Nitrogen Cycle Stages Explained: Fixation to Denitrification

The Nitrogen Cycle: Essential Biogeochemical Process

Question: Briefly explain the nitrogen cycle with a diagram.

Answer: The nitrogen cycle is a biogeochemical process where nitrogen is converted between different chemical forms as it moves through living and non-living components. It plays a crucial role in maintaining the nitrogen balance in ecosystems.

Stages of the Nitrogen Cycle

The nitrogen cycle consists of several important steps:

1. Nitrogen Fixation
2. Nitrification
3. Assimilation
4. Ammonification (Decay)
5. Denitrification

1. Nitrogen Fixation

Atmospheric nitrogen ($ ext{N}_2$) is inert, meaning plants and animals cannot directly use it.

• Certain bacteria, such as Rhizobium & Azotobacter, known as nitrogen-fixing bacteria, convert $ ext{N}_2$ gas into ammonia ($ ext{NH}_3$), which can be used by plants.

Nitrogen fixation can also occur through:

• Atmospheric Fixation: Lightning converts $ ext{N}_2$ into nitrogen oxides ($ ext{NO}_x$), which dissolve in rain and enter the soil.
• Industrial Fixation: Factories produce ammonia for fertilizers using the Haber-Bosch process.

2. Nitrification

Nitrification is the biological process in which ammonia ($ ext{NH}_3$) or ammonium ($ ext{NH}_4^+$) is converted into nitrites ($ ext{NO}_2^-$) and then into nitrates ($ ext{NO}_3^-$) by specific types of bacteria. This process is essential for making nitrogen available for plants.

Ammonia ($ ext{NH}_3$) in the soil is toxic to plants, so it is converted into nitrates ($ ext{NO}_3^-$) by bacteria using the following steps:

1. Step 1: Nitrosomonas bacteria convert ammonia into nitrites ($ ext{NO}_2^-$).
2. Step 2: Nitrobacter bacteria convert nitrites into nitrates ($ ext{NO}_3^-$), which plants can absorb through their roots.

3. Assimilation

Assimilation is the process where plants absorb nitrogen compounds (such as nitrates, nitrites, or ammonium) from the soil and use them to form essential organic molecules, like proteins and nucleic acids. This is a crucial step because it allows nitrogen to enter the food chain.

When animals eat these plants, they obtain nitrogen for their own biological processes.

4. Ammonification (Decay)

When plants and animals die or produce waste (e.g., urine, feces), decomposers like bacteria and fungi break down the organic matter.

This process releases ammonia ($ ext{NH}_3$) or ammonium ($ ext{NH}_4^+$) back into the soil.

5. Denitrification

Denitrification is the final stage of the nitrogen cycle, where nitrate ($ ext{NO}_3^-$) and nitrite ($ ext{NO}_2^-$) are converted back into gaseous nitrogen ($ ext{N}_2$) or nitrous oxide ($ ext{N}_2 ext{O}$) by anaerobic bacteria.

This process completes the nitrogen cycle, ensuring that excess nitrogen does not accumulate in the soil.

This can be explained with the help of the following diagram:

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