Self Study Notes for Environment | Environment Notes for UPSC & UPPCS | Nitrogen Cycle UPSC | Nitrogen Cycle UPPCS | Nitrogen Cycle Tricks to Remember | Steps of Nitrogen Cycle UPSC | Nitrogen Cycle Diagram
Master the Nitrogen Cycle for UPSC & UPPCS Exams! The Nitrogen Cycle is a crucial concept in environmental science and a recurring, high-scoring topic in both Prelims and Mains.
The Nitrogen Cycle is one of the major Nutrient Cycles that regulate the flow of essential elements through the environment. It ensures the continuous movement of nitrogen between the atmosphere, soil, plants, animals, and microbes.
This post by Self Study Wala provides clear, NCERT-based notes to help you master the cycle’s key processes—nitrogen fixation, nitrification, assimilation, ammonification, and denitrification—along with human impacts and related PYQs. Ideal for quick revision and strong conceptual clarity.
Table of Contents
Nitrogen Cycle UPSC & UPPCS
Nitrogen (N) is a vital component of amino acids, proteins, nucleic acids, and chlorophyll, forming about 16% of protein mass.
Although ~78% of the Earth’s atmosphere is nitrogen, most organisms cannot utilize atmospheric N₂ directly.
Hence, nitrogen must undergo a cyclical transformation into usable forms through the Nitrogen Cycle.
Steps involve in Nitrogen Cycle UPSC
The nitrogen cycle consists of five key processes:
1. Nitrogen Fixation – Step I of Nitrogen Cycle
Conversion of inert atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium (NH₄⁺) — a form usable by plants.
Types of Nitrogen Fixation:
Biological Fixation:
- By free-living bacteria: Azotobacter, Clostridium
- Symbiotic bacteria in legume root nodules: Rhizobium
- Cyanobacteria: Anabaena, Nostoc, Spirulina
Industrial Fixation:
- Haber-Bosch Process:
- N₂ + 3H₂ → 2NH₃
- (Ammonia used in fertilizers)
Atmospheric Fixation:
- Lightning and thunder convert N₂ into nitric oxide (NO) → nitrogen dioxide (NO₂) → nitric acid (HNO₃)
- Washed into soil via precipitation (acid rain)
2. Nitrification – Step II of Nitrogen Cycle
Conversion of ammonia (NH₃) into nitrite (NO₂⁻) and then into nitrate (NO₃⁻) by aerobic bacteria.
Two-step Process:
Step 1 – Ammonia → Nitrite
- Key Microorganisms: Nitrosomonas, Nitrococcus
- 2NH₃ + 3O₂ → 2NO₂⁻ + 2H⁺ + 2H₂O
Step 2 – Nitrite → Nitrate
- Key Microorganisms: Nitrobacter
- 2NO₂⁻ + O₂ → 2NO₃⁻
Plants prefer NO₃⁻ for uptake, though some can use NH₄⁺.
3. Assimilation – Step III of Nitrogen Cycle
- Absorption of nitrates (NO₃⁻) and ammonium (NH₄⁺) by plant roots to form organic nitrogen compounds like amino acids and proteins.
- Animals obtain nitrogen by consuming plants or herbivores.
- Nitrogen becomes part of the food chain.
4. Ammonification (Mineralization) – Step IV of Nitrogen Cycle UPSC
- Conversion of organic nitrogen (from dead organisms and excreta) into ammonia (NH₃) or ammonium (NH₄⁺) by decomposers (bacteria and fungi).
- Releases nitrogen back into the soil.
5. Denitrification – Step V of Nitrogen Cycle
- Conversion of nitrates (NO₃⁻) back into atmospheric nitrogen (N₂) under anaerobic conditions by denitrifying bacteria.
- Example organisms: Pseudomonas, Thiobacillus
- Occurs in waterlogged, oxygen-poor soils and marine sediments.
- Closes the nitrogen cycle loop.
