The percentage of nitrogen in the atmosphere is 78 %. Nitrogen is very essential for living organisms as it is a constituent of proteins, vitamins, Chlorophylls, nucleic acids like D.N.A and R.N.A. The atmospheric nitrogen cannot be taken directly by plants and animals. Certain bacteria and Cynobacteria (blue green algae) fix the atmospheric nitrogen in the soil in form of nitrogenous compounds. In factories when chemical fertilizers are prepared and also during lightening, nitrogen is fixed in nitrogenous compounds.

Nitrogen Cycle (Source and Copywrite of

The nitrogenous compound present in the soil is taken by plants along with water absorption. Nitrogen is then used to prepare vitamins, chlorophylls, nucleic acids and proteins. The nitrogen comes to animals when they eat plants and their products. When plants and animals die, decomposers like bacteria and fungi release the nitrogenous compounds like protein in the soil. The conversion of atmospheric nitrogen into nitrogenous compounds like nitrates and nitrites by some bacteria is called nitrification. The process of conversion of proteins present in dead plants and animals into ammonia is called ammonification. Another kind of bacteria called denitrifying bacteria release the nitrogen from the nitrogenous compounds. Thus, the percentage of nitrogen remains almost constant in the atmosphere.


(i) A bacterium that leaves in the root nodules of leguminous plants and help in nitrogen fixation from atmosphere into the soil –

(a) Blue green Algae (Cyanobacteria)          (b) Rhizobium               (c) Yeasts               (d) Lactobacillus

(ii) The process of absorbing atmospheric nitrogen by plants through the roots along with water is called as-

(a) Nitrification              (b) Ammonification        (c) Nitrogen Fixation                 (d) Denitrification

(iii) The breakdown of nitrogenous compounds to form nitrogen is called as?

(a)Ammonification       (b) Nitrification                (c) Denitrification                      (d) Nitrogen fixation

(iv) The breakdown of dead plants, animals and their products by decomposers like-

(a) Algae                     (b) Bacteria                (c) Fungi                     (d) Bacteria and fungi

(v) Select the correct responses for the statements.

S.N. StatementResponseResponse
1.  Preparation of chemical fertilizers in factories is chemical nitrogen fixation.YesNo
2.  Nitrates and nitrites are nitrogenous compounds formed in soilYesNo

Q. NoResponse
1 (b) Rhizobium
2 (c) Nitrogen Fixation
3 (c) Denitrification
4 (d) Bacteria and fungi
51. Yes
2. Yes

2 thoughts on “NITROGEN CYCLE”

Leave a Comment

Your email address will not be published. Required fields are marked *