Chapter 13 – Nitrogen Cycle

Are you finding the Nitrogen Cycle difficult to understand? Don’t worry—this chapter is very important for your WBBSE Class 10 Geography exam. On this page, you will find important questions and answers explained in simple and clear English. These questions are based on important topics and exam patterns.
This will help you learn easily, revise quickly, and score better marks in your Madhyamik exam.

Content Structure

1. Multiple choice questions and answers (MCQs)
2. Very short-type questions and answers
- 2A. Fill in the blanks
- 2B. True or false
- 2C. Match the following
- 2D. Single word questions and answers
3. Short-type questions and answers
4. Long-type questions and answers

1. Multiple choice questions and answers (MCQs)

1. The approximate percentage of N2 gas in the atmosphere is-
(a) 77.17%
(b) 79.17%
(c) 80%
(d) 78.17%

Ans: (d) 78.17%

2. The process of release of ammonia and its formation to ammonium ions is known as-
(a) Ammonification
(b) Nitrification
(c) Denitrification
(d) Nitrogen assimilation

Ans: (a) Ammonification

3. The process of converting ammonia to nitrate is known as –
(a) Ammonification
(b) Denitrification
(c) Nitrification
(d) Sedimentation

Ans: (c) Nitrification

4. The percentage of nitrogen in atmosphere is?
(a) 0.03
(b) 77.17
(c) 20.60
(d) 0.04

Ans: (c) 20.60

5. The most abundant gas in the atmosphere is-
(a) Oxygen
(b) Nitrogen
(c) Hydrogen
(d) Carbon dioxide

Ans: (b) Nitrogen

6. Plants can absorb nitrogen from soil in the from of-
(a) N2o
(b) NO2
(c) NO
(d) NO2 and NO3

Ans: (d) NO2 and NO3

7. A nitrogen-fixing bacterium is-
(a) Azotobacter
(b) Mycobacterium
(c) Nitrobacter
(d) Nitrosomonas

Ans: (a) Azotobacter

8. The microorganism inhabiting the root nodules of sweet pea plant is-
(a) Nostoc
(b) Anabaena
(c) Rhizobium
(d) Chlamydomonas

Ans: (c) Rhizobium

9. Amonification in the formation of-
(a) Ammonia from nitrogen
(b) Ammonia from nitrates by decomposers
(c) Ammonia from amino acid
(d) Ammonia from nitrates by nitrogen fixers

Ans: (c) Ammonia from amino acid

10. Which of the following is not involved with nitrogen cycle?
(a) Nitrification
(b) Ammonification
(c) Eutrophication
(d) Denitrification

Ans: (c) Eutrophication

11. The plant, which traps and consumes insects to fulfil the need for nitrogen, is-
(a) Nayantara
(b) Sarpagandha
(c) Pitcher plant
(d) Sunflower

Ans: (c) Pitcher plant

12. Which of the following is not a nitrogen-fixing bacterium?
(a) Azotobacter
(b) Mycobacterium
(c) Nostoc
(d) Rhizobium

Ans: (b) Mycobacterium

13. Which of the following components is increased in soil due to cultivation of leguminous plants in it?
(a) Sulphar dioxide
(b) Oxygen
(c) Carbon dioxide
(d) Nitrogen

Ans: (d) Nitrogen

14. Application of nitrogen in soil-
(a) Reduces fertality
(b) Increases fertality
(c) Increases PH
(d) Has no effect on fertality

Ans: (b) Increases fertility

15. The percentage of nitrogen in atmosphere is about-
(a) 78.09
(b) 1.40
(c) 0.08
(d) 20.60

Ans: (a) 78.09

16. The component of air, which neither plants nor animals can absorb directly, is-
(a) Oxygen
(b) Carbon dioxide
(c) Nitrogen
(d) Both a & b

Ans: (c) Nitrogen

17. The enzyme by which nitrogen-fixing bacteria anaerobically convert atmosphare nitrogen into ammonia is-
(a) Nitrogenase
(b) Amylase
(c) Galactase
(d) Pectinase

Ans: (a) Nitrogenase

18. Coversation of ammonia into nitrate is called-
(a) Denitrification
(b) Nitrification
(c) Calcification
(d) Ammonification

Ans: (b) Nitrification

19. The blue-green algae capable of absorbing nitrogen from atmosphere is-
(a) Clostridium
(b) Rhodochorton
(c) Anabaena
(d) Chlamydomonas