Summary: Five Steps involve in Nitrogen Cycle
| Process | Conversion | Key Agents |
| 1. Nitrogen Fixation | Inert atmospheric Nitrogen (N₂) → Ammonia (NH₃) or Ammonium (NH₄⁺) | 1. Biological Fixation: Free-living bacteria: Azotobacter, ClostridiumSymbiotic bacteria: RhizobiumCyanobacteria: Anabaena, Nostoc, Spirulina 2. Atmospheric Fixation: Lightning and Thunder 3. Industrial Fixation: Haber-Bosch Process |
| 2. Nitrification | Ammonia (NH₃) → Nitrite (NO₂⁻) → Nitrate (NO₃⁻) | Aerobic bacteria Ammonia → Nitrite: Nitrosomonas, Nitrococcus Nitrite → Nitrate: Nitrobacter |
| 3. Assimilation | Nitrate (NO₃⁻) or Ammonium (NH₄⁺) → Organic N (Amino acids and Proteins) | By Plants |
| 4. Ammonification (Mineralization) | Organic N → Ammonia (NH₃) or Ammonium (NH₄⁺) | By Decomposers (bacteria and fungi) |
| 5. Denitrification | Nitrate (NO₃⁻) → Atmospheric Nitrogen (N₂) | Anaerobic bacteria – Pseudomonas, Thiobacillus |
Human Impact on the Nitrogen Cycle
Nitrogen cycle is the most human-impacted biogeochemical cycle. Human activities are significantly altering the balance of the nitrogen cycle:
1. Excessive Use of Nitrogen Fertilizers
- Leads to leaching of nitrates into groundwater.
- Causes eutrophication in aquatic ecosystems.
- Increases nitrous oxide (N₂O) emissions – a potent greenhouse gas.
2. Fossil Fuel Combustion
- Releases NOx gases (NO and NO₂), contributing to:
- Acid rain
- Photochemical smog
- Tropospheric ozone formation
3. Soil Degradation
- Nutrient imbalances (loss of Ca²⁺, K⁺)
- Soil acidification
- Loss of fertility and microbial diversity
4. Biodiversity Loss
- Excess nitrogen favors fast-growing invasive species, displacing native flora.
- Alters food chains and disrupts ecological balance.
5. Marine Ecosystem Disruption
Nutrient runoff causes:
- Eutrophication
- Algal blooms
- Hypoxia (oxygen depletion)
- Collapse of coastal fisheries
Self Study Notes for Environment | Environment Notes for UPSC & UPPCS | Nitrogen Cycle UPSC | Nitrogen Cycle UPPCS | Nitrogen Cycle Tricks to Remember | Steps of Nitrogen Cycle UPSC | Nitrogen Cycle Diagram
Frequently Asked Questions (FAQs) on the Nitrogen Cycle
1. Why is the nitrogen cycle essential for life on Earth?
The nitrogen cycle ensures the conversion of inert atmospheric nitrogen (N₂) into usable forms like ammonia, nitrate, and nitrite, which are essential for building proteins, amino acids, DNA, and chlorophyll in plants and animals.
2. Despite being 78% of the atmosphere, why can’t organisms use atmospheric nitrogen (N₂) directly?
Atmospheric N₂ is inert and non-reactive due to its triple bond, making it unusable by most organisms. It must be fixed into reactive forms like NH₃ or NO₃⁻ through the Nitrogen Fixation process.
3. What are the major steps of the nitrogen cycle?
The nitrogen cycle has five key steps:
- i. Nitrogen Fixation
- ii. Nitrification
- iii. Assimilation
- iv. Ammonification
- v. Denitrification
4. What is nitrogen fixation and which organisms perform it?
Nitrogen fixation converts atmospheric N₂ → NH₃/NH₄⁺. It is done by:
- Free-living bacteria (Azotobacter, Clostridium)
- Symbiotic bacteria (Rhizobium in legumes)
- Cyanobacteria (Anabaena, Nostoc)
- Lightning (atmospheric fixation)
- Haber-Bosch process (industrial fixation)
5. What role does nitrification play in the nitrogen cycle?
Nitrification converts ammonia (NH₃) into nitrite (NO₂⁻) and then nitrate (NO₃⁻), primarily by aerobic bacteria:
- NH₃ → NO₂⁻: Nitrosomonas, Nitrococcus
- NO₂⁻ → NO₃⁻: Nitrobacter
6. What is assimilation in the nitrogen cycle?
Assimilation is the uptake of NO₃⁻ or NH₄⁺ by plants, which convert them into organic nitrogen (amino acids, proteins). Animals obtain nitrogen by eating plants or herbivores.
7. What is ammonification (mineralization) and who performs it?
Ammonification is the conversion of organic nitrogen from dead organisms and waste into ammonia (NH₃) or ammonium (NH₄⁺) by decomposer bacteria and fungi.
