Oxygen and Nitrogen Cycle — REET Science Notes
This page covers the 'Oxygen and Nitrogen Cycle' topic from REET Level 2 Science, aligned with the Class 9 NCERT syllabus — the stages of both cycles, the role of the bacteria involved, and the balance maintained in nature. In the exam, questions on the correct order of the stages and the names of the bacteria come up often, so every stage is explained here in clear sequence.
- 0 questions in REET 2024
- Class 6–8 level
- 7 min read
How this topic is asked in REET
No direct question from the 'Oxygen and Nitrogen Cycle' topic was recorded in the REET 2024 paper, so the question pattern here is based on the language of the Class 9 NCERT syllabus ('Natural Resources'), not on any verified previous question. Typically, questions are likely to be based on the correct sequence of the cycle's stages, the names of the bacteria involved in nitrogen fixation, and the role of oxygen in photosynthesis and respiration.
Nitrogen in the atmosphere and why it's needed
About 78% of the atmosphere is nitrogen gas (N₂), making it the most abundant gas. Yet most organisms cannot directly use this free nitrogen gas, because the two nitrogen atoms in an N₂ molecule are joined by an extremely strong triple bond, which makes it chemically inert. This is why organisms need nitrogen in a combined form — such as ammonia and nitrates — to build essential molecules like proteins and DNA.
Nitrogen is an extremely essential element for living organisms because it is a major component of important biological molecules such as proteins, DNA, RNA and chlorophyll. This is why the availability of adequate amounts of usable nitrogen compounds in the soil is essential for the proper growth and development of plants; a shortage of nitrogen causes plant leaves to turn yellow, a condition called chlorosis.
Main stages of the nitrogen cycle
- 1. Nitrogen fixation
- Converting free N₂ into usable compounds — biologically by Rhizobium bacteria (in the root nodules of leguminous plants) and free-living bacteria such as Azotobacter; abiotically through lightning; industrially through the Haber process (used in fertiliser manufacturing)
- 2. Nitrification
- Conversion of ammonia into nitrite by Nitrosomonas bacteria, then into nitrate by Nitrobacter bacteria
- 3. Assimilation
- Plants absorb nitrate/ammonium through their roots and build proteins and amino acids; animals obtain nitrogen by eating plants
- 4. Ammonification
- Decomposers (bacteria and fungi) break down dead organisms and waste, returning nitrogen to the soil in the form of ammonia
- 5. Denitrification
- Denitrifying bacteria (such as Pseudomonas) convert nitrate back into free N₂ gas, returning it to the atmosphere — this completes the cycle
The oxygen cycle
About 21% of the atmosphere is oxygen gas (O₂). The oxygen cycle is mainly based on the balance between photosynthesis and respiration. Green plants, algae, and some bacteria, in the process of photosynthesis, make glucose from carbon dioxide and water in the presence of sunlight and release oxygen gas, which keeps replenishing the atmosphere's oxygen.
In contrast, all living organisms (both plants and animals) use oxygen in respiration to obtain energy from food and release carbon dioxide gas. In addition, combustion (burning) and decomposition also use up atmospheric oxygen. In this way, the production of oxygen through photosynthesis and its consumption through respiration, combustion and decomposition together keep the level of oxygen in the atmosphere balanced. The oxygen cycle is deeply linked to the carbon cycle.
A large amount of the earth's oxygen is also produced by marine algae (phytoplankton). In addition, forests, especially tropical rainforests, also produce large quantities of oxygen, which is why deforestation has an adverse effect on the atmospheric oxygen-carbon dioxide balance.