Ans: (c) Anabaena

20. A nitrogen-releasing bacterium is-
(a) Rhizobium
(b) Diatom
(c) Nostor
(d) Thiobacillus

Ans: (d) Thiobacillus

21. The nitrogenous compound responsible for acid rain is-
(a) NO
(b) HNO3
(c) NO2
(d) All of these

Ans: (d) All of these

22. The nitrogenous compounds responsible for ozone depletion are-
(a) NO and NO2
(b) NH3 and HNO2
(c) NH4OH and HNO3
(d) HNO2 and HNO 3

Ans: (a) NO and NO₂

23. A nitrogenous greenhouse gas is-
(a) N2O
(b) NH3
(c) HNO3
(d) All of these

Ans: (a) N2O

24. Nitrate in soil is transformed into ammonia and then into free nitrogen by bacterial action. This process is called-
(a) Nitrification
(b) Ammonification
(c) Symbiosis
(d) Denitrification

Ans: (d) Denitrification

25. A soil-dweling free-living nitrogen fixing bacterium is-
(a) Nostac
(b) Diatom
(c) Clostridium
(d) Anabaena

Ans: (c) Clostridium

26. Bacillus mycoides is a type of-
(a) Symbiotic bacterium
(b) Denitrifying bacterium
(c) Nitrifying bacterium
(d) Ammonifying bacterium

Ans: (d) Ammonifying bacterium

27. Which of the following microbes take part in nitrification? [Madhyamik 18]
(a) Nitrosomonas
(b) Thiobacillus
(c) Pseudomonas
(d) Azotobacter

Ans: (a) Nitrosomonas

28. Which of the following phase of nitrogen cycle pseudomonas is associated with? [Madhyamik 17]
(a) Nitrification
(b) Ammonification
(c) Nitrogen fixation
(d) Denitrification

Ans: (d) Denitrification

2. Very short-type questions and answers

2A. Fill in the blanks

1. _____________ is a kind of nitrifying bacteria. (Nitrosomonas)
2. In the nitrogen cycle Nitrobacter converts nitrites into _____________. (nitrates)
3. __________ is an ammonifying bacteria. (Bacillus)
4. Denitrifying bacteria converts nitrate and nitrites into free ___________. (nitrogen)
5. The process of trapping atmospheric nitrogen in soil is known as ________. (nitrogen fixation)
6. ____________ develops root nodules in the root system of leguminous plants. (Rhizobium)
7. Anabaena is an example of ___________. (cyanobacteria)
8. During lightning, _____________ is produced in atmosphere by the combination of nitrogen and oxygen. (nitric oxide)
9. Nitrosomonas is a type of ______________ bacterium. (nitrifying)
10. ____________ is a bacterium which helps in conversion of nitrite into nitrate. (Nitrobacter)
11. Pea, gram and other pulses are known as ___________ plants. (leguminous)
12. Rhizobium is a ___________ nitrogen fixing bacterium. (symbiotic)
13. NO₂ of atmosphere mixes with rain water to produce __________ and nitric acid. (nitrous acid)
14. Azotobacter is a ___________ nitrogen-fixing bacterium. (free-living)
15. At the ____________ phase of nitrogen cycle ammonia is converted into nitrite and nitrate by the action of some bacteria. (nitrification) [Madhyamik 20]

2B. True or false

1. Nitrosomonas is a denitrifying bacterium. (False)
2. Haber-Bosch reaction is a natural process of nitrogen fixation. (False)
3. Clostridium is a free-living nitrogen-fixing bacterium. (True)
4. Ammonia dissolves in water to produce nitrous acid and nitric acid. (False)
5. The bacterial species helping in ammonification process is Bacillus mycoides. (True)
6. Nostoc is capable of absorbing atmospheric nitrogen. (True)
7. Azotobacter releases free nitrogen from nitrates trapped in soil. (False)
8. Carbon dioxide is a nitrogenous greenhouse gas. (False)
9. The other name of ammonification is mineralisation. (True)
10. Anabaena is an example of ammonification is mineralisation. (False)

2C. Match the following

A	B
1. Free living nitrogen fixing bacteria	A. Pseudomonas
2. Nitrifying bacteria	B. Azotobacter
3. Symbiotic bacteria	C. Nitrosomonas
4. Denitrifying bacteria	D. Rhizobium
5. Symbiotic cyanobacteria	E. Anabaena
6. Free-living nitrogen fixing cyanobacteria	F. Agrobacterium
	G. Nostoc

