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Class 8, Science, Chapter-2, Lecture-5, Nitrogen Cycle (Notes)

Soil Fertility and Microorganisms

Symbiotic bacteria (Rhizobium) in the root nodules of leguminous plants absorb atmospheric nitrogen and convert it into nitrogenous compounds. 
Blue-green algae also fixes the atmospheric nitrogen as nitrogen compounds in the soil, enriching the soil and thus increasing its fertility. 
These microorganisms are commonly known as biological nitrogen fixers.

NITROGEN CYCLE: 

A constant interaction in which continuous transfer of nitrogen between the biotic and abiotic components of the biosphere occur which makes the atmosphere a dynamic but stable system and maintains nitrogen balance, is known as nitrogen cycle.

STEPS:

  1. Nitrogen Fixation: 
    The process of conversion of free nitrogen into useful nitrogenous compound is called nitrogen fixation. 
    i) Natural Fixation: 
        (a) Biological Fixation of N2 to NO3 by Rhizobium-Legume Symbiosis and Cyanobacterium in soil 
        (b) Atmospheric Fixation of N2 to NO by Lightening and then to NO3 and Ca(NO3)2
    ii) Artificial Fixation: 
    Industrial Fixation of N2 to NH3 and fertilizers.
  2. Assimilation: 
    Conversion of Inorganic Nitrogenous compounds (NO3 to NO2) to Organic compounds (Amino acids, Nucleic Acids or Chlorophyll) is called assimilation.
  3. Ammonification: 
    Conversion of Organic Nitrogenous compounds to Inorganic ammonia (NH4+) is called Ammonification.
  4. Nitrification 
    Conversion of ammonia to Nitrites by Nitrosomonas and Nitrites to Nitrates by Nitrobacter is called Nitrification.
  5. Denitrification: 
    Conversion of Nitrates (NO3) to atmospheric Nitrogen (N2) by Clostridium, Pseudomonas and Bacillus is called denitrification.
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