After reading this you should be able to:

1. describe the nitrogen cycle.

2. explain the importance of nitrogen cycle to plants and animals.
3. Nitrogen cycle

4. Importance of nitrogen cycle


The nitrogen cycle indicates the circulation of nitrogen in nature. It shows how nitrogen moves through both the living and non-living environments. Nitrogen is one of the essential elements for plant growth. This element can however, not be used by plants directly from the atmosphere. It has to be converted first into nitrates. Certain free-living nitrogen fixing bacteria (for example, Asotobacter) reduce the atmospheric nitrogen into ammonium compounds. This process is referred to as nitrogen-fixation. Another nitrogenfixing bacterium known as Rhizobium, lives in symbiotic association in the root nodules of guminous plants such as groundnuts.

The Nitrogen Cycle

The circulation of nitrogen and its compounds in the environment is called the nitrogen cycle. The diagram below shows the nitrogen cycle. Study it carefully. Do plants and animals use the nitrogen in the air directly?


Nitrogen fixation can also occur as a result of electrical discharge during thunderstorms. In the process, oxides of nitrogen are produced which in turn enters the soil. The acid combines with other salts to form nitrates.

During the process of decomposition bacteria and fungi, break down the dead remains and waste products of other organisms into ammonium compounds. In crop production, artificial fertilizers are added to the soil and these sometimes include ammonium compounds.

The ammonium compounds formed by the action of nitrifying bacteria, putrefying bacteria and fungi, or those added as fertilizers, are oxidized to nitrates by nitrifying bacteria such as nitrosomonas. The nitrtes in turn are oxides to nitrates by other nitrifying bacteria such as Nitrobacter. This whole process of converting nitrogen and nitrogen-based compounds to nitrates is known as nitrification.

Certain bacteria and fungi however reduce nitrates in the soil to nitrogen gas. This process is called dentrification.

These micro-organisms are harmful because they reduce or diminish the amount of nitrates available to plants in the soil. Nitrates are also lost from the topsoil through teaching and evaporation, as a result of water action and bush burning respectively.


     Nitrogen is essential for plant growth and reproduction because it is a constituent element of protein and hence of the protoplasm of all living cells. It is also found in many other organic compounds associated with plant. Nitrogen therefore helps plants to grow. Animals cannot prepare their own food but depend on plants for food. One of the essential food nutrients which helps in growth and repair of worn out cells is protein and this forms one of the constituents of plant food.


1. To which of the following classes of crops is one of the niteogen-fixing bacteria species associated with?
a. cereals
b. roots crops
c. legumes
d. stem tubers

2. Which of these bacterium converts ammonium compounds into nitrites in the nitrogen cycle? a. agotobacer b. nitrogen c. rhizobium d. nitrogomonas.

3. Dentrification involves:

a. reduction of nitrates into gaseous nitrogen.

b. Oxidation of ammonium copounds into nitrates. c. Oxidation of nitrates into nitrites.

d. None of the above.

4. How many stages are involved in the nitrification process in the nitrogen cycle?
a. 2     b. 4     c. 3    d. 5.

5, The bacteria involeved in the nitrification Process are:
a. rhizobium and azotobacter.
b. azotobacter and clostridium.
c. nitrobacter and nitrosomouas.
d. nitrobacter and rhizobium.

6. In the nitrogen cycle, ammonium ions are deriVed from
a. stems.
b. roots.
c. leaves.
d.dead plants and animals,

7. The conversion of nitrates into free nitrogen is referred to as

a. nitrification.
b. denitrilication.
c. nitrogen-fixing
d. none of the above.

8. The species of the nitrogen-hing bacterium that is associated with root modules of leguminous plants is called
a. rhizobium.
b. nitrobacter.
c. azotobacter.
d. clostridium.

9. a. Explain what is meant by nitrogen cycle.
b. State three means by which nitrogen is continuously lost from the soil.

10. Briefly exlplain how nitrogen is fixed into soil besides the nitrogen iixing bacteria.

11. Draw the nitrogen cycle.

12. Explain what notification and dentrification are.

13. Describe four different roles played by bacteria in the nitrogen cycle.

14. How do animals obtain nitrogenous

15. State the importance of nitrogen to plants and animals.

16. Describe the chemical fixation of nitrogen during lightening.