Key bacteria involved in the nitrogen cycle
| Bacteria | Role |
|---|---|
| Rhizobium | Biological nitrogen fixation in the root nodules of leguminous plants |
| Azotobacter | Free-living nitrogen fixation in the soil |
| Nitrosomonas | Converting ammonia into nitrite (the first stage of nitrification) |
| Nitrobacter | Converting nitrite into nitrate (the second stage of nitrification) |
| Denitrifying bacteria (such as Pseudomonas) | Converting nitrate back into free N₂ gas |
| Clostridium | Another free-living bacterium in the soil that carries out nitrogen fixation |
Key facts about the oxygen cycle
- Equation for photosynthesis (in simple form)
- Carbon dioxide + water + sunlight → glucose + oxygen (in the presence of chlorophyll)
- Use of oxygen in respiration
- Energy is obtained from the oxidation of food (glucose), and carbon dioxide and water are released
- Role of oxygen in combustion
- Oxygen is essential for fuel to burn; combustion is not possible without oxygen
- Oxygen dissolved in water
- Aquatic organisms (such as fish) respire using oxygen dissolved in water
- The ozone layer (O₃)
- Ozone gas, made of three oxygen atoms, in the upper atmosphere (stratosphere) absorbs the sun's harmful ultraviolet (UV) rays and protects life
The importance of these cycles in nature
The nitrogen and oxygen cycles ensure the continuous recycling of elements in nature, so that the nutrients and gases essential for life remain constantly available. If these cycles are thrown out of balance — for example, photosynthesis decreasing due to excessive deforestation, or disruption of the nitrogen cycle from excessive fertiliser use — it can have a serious impact on the environment. This is why growing leguminous plants (such as chickpea, moong) as part of crop rotation is a traditional and useful way to naturally increase nitrogen in the soil.
Farmers often find that soil fertility decreases after growing the same type of crop (like wheat) continuously in a field, because plants keep taking nitrogen-rich nutrients from the soil. In such cases, growing leguminous crops periodically as part of crop rotation naturally replenishes nitrogen in the soil through Rhizobium bacteria, which also reduces dependence on chemical fertilisers — this is an important practice in sustainable farming.
In addition, excessive and unbalanced use of nitrogen-based chemical fertilisers (such as urea) increases the amount of nitrate in soil and water sources, which can cause problems such as water pollution and eutrophication. This is why balanced fertiliser use and promoting organic farming are important for keeping the nitrogen cycle healthy.
How to teach this to students
Classroom activity
Show students the nodules on the roots of a leguminous plant (chickpea/moong) and explain that Rhizobium bacteria live in them and carry out nitrogen fixation.
Teaching aids
Have students draw the nitrogen cycle as a circular diagram (chart) and write out the stages in order themselves — this makes the sequence easier to remember.
Common misconception
Students often think that plants directly absorb atmospheric N₂ gas. Make it clear that plants can only absorb nitrogen in a combined form, such as nitrate or ammonium, not free N₂ gas.
Assessment idea
Ask students what effect continuously growing the same crop (like wheat) in a field would have on soil fertility, and why rotating leguminous crops is necessary — this checks practical understanding.
A trick to remember this
Remember the five stages of the nitrogen cycle in order: 'fixation → nitrification → assimilation → ammonification → denitrification.' To remember the bacteria, keep in mind: 'Nitrosomonas makes nitrite first, Nitrobacter then makes nitrate' — the order of the names also helps you remember the order of the stages.
Practice questions
Question 1
What is the approximate percentage of nitrogen gas in the atmosphere?
- A21%
- B0.03%
- C50%
- D78%
Explanation
Nitrogen gas (N₂) makes up about 78% of the atmosphere, making it the most abundant atmospheric gas.
Question 2
In which form do plants absorb nitrogen?
- Aमुक्त N₂ गैस के रूप में
- Bकार्बन डाइऑक्साइड के रूप में
- Cनाइट्रेट/अमोनियम जैसे संयुक्त रूप में
- Dऑक्सीजन के रूप में
Explanation
Since free N₂ gas is chemically inert, plants cannot use it directly, so they absorb nitrogen in a combined form, such as nitrate or ammonium.
Question 3
Where is the Rhizobium bacterium found?
