Nitrogen is an important constituent of our atmosphere incorporating 75% of the gases in our atmosphere. Nitrogen atoms are found in all DNA and proteins, that is why Nitrogen is an integral element of the body of all living organisms. Candidates can read other NCERT Geography notes to prepare for UPSC IAS Prelims and Mains exam.

In the atmosphere, Nitrogen exists as N2 and is of great significance for the growth of plants. However, though the gas is abundant in the atmosphere, it still cannot be directly used by either plants or animals. Herein, the role of the nitrogen cycle comes in.

Nitrogen cycle is a biogeochemical cycle. It converts the inert nitrogen that is present in the atmosphere into nitrogen that is useful to the plants and animals. Through the cycle, nitrogen passes from the atmosphere to the soil. From there, it goes to the organism and then back to the atmosphere. There are several processes in the Nitrogen cycle. These include nitrogen fixation, nitrification, denitrification, decay and putrefaction. Nitrogen goes through various processes and transformation in order to maintain a balance in the ecosystem.

Steps of the Nitrogen Cycle

Step 1 - Nitrogen Fixation

• It is the first step of the nitrogen cycle.
• Herein, the inert nitrogen available in the atmosphere around us is converted into ammonia (NH3) that is usable.
• The gas is deposited from the atmosphere into the soils and surface waters, mainly through the process of precipitation.
• The whole process of the nitrogen fixation is completed by the symbiotic bacteria. This Nitrogen Fixation bacteria is known as Diazotrophs.
• There are various types of nitrogen fixation such as atmospheric fixation, Industrial nitrogen fixation, and Biological nitrogen fixation

Step 2 - Nitrification

• Herein, in this process, ammonia is converted into nitrate. This is done due to the presence of a bacteria that is in the soil.
• The nitrites are formed by the process of oxidation of ammonia that is done through the help of Nitrification bacteria species known as Nitrosomonas.
• The nitrites produced are then converted to nitrates by Nitrobacter. This conversion is essential because the ammonia gas is toxic for the plants.

2NH₄⁺ + 3O₂ → 2NO₂^– + 4H⁺ + 2H₂O

2NO₂^– + O₂→ 2NO₃^–

Step 3 - Assimilation

• Plants which are the primary producers take in the compounds of nitrogen from soil. They do this through their roots.
• Nitrogen is present in the form of ammonia, nitrite ions, nitrate ions or ammonium ions. These are then used in the formation of the animal and plant proteins.
• The nitrogen is then used in chlorophyll, amino acids and nucleic acids.
• Hence, when a primary consumer eats the plant, nitrogen enters the food web.
• Only plants get nitrogen through the process of assimilation.

Step 4 - Ammonification

• Ammonification is part of the decaying process.
• On the death of animals and plants, the nitrogen that is present in the organic matter eventually is released back to the soil.
• Fungi or bacteria, the decomposers that are present in the soil then convert the organic matter released back into ammonium.
• This decomposition process leads to the production of ammonia, which is used further for other biological processes.

Step 5 - Denitrification

• The process in which the compounds of nitrogen make their way back to the atmosphere is known as denitrification.
• Nitrate (NO3-) is converted into gaseous nitrogen (N).
• This is the final stage of the nitrogen cycle.
• This process happens in absence of oxygen.
• Denitrifying bacteria species carry out the process of denitrification. These bacteria in turn process nitrate to simultaneously gain oxygen and give out free nitrogen gas as a byproduct of the process.

Importance of Nitrogen Cycle

1. The Nitrogen Cycle helps the plants in synthesizing chlorophyll from the compounds of nitrogen.
2. During the process of ammonification, the bacteria involved in the process helps to decompose the animal and the plant matter. This eventually helps in cleaning our environment.
3. The nitrogen cycle is useful as it also helps in converting the inert nitrogen gas that can’t be used directly into a usable form for the plants through biochemical processes.
4. During the cycle, nitrites and nitrates are released back into the soil. This release helps in enriching the soil for cultivation by the release of the necessary nutrients required for the process.
5. Nitrogen is a very significant part of the cell. It is a part of many integral compounds and important elements.

To get more information and study materials on Physical Geography, Human Geography, and Environmental Geography for UPSC IAS Prelims and Mains exam, candidates can check NCERT Geography Notes right here.

