- Under anaerobic conditions (low oxygen levels), certain bacteria perform denitrification, converting nitrate back into atmospheric nitrogen gas (N2).
- Denitrification is an essential step in the nitrogen cycle as it balances the amount of nitrogen in the environment and prevents its overaccumulation.
The continuous cycling of nitrogen among these steps ensures that plants and animals have access to the essential nitrogen they need for growth, development, and reproduction. Additionally, human activities such as excessive fertilizer application and industrial activities can disrupt the natural nitrogen cycle, leading to environmental issues like groundwater nitrate contamination and eutrophication of water bodies.