- Aफलीदार पौधों की जड़ ग्रंथियों में
- Bवायुमंडल में
- Cसमुद्र में
- Dमनुष्य की आँत में
Explanation
Rhizobium bacteria is found in the root nodules of leguminous plants and carries out biological nitrogen fixation.
Question 4
The bacterium that converts ammonia into nitrite is
- Aनाइट्रोबैक्टर
- Bराइजोबियम
- Cनाइट्रोसोमोनास
- Dएजोटोबैक्टर
Explanation
In the first stage of nitrification, Nitrosomonas bacteria convert ammonia into nitrite, and then Nitrobacter converts it into nitrate.
Question 5
The process of converting nitrate back into free nitrogen gas is called
- Aनाइट्रीकरण
- Bस्वांगीकरण
- Cअमोनीकरण
- Dविनाइट्रीकरण
Explanation
Denitrifying bacteria convert soil nitrate back into free N₂ gas and return it to the atmosphere — this is called denitrification.
Question 6
What is the approximate percentage of oxygen gas in the atmosphere?
- A21%
- B78%
- C0.9%
- D4%
Explanation
Oxygen gas (O₂) makes up about 21% of the atmosphere, making it the second most abundant gas after nitrogen (78%).
Question 7
Nitrogen fixation occurring during lightning is an example of which type of fixation?
- Aजैविक स्थिरीकरण
- Bऔद्योगिक स्थिरीकरण
- Cअजैविक (वायुमंडलीय) स्थिरीकरण
- Dविनाइट्रीकरण
Explanation
The energy generated by lightning combines atmospheric nitrogen and oxygen to form nitrogen oxides, which reach the soil with rainwater — this is abiotic (atmospheric) nitrogen fixation.
Question 8
The process of returning nitrogen to the soil as ammonia through the decomposition of dead organisms and waste is called
- Aनाइट्रीकरण
- Bअमोनीकरण
- Cस्वांगीकरण
- Dविनाइट्रीकरण
Explanation
Decomposer bacteria and fungi break down dead organisms and waste, returning nitrogen to the soil as ammonia — this is called ammonification.
Question 9
The most effective way to help students remember the sequence of steps while teaching the nitrogen cycle in class is
- Aकेवल चरणों के नाम रटवाना
- Bगोलाकार आरेख बनवाकर चरणों को क्रम में स्वयं लिखवाना
- Cकेवल परीक्षा प्रश्न हल करवाना
- Dटॉपिक को छोड़ देना
Explanation
Drawing the diagram themselves helps students deeply understand the logical order and relationship of each stage of the cycle.
Question 10
If a student thinks that plants directly absorb N₂ gas from the atmosphere, the teacher should clarify that
- Aयह सही है
- Bपौधों को नाइट्रोजन की आवश्यकता नहीं होती
- Cयह केवल फलीदार पौधों पर लागू होता है
- Dपौधे केवल नाइट्रेट/अमोनियम जैसे संयुक्त रूप में ही नाइट्रोजन ले सकते हैं, मुक्त गैस को नहीं
Explanation
The triple bond in the N₂ molecule makes it chemically inert, so plants cannot use it directly and instead need nitrogen in a combined form such as nitrate or ammonium.
Frequently asked questions
What percentage of the atmosphere is nitrogen?+
About 78% of the atmosphere is nitrogen gas (N₂), making it the most abundant atmospheric gas.
What is nitrogen fixation?+
The process of converting free atmospheric nitrogen gas (N₂) into useful compounds such as ammonia or nitrate is called nitrogen fixation.
Where is Rhizobium bacteria found and what does it do?+
Rhizobium bacteria is found in the root nodules of leguminous plants (such as chickpea, pea), and it carries out biological fixation of atmospheric nitrogen.
What is denitrification?+
Denitrification is the process in which denitrifying bacteria convert soil nitrate back into free nitrogen gas, returning it to the atmosphere.
What role does photosynthesis play in the oxygen cycle?+
In photosynthesis, green plants make food from carbon dioxide and water and release oxygen, which keeps continuously replenishing the atmosphere's oxygen.
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Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked