Steps of Nitrogen Cycle

The stages of nitrogen cycle are explained as follows:

Nitrogen Fixation

Atmospheric nitrogen (N₂) is converted into ammonia (NH₃)or ammonium ions (NH4⁺) through biological, industrial, or natural processes. Nitrogen-fixing bacteria, such as those in the genera Rhizobium and Azotobacter, play an important role. These bacteria either live in the soil or form symbiotic relationships with plants. They possess the enzyme nitrogenase, which can break the strong triple bond of atmospheric nitrogen and convert it into ammonia.

Types of Nitrogen Fixation

1. Atmospheric Fixation: It is a natural phenomenon in which the lightning energy breaks the nitrogen into nitrogen oxide, which circulates in the ecosystem. Nitrogen enters the ecosystem with the help of plants.
2. Industrial Nitrogen Fixation: Ammonia is formed from the direct combination of nitrogen and hydrogen, which is further converted into different fertilizers. Example- Urea.
3. Biological Nitrogen Fixation: Nitrogen in the air is not directly taken by the plants. Plants take the nitrogen from the soil with the help of roots and bacteria (rhizobium and blue-green algae), which make the symbiosis interaction between the plant and animal. Bacteria convert the unusable nitrogen into usable compounds taken by the plant.

Ammonification

When plants and animals die, and when waste products decompose, organic nitrogen compounds are released. Decomposer microorganisms, including bacteria and fungi, break down these organic materials and convert them into ammonia (NH3). This process replenishes the soil with ammonium ions.

Nitrification

Nitrification is a two-step process involving different types of bacteria:

• Ammonium Oxidation: Ammonium ions (NH4+) are converted into nitrite ions (NO2-) by ammonium-oxidizing bacteria such as Nitrosomonas.
• Nitrite Oxidation: Nitrite ions (NO2-) are further converted into nitrate ions (NO3-) by nitrite-oxidizing bacteria like Nitrobacter. Nitrates are more mobile in soils and can be taken up by plants.

Assimilation

Plants take up ammonium (NH4+) and nitrate (NO3-) ions from the soil through their roots. These ions are then used to synthesize amino acids, proteins, nucleic acids, and other nitrogen-containing organic molecules essential for plant growth.

• Consumption: Herbivores and other animals consume plants to obtain their nutrients, including the nitrogen compounds present in the plant tissues. Carnivores, in turn, consume other animals. This transfers the nitrogen through the food chain.
• Decomposition: The remains of plants and animals, including their nitrogen-rich compounds, undergo decomposition by decomposer microorganisms. During this process, organic nitrogen is broken down into simpler nitrogen compounds, such as ammonia (NH3), which can be reused in the cycle.

Denitrification

Denitrification is the process by which nitrate ions (NO3-) are converted back into nitrogen gas (N₂) or nitrous oxide (N₂O) by denitrifying bacteria, such as Pseudomonas and Paracoccus. This occurs under anaerobic conditions. It helps to continue the nitrogen cycle by releasing nitrogen back into the atmosphere.

Nitrogen cycle is the natural process by which atmospheric nitrogen is converted into various usable forms, like ammonia and nitrates, by bacteria for plant and animal consumption and then returned to the atmosphere through decomposition and denitrification, maintaining the nutrient balance in ecosystems. Nitrogen is an essential element for life as it is a key component of amino acids, proteins, DNA, and other biological molecules. Human activities, like agriculture and industry, can disrupt this cycle, leading to environmental issues.