FAQs

MCQs

1. Which process in the Nitrogen Cycle involves the conversion of atmospheric nitrogen into ammonia?

A) Nitrification
B) Denitrification
C) Nitrogen Fixation
D) Ammonification

2. What is the primary role of nitrifying bacteria in the Nitrogen Cycle?

A) Converting nitrates into nitrogen gas
B) Breaking down organic matter into ammonia
C) Converting ammonia into nitrites and nitrates
D) Fixing atmospheric nitrogen into organic compounds

3. Which human activity has significantly disrupted the natural Nitrogen Cycle?

A) Deforestation
B) Industrial nitrogen fixation for fertilizers
C) Urbanization
D) Mining

4. In the Nitrogen Cycle, what is ammonification?

A) Conversion of ammonia into nitrites
B) Conversion of nitrates into nitrogen gas
C) Decomposition of organic nitrogen into ammonia
D) Fixation of atmospheric nitrogen into organic compounds

5. Why is denitrification an essential process in the Nitrogen Cycle?

A) It converts ammonia into nitrites
B) It releases nitrogen gas back into the atmosphere
C) It fixes atmospheric nitrogen into the soil
D) It converts nitrates into ammonia

GS Mains Questions and Model Answers

Q1: Discuss the significance of the Nitrogen Cycle in maintaining soil fertility and supporting plant growth.

Answer: The Nitrogen Cycle is vital for maintaining soil fertility and supporting plant growth. It transforms inert atmospheric nitrogen into bioavailable forms like ammonia and nitrates through processes such as nitrogen fixation and nitrification. Plants absorb these compounds to synthesize essential biomolecules, including proteins and nucleic acids. Additionally, processes like ammonification and denitrification recycle nitrogen within the ecosystem, ensuring a continuous supply. Disruptions in this cycle, often due to human activities, can lead to soil degradation and reduced agricultural productivity. Sustainable agricultural practices and reduced reliance on chemical fertilizers are key to maintaining balance in the Nitrogen Cycle.

Q2: Explain how human activities have impacted the Nitrogen Cycle and suggest measures to mitigate these effects.

Answer: Human activities, such as the excessive use of synthetic nitrogen fertilizers, fossil fuel combustion, and industrial processes, have significantly altered the Nitrogen Cycle. These actions have increased the amount of reactive nitrogen in the environment, leading to issues such as soil acidification, water pollution, and increased greenhouse gas emissions. To mitigate these effects, measures like adopting sustainable agricultural practices, reducing industrial nitrogen emissions, promoting organic farming, and restoring natural ecosystems should be prioritized. Policy interventions and international cooperation are also crucial to minimizing human impact on the Nitrogen Cycle.

Q3: Analyze the role of microorganisms in the Nitrogen Cycle and their importance in ecosystem functioning.

Answer: Microorganisms play a crucial role in the Nitrogen Cycle, facilitating processes such as nitrogen fixation, nitrification, ammonification, and denitrification. Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia, making it accessible for plant uptake. Nitrifying bacteria oxidize ammonia into nitrites and nitrates, while denitrifying bacteria convert nitrates back into nitrogen gas, completing the cycle. These microbial processes ensure the availability of nitrogen in forms that can be used by plants and other organisms, maintaining ecosystem balance and productivity. Without these microorganisms, nitrogen would remain inaccessible, disrupting nutrient cycles and ecosystem functioning.

Previous Year Questions on the Nitrogen Cycle

1. UPSC CSE Prelims 2019:

Question: Which process in the Nitrogen Cycle converts nitrates back into nitrogen gas?

A) Nitrification
B) Ammonification
C) Denitrification
D) Nitrogen Fixation

Answer: (C)

Explanation: Denitrification is the process that converts nitrates back into nitrogen gas, releasing it into the atmosphere and completing the Nitrogen Cycle.

2. UPSC CSE Mains 2018 (GS Paper 3):

Question: "Evaluate the impact of human activities on the Nitrogen Cycle and suggest sustainable practices to restore its balance."

Answer: Human activities, such as excessive fertilizer use, fossil fuel combustion, and industrial emissions, have disrupted the Nitrogen Cycle, leading to environmental issues like soil degradation, water pollution, and increased greenhouse gas emissions. Sustainable practices to restore balance include reducing synthetic fertilizer usage, adopting organic farming, implementing crop rotation with nitrogen-fixing plants, and reducing nitrogen emissions from industrial sources. Promoting awareness and international cooperation is also essential to mitigate human impact on this vital cycle.