Ans:

A	B
1. Free-living nitrogen fixing bacteria	B. Azotobacter
2. Nitrifying bacteria	C. Nitrosomonas
3. Symbiotic bacteria	D. Rhizobium
4. Denitrifying bacteria	A. Pseudomonas
5. Symbiotic cyanobacteria	E. Anabaena
6. Free-living nitrogen fixing cyanobacteria	G. Nostoc

A	B
1. Nitrogen fixing plants	A. Industrial fixation of nitrogen
2. Nostoc absorbs atmospheric nitrogen and traps it in soil	B. Ammonification
3. Bacillus mycoides	C. Fixation of nitrogen by free living microorganism
4. Production of ammonia by Haber-Bosch process	D. Leguminous plants
5. Conversion of nitrate into free nitrogen	E. Denitrification
6. Product of reaction between oxygen and nitrogen	F. Nitric oxide
	G. Insectivorous plants

Ans:

A	B
1. Nitrogen fixing plants	D. Leguminous plants
2. Nostoc absorbs atmospheric nitrogen and traps it in soil	C. Fixation of nitrogen by free-living microorganism
3. Bacillus mycoides	B. Ammonification
4. Production of ammonia by Haber–Bosch process	A. Industrial fixation of nitrogen
5. Conversion of nitrate into free nitrogen	E. Denitrification
6. Product of reaction between oxygen and nitrogen	F. Nitric oxide

2D. Single word questions and answers

1. Mention the percentage of N2 in the atmosphere.
Ans: 78% nitrogen (N₂) is present in the atmosphere.

2. Name one nitrifying bacteria.
Ans: Nitrosomonas is one nitrifying bacterium.

3. Name one denitrifying bacteria.
Ans: Pseudomonas is a denitrifying bacterium.

4. Name a bacterium which can fix nitrogen from the air.
Ans: Azotobacter is a bacterium that can fix nitrogen from the air.

5. What is Rhizobium?
Ans: Rhizobium is a symbiotic bacterium that lives in the root nodules of leguminous plants and fixes atmospheric nitrogen.

6. Name an algae which can trap atmospheric nitrogen.
Ans: Anabaena is an alga that can trap atmospheric nitrogen.

7. Which is the general source of nitrogen for higher plants?
Ans: The general source of nitrogen for higher plants is nitrates present in the soil.

8. From where do the animals get nitrogen?
Ans: Animals get nitrogen from plants or other animals through food.

9. Name the process of cyclic rotation of various elements and inanimate matters in the biosphere.
Ans: The cyclic rotation of elements and inanimate matter in the biosphere is called the biogeochemical cycle.

10. Name a nitrifying bacterium.
Ans: Nitrobacter is a nitrifying bacterium.

11. In which form does nitrogen stay in soil?
Ans: Nitrogen stays in the soil mainly in the form of nitrates and ammonium salts.

12. Give examples of two cyanobacteria which are capable of fixing nitrogen from atmosphere.
Ans: Anabaena and Nostoc are two cyanobacteria capable of fixing atmospheric nitrogen.

13. Which role is played by fungi in the environment?
Ans: Fungi act as decomposers and help in breaking down dead and decaying organic matter.

14. Name two plants which feed on insects to obtain nitrogen.
Ans: Pitcher plant and Venus flytrap are two plants that feed on insects to obtain nitrogen.

15. Name the process where nitrates are converted into free nitrogen.
Ans: The process in which nitrates are converted into free nitrogen is called denitrification.

16. Name the two steps of nitrification.
Ans: The two steps of nitrification are formation of nitrite and formation of nitrate.

17. By which process of nitrification, nitrite is produced from ammonia?
Ans: In nitrification, ammonia is converted into nitrite by Nitrosomonas.

18. Mention the role of bio-geochemical cycle.
Ans: The biogeochemical cycle helps in maintaining balance and recycling nutrients in nature.

19. Name a gas that is essential for all plants but they cannot absorb it directly from the atmosphere.
Ans: Nitrogen gas is essential for plants but they cannot absorb it directly from the atmosphere.