8. How does denitrification complete the nitrogen cycle?
Denitrification converts nitrate (NO₃⁻) back to atmospheric nitrogen (N₂) under anaerobic conditions, by bacteria like Pseudomonas and Thiobacillus—thus closing the nitrogen loop.
9. How do human activities affect the nitrogen cycle?
Human interference includes:
- Overuse of fertilizers → eutrophication, groundwater pollution, N₂O emissions.
- Fossil fuel burning → NOx gases → smog, acid rain.
- Soil degradation and biodiversity loss due to nitrogen imbalance.
10. What are the ecological consequences of nitrogen cycle disruption?
- Disruption causes:
- Algal blooms, hypoxia, and collapse of aquatic life.
- Soil acidification and loss of fertility.
- Invasion by nitrogen-loving species, threatening native biodiversity.
Self Study Notes for Environment | Environment Notes for UPSC & UPPCS | Nitrogen Cycle UPSC | Nitrogen Cycle UPPCS | Nitrogen Cycle Tricks to Remember | Steps of Nitrogen Cycle UPSC
Nitrogen Cycle UPSC – PYQ Compilation (UPSC & UPPCS)
UPPCS Prelims 2008
Q. Match the process with its corresponding microorganism:
List-I (Process) — List-II (Microorganism)
A. Nitrogen fixation — 1. Nitrosomonas
B. Nitrification — 2. Rhizobium
C. Denitrification — 3. Pseudomonas
Code:
(a) A-2, B-1, C-3
(b) A-1, B-3, C-2
(c) A-3, B-2, C-1
(d) A-2, B-3, C-1
Answer: (a) A-2, B-1, C-3
Explanation:
Rhizobium fixes nitrogen
Nitrosomonas performs nitrification
Pseudomonas is a denitrifier
UPPCS Prelims 2012
Q. The cycling of elements in an ecosystem is called:
(A) Chemical cycles
(B) Biogeochemical cycles
(C) Geological cycles
(D) Geochemical cycles
Answer: (B) Biogeochemical cycles
Explanation:
Biogeochemical cycles describe the natural pathways by which essential elements of living matter (like nitrogen, carbon, phosphorus, sulfur) circulate through both biotic (living) and abiotic (non-living) components of the Earth. The Nitrogen Cycle is a key example of such a cycle.
UPPCS Prelims 2020
Q. The movement of nutrient elements through various components of an ecosystem is called:(A) Biogeochemical Cycle
(B) Geological Cycle
(C) Organic Cycle
(D) Hydrological Cycle
Answer: (A) Biogeochemical Cycle
Explanation:
In the Nitrogen Cycle, nitrogen transitions through atmosphere → soil (lithosphere) → plants and animals (biosphere) → back to the atmosphere, making it a perfect example of a biogeochemical nutrient movement. This process maintains ecosystem stability and nutrient availability.
UPPCS Prelims 2015
Q. Which of the following is not a part of the Nitrogen Cycle?
(a) Assimilation
(b) Eutrophication
(c) Denitrification
(d) Nitrification
Answer: (b) Eutrophication
Explanation:
Eutrophication is a nutrient pollution-related process, not a step of the nitrogen cycle itself.
UPPCS Prelims 2003
Q. Which of the following is a legume root-nodulating nitrogen-fixing bacteria?
(a) Azotobacter
(b) Nitrobacter
(c) Rhizobium
(d) Pseudomonas
Answer: (c) Rhizobium
Explanation:
Rhizobium fixes nitrogen in symbiosis with leguminous plants.
IAS Prelims 1998
Q. Consider the following statements:
1. Ammonification releases ammonia from dead organic matter.
2. Nitrification converts ammonia into nitrites and nitrates.
Choose the correct statement(s):
(a) Only 1
(b) Only 2
(c) Both 1 and 2
(d) Neither 1 nor 2
Answer: (c) Both 1 and 2
Explanation:
Ammonification and nitrification are both essential microbial processes in the nitrogen cycle.
IAS Prelims 2003
Assertion (A): Plants cannot directly use atmospheric nitrogen.
Reason (R): Atmospheric nitrogen is inert and requires fixation into ammonia or nitrates.
Code:
(a) Both A and R are true, and R is the correct explanation.
(b) Both A and R are true, but R is not the correct explanation.
(c) A is true, R is false.
(d) A is false, R is true.
Answer: (a) Both A and R are true, and R is the correct explanation.
Explanation:
Plants absorb nitrogen as nitrate or ammonium ions, not as atmospheric N₂.