20. Name a denitrifying bacterium.
Ans: Pseudomonas is a denitrifying bacterium.

21. Mention the types of microorganisms which help in biological fixation of nitrogen in soil.
Ans: Bacteria and cyanobacteria help in biological fixation of nitrogen in soil.

22. Name the pinkish red pigment present in the root nodules of leguminous plants.
Ans: Leghaemoglobin is the pinkish-red pigment present in the root nodules of leguminous plants.

23. Name a nitrogen fixing free living bacterium.
Ans: Azotobacter is a free-living nitrogen-fixing bacterium.

24. Name a nitrogen-fixing symbiotic bacterium.
Ans: Rhizobium is a nitrogen-fixing symbiotic bacterium.

25. Which type of bacteria convert nitrates into free nitrogen?
Ans: Denitrifying bacteria convert nitrates into free nitrogen.

26. Name a process by which industrial nitrogen fixation occurs?
Ans: Industrial nitrogen fixation occurs by the Haber process.

27. Name the process of ammonia formation by decomposition of dead and decaying organic matters.
Ans: The formation of ammonia from dead and decaying organic matter is called ammonification.

28. Name two non-leguminous nitrogen-fixing plants.
Ans: Alnus and Casuarina are two non-leguminous nitrogen-fixing plants.

29. Which enzyme helps to catalyse the reactions for N2 fixation?
Ans: Nitrogenase enzyme catalyses the reactions for nitrogen fixation.

30. Write down names of two nitrogen containing chemical fertiliser.
Ans: Urea and ammonium sulphate are two nitrogen-containing chemical fertilisers.

3. Short-type questions and answers

1. Name two nitrifying bacteria.
Ans: Nitrosomonas and Nitrobacter are two nitrifying bacteria this converts ammonia into nitrite, while Nitrobacter converts nitrite into nitrate in the soil.

2. Name two denitrifying bacteria.
Ans: Pseudomonas and Thiobacillus are denitrifying bacteria these bacteria convert nitrates present in the soil into free nitrogen gas and release it into the atmosphere.

3. What is nitrogen cycle?
Ans: The nitrogen cycle is a natural and continuous process in which nitrogen circulates between the atmosphere, soil, plants, animals and microorganisms. It helps in maintaining the balance of nitrogen in nature.

4. Mention importance of nitrogen for the organism.
Ans: Nitrogen is very important for living organisms because it is a main component of proteins, enzymes, nucleic acids and chlorophyll. Without nitrogen, normal growth and development of plants and animals is not possible.

5. Define biogeochemical cycle.
Ans: A biogeochemical cycle is the cyclic movement of chemical elements like carbon, oxygen and nitrogen between living organisms and the non-living environment such as air, water and soil.

6. What is nitrogen cycle? What is nitrogen fixation?
Ans: The nitrogen cycle is the continuous movement of nitrogen through different forms in nature. Nitrogen fixation is the process by which atmospheric nitrogen is converted into ammonia or nitrates by bacteria, lightning or industrial methods.

7. Discuss the significance cycle relation of the elements like C, O2 and N2 in the environment.
Ans: The cyclic relation of carbon, oxygen and nitrogen helps in recycling these elements in nature. Carbon cycle supports photosynthesis and respiration, oxygen cycle helps in breathing and combustion, and nitrogen cycle maintains soil fertility and plant growth.

8. Write the percentage of nitrogen, oxygen and carbon in the atmosphere.
Ans: The atmosphere contains about 78% nitrogen, 21% oxygen and nearly 0.03% carbon dioxide.

9. Mention the importance of nitrogen in living organisms.
Ans: Nitrogen is essential for the formation of proteins, DNA, RNA and enzymes. It also plays an important role in plant growth by helping in the formation of chlorophyll.

10. What is ammonification?
Ans: Ammonification is the process by which decomposer bacteria and fungi convert organic nitrogen from dead plants and animals into ammonia and ammonium compounds in the soil.

11. What is nitrification?
Ans: Nitrification is a biological process in which ammonia is first converted into nitrite by Nitrosomonas and then into nitrate by Nitrobacter in the presence of oxygen.

12. What is denitrification?
Ans: Denitrification is the process in which denitrifying bacteria convert nitrates in the soil into free nitrogen gas and return it to the atmosphere.

13. Mention the role of bacteria in nitrogen cycle.
Ans: Bacteria play an important role in the nitrogen cycle by carrying out nitrogen fixation, ammonification, nitrification and denitrification, thus keeping nitrogen available in usable forms.