IAS Prelims 2005
Consider the following:
1. Denitrification adds nitrogen to the soil.
2. Nitrogen-fixing bacteria live in root nodules.
3. Lightning can fix atmospheric nitrogen.
Which of the above are correct?
(a) 1 and 2
(b) 2 and 3
(c) 1 and 3
(d) 1, 2, and 3
Answer: (b) 2 and 3
Explanation:
Denitrification removes nitrogen from soil. Statements 2 and 3 are correct.
IAS Prelims 2013
Q. Concerning the food chains in ecosystems, which of the following kinds of organism is/are known as decomposer organism(s)?
- Virus
- Fungi
- Bacteria
(A) Only 1
(B) 2 and 3
(C) 1 and 3
(D) All of these
Answer: (B) 2 and 3
Explanation:
In the ammonification step of the Nitrogen Cycle, fungi and bacteria decompose organic nitrogen (from dead plants, animals, and waste) into ammonia (NH₃).
Viruses are not decomposers; they are obligate parasites and cannot decompose organic matter independently.
Self Study Notes for Environment | Environment Notes for UPSC & UPPCS | Nitrogen Cycle UPSC | Nitrogen Cycle UPPCS | Nitrogen Cycle Tricks to Remember | Steps of Nitrogen Cycle UPSC
Model questions on the Nitrogen Cycle UPSC and UPPCS Prelims
1. Which of the following processes converts atmospheric nitrogen into usable forms by plants?
(a) Denitrification
(b) Nitrification
(c) Nitrogen Fixation
(d) Ammonification
Answer: (c) Nitrogen Fixation
Explanation:
Nitrogen fixation is the process where nitrogen gas (N₂) is converted into ammonia (NH₃) by bacteria like Rhizobium, making it usable by plants.
2. Arrange the following steps of the nitrogen cycle in correct order:
I. Nitrification
II. Nitrogen fixation
III. Assimilation
IV. Denitrification
Code:
(a) II → I → III → IV
(b) I → II → IV → III
(c) II → III → I → IV
(d) IV → I → II → III
Answer: (a) II → I → III → IV
Explanation:
Nitrogen is first fixed, then nitrified, assimilated by plants, and finally returned to atmosphere via denitrification.
3. Which compound is formed during ammonification in Nitrogen Cycle?
(a) NO₃⁻
(b) NH₃
(c) NO₂⁻
(d) N₂
Answer: (b) NH₃
Explanation:
Ammonification is the conversion of organic nitrogen into ammonia (NH₃) by decomposers.
4. Choose the correct statement about Nitrogen Cycle:
(a) It is entirely a physical process.
(b) Plants use nitrogen gas directly.
(c) Bacteria play a key role.
(d) Nitrogen never returns to the atmosphere.
Answer: (c) Bacteria play a key role.
Explanation:
Multiple bacteria are involved in fixation, nitrification, ammonification, and denitrification.
5. Consider the following statements:
1. Nitrobacter converts ammonia to nitrite.
2. Nitrosomonas converts nitrite to nitrate.
Which is correct?
(a) Only 1
(b) Only 2
(c) Both
(d) Neither
Answer: (d) Neither
Explanation:
Nitrosomonas converts ammonia to nitrite, Nitrobacter converts nitrite to nitrate.
6. What are the outcomes of denitrification?
A. Nitrogen gas released
B. Fertility increases
Code:
(a) Only A
(b) Only B
(c) A and B
(d) Neither A nor B
Answer: (a) Only A
Explanation:
Denitrification leads to loss of usable nitrogen as it returns to gaseous form, reducing fertility.
7. Match the compound to the process that produces it:
A. Ammonia — 1. Nitrification
B. Nitrite — 2. Ammonification
C. Nitrate — 3. Oxidation of nitrite
Code:
(a) A-2, B-1, C-3
(b) A-1, B-2, C-3
(c) A-2, B-3, C-1
(d) A-2, B-1, C-3
Answer: (d) A-2, B-1, C-3
Explanation:
Ammonification produces ammonia
Nitrification (first step) → nitrite
Second step of nitrification → nitrate
8. Which of the following processes involve bacteria?
1. Nitrification
2. Assimilation
3. Nitrogen Fixation
4. Denitrification
(a) 1, 3, and 4 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2, 3, and 4
Answer: (a) 1, 3, and 4 only
Explanation:
Assimilation is carried out by plants; all others are bacterial processes.
9. Which of the following statements is incorrect about Nitrogen Cycle?
(a) It involves both aerobic and anaerobic bacteria.
(b) Nitrification occurs in water-logged conditions.
(c) Lightning helps in nitrogen fixation.
(d) Denitrification converts nitrate to nitrogen gas.
Answer: (b) Nitrification occurs in water-logged conditions.
Explanation:
Nitrification is an aerobic process and is inhibited in water-logged (anaerobic) conditions.
10. Which of the following human activities contributes significantly to nitrogen pollution in aquatic ecosystems?
(a) Deforestation
(b) Use of chemical fertilizers
(c) Construction of dams
(d) Urban expansion
Answer: (b) Use of chemical fertilizers
Explanation:
Excessive use of nitrogen-rich fertilizers leads to runoff into water bodies, causing eutrophication, algal blooms, and aquatic ecosystem degradation.
11. Which of the following is true about denitrification?
(a) It converts nitrates into nitrogen gas
(b) It adds nitrogen to the soil
(c) It helps in ammonification
(d) It is carried out by Rhizobium
Answer: (a) It converts nitrates into nitrogen gas
Explanation:
Denitrification is the process by which nitrate (NO₃⁻) is reduced to nitrogen gas (N₂) by bacteria like Pseudomonas and Clostridium, releasing it back into the atmosphere.
12. Assertion (A): Nitrification increases soil fertility.
Reason (R): Nitrates formed are easily absorbed by plants.
Code:
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, R is false
(d) A is false, R is true
Answer: (a) Both A and R are true, and R is the correct explanation of A
Explanation: Nitrification forms nitrates, which are highly water-soluble and can be easily absorbed by plants, improving soil fertility.
13. Which of the following pairs is correctly matched?
(a) Azotobacter – Symbiotic nitrogen fixer
(b) Nitrosomonas – Denitrifying bacteria
(c) Pseudomonas – Denitrifying bacteria
(d) Nitrobacter – Ammonifying bacteria
Answer: (c) Pseudomonas – Denitrifying bacteria
Explanation:
Azotobacter is a free-living, not symbiotic fixer
Nitrosomonas is a nitrifying bacterium
Pseudomonas performs denitrification
Nitrobacter is involved in nitrification, not ammonification
Self Study Notes for Environment | Environment Notes for UPSC & UPPCS | Nitrogen Cycle UPSC | Nitrogen Cycle UPPCS | Nitrogen Cycle Tricks to Remember | Steps of Nitrogen Cycle UPSC
Self Study Notes for Environment | Environment Notes for UPSC & UPPCS | Nitrogen Cycle UPSC | Nitrogen Cycle UPPCS | Nitrogen Cycle Tricks to Remember | Steps of Nitrogen Cycle UPSC | Nitrogen Cycle Diagram
Powered by Self Study Wala – Trusted by 10,000+ UPSC/UPPCS Aspirants
About Self Study Wala
Self Study Wala – India’s Leading Self-Study Platform for UPSC, UPPCS & Other State PCS Preparation
Self Study Wala is a bootstrapped EdTech startup, self-funded and founded by Mohit Sharma, who also serves as the Director of Sankalp IAS Academy.
Self Study Wala is a trusted, top-rated online learning platform designed exclusively for UPSC, UPPCS, and other state PCS aspirants. Built for serious self-learners, it offers affordable, bilingual, and expert-curated study material — without the noise and distractions of traditional coaching models.
Founded and led by Mohit Sharma, the Director of Sankalp IAS Academy, Self Study Wala is a bootstrapped EdTech startup dedicated to making high-quality, structured, and exam-oriented content accessible to every aspirant — especially those from Tier-2 and Tier-3 cities of India. With over 14+ years of mentorship experience, Mohit Sharma has personally guided thousands of students, many of whom are now serving as administrators and officers across the country.
Self Study Wala is best known for:
📚 Regularly updated eBooks & test series for Prelims & Mains
🧠 Smart GK Tricks and simplified UPSC content
💸 Low-cost, high-impact resources trusted by 10,000+ aspirants
📚 UPPCS-focused Bilingual Current Affairs & monthly magazines in Hindi-English
📲 Mobile-friendly PDFs, infographics, and quick-revision reels
Whether you’re a first-timer or a seasoned repeater, Self Study Wala empowers aspirants with the tools and strategy to crack civil services exams.
👉 Join the revolution of self-learners who trust Self Study Wala for reliable, relevant, and result-driven preparation.
Self Study Wala – अब कामयाबी अपने दम पर !