14. Mention the role oof bacteria in nitrification.
Ans: In nitrification, Nitrosomonas converts ammonia into nitrite and Nitrobacter converts nitrite into nitrate, which can be absorbed by plants.

15. Mention the importance of soil-living microbes.
Ans: Soil-living microbes decompose organic matter, improve soil fertility and help in nutrient recycling, especially nitrogen, phosphorus and carbon.

16. Write names of two microorganisms participating in the process of biological nitrogen fixation. [Madhyamik 17]
Ans: Rhizobium and Azotobacter are two microorganisms that take part in biological nitrogen fixation.

17. Mention the significance of nitrogen cycle.
Ans: The nitrogen cycle maintains soil fertility, ensures continuous supply of nitrogen to living organisms and keeps the balance of nitrogen in nature.

18. Relate the following phenomena with the trend of disturbance in nitrogen cycle resulting from different human activities – Global warming, Acidification of soil and water of river and lake. [Madhyamik 20]
Ans: Excessive use of fertilizers and burning of fossil fuels increase nitrogen oxides and nitrous oxide in the atmosphere. These gases cause global warming and form acids in rainwater, leading to acidification of soil, rivers and lakes and disturbing the nitrogen cycle.

4. Long-type questions and answers

1. Analyse the role of bacteria at any three steps of nitrogen cycle. [Madhyamik 18,19]
Ans: Bacteria play a very important role in maintaining the nitrogen cycle in nature. Their role in three major steps is as follows:

Nitrogen fixation: Certain bacteria like Rhizobium and Azotobacter convert atmospheric nitrogen into ammonia or ammonium compounds, which can be absorbed by plants.
Nitrification: Nitrifying bacteria such as Nitrosomonas convert ammonia into nitrite, and Nitrobacter converts nitrite into nitrate, making nitrogen available to plants.
Denitrification: Denitrifying bacteria like Pseudomonas convert nitrates present in soil into free nitrogen gas, which returns to the atmosphere and completes the nitrogen cycle.

2. Explain the role of bacteria in fixing free nitrogen in the environment.
Ans: Atmospheric nitrogen is abundant but cannot be used directly by plants and animals. Nitrogen-fixing bacteria play an important role in converting free nitrogen into usable forms. Some bacteria like Rhizobium live symbiotically in the root nodules of leguminous plants and fix nitrogen with the help of nitrogenase enzyme. Free-living bacteria such as Azotobacter and Clostridium also fix nitrogen independently in the soil. These bacteria convert atmospheric nitrogen into ammonia, which later forms nitrates. This process increases soil fertility and supports plant growth.

3. Discuss two important roles of organism in supplying carbon dioxide in the environment.
Ans: Organisms play an important role in maintaining the level of carbon dioxide in the environment.

Respiration: All living organisms respire and release carbon dioxide as a by-product of respiration. This carbon dioxide returns to the atmosphere and is used by green plants during photosynthesis.
Decomposition: Decomposer organisms like bacteria and fungi break down dead plants and animals. During this process, carbon dioxide is released into the atmosphere, contributing to the carbon cycle.

4. Discuss briefly the role of bacteria in releasing and fixing nitrogen in the environment.
Ans: Bacteria are essential for both the fixation and release of nitrogen in the environment. Nitrogen-fixing bacteria like Rhizobium, Azotobacter and Clostridium convert atmospheric nitrogen into ammonia, which is later used by plants. On the other hand, denitrifying bacteria such as Pseudomonas convert nitrates present in soil into free nitrogen gas and release it back into the atmosphere. Through these activities, bacteria help in maintaining the balance of nitrogen in nature.

5. Explain any three incidence which are occuring as a result of disruption of nitrogen cycle. [Madhyamik 19]
Ans: Disruption of the nitrogen cycle due to human activities leads to several environmental problems. Three major incidences are:

Global warming: Excessive release of nitrogenous greenhouse gases like nitrous oxide increases the greenhouse effect, leading to global warming.
Acidification of soil and water: Nitrogen oxides mix with rainwater to form nitric acid, resulting in acid rain. This causes acidification of soil, rivers and lakes, harming plants and aquatic life.
Eutrophication: Excessive use of nitrogen fertilizers increases nitrate levels in water bodies, leading to rapid growth of algae. This reduces oxygen content and kills aquatic organisms.