Nitrogen Fixation

Nitrogen is an essential constituent of proteins, which are the building blocks of all the living beings. Our atmosphere contains 78% of Nitrogen but it can not be used directly by the majority of living organisms.

 There are Five main processes on the basis of which the entire nitrogen cycle can be explained.

1.Nitrogen Fixation: This process involves conversion of gaseous nitrogen into ammonia or nitrates which can be used by plants. It involves the following two steps:

a.) Activation: activation of nitrogen requires 169kcal of energy fixation of each mol of nitrogen(28g) & the molecular nitrogen is converted into atomic nitrogen.

         N^2---> 2N

b.) Fixation: Activated nitrogen combines with hydrogen to from ammonia. In this step per mol synthesis of ammonia liberates 13kcal of energy.

2N+3H^2------> 2NH^3

Atmospheric nitrogen can be fixed to converted to biologically usable form:

1.Atmospheric Fixation: Lightening, combustion& volcanic activity helps in the fixation of nitrogen. With this method of fixation atmospheric oxygen & nitrogen combine to form nitrate which comes to the earth with rain as nitric acid.

2. Industrial fixation: At high temperature, 400degree celcius& high pressure 200atm., molecular nitrogen is broken into atomic nitrogen which then combines with hydrogen to form ammonia.

3. Bacterial fixation: This is also known as biological fixation of nitrogen and makes up 100-200kg N/ha. Thus 90% of the fixed nitrogen to the earth is contributed by this method every year. There are two types of bacteria that help in biological fixation of nitrogen, symbiotic and asymbiotic.

 Free living and the symbiotic bacteria both require molybdenum as an activator. An accumulation of ammonia and nitrates in the soil inhibits the activity of these bacteria. 

c.) Nitrification: It is a process by which ammonia is converted into salts of nitrate or nitrite by Nitrosomonas and nitrococcus bacteria respectively. Another soil bacteria Nitrobacter can convert nitrateinto nitrite.

NH^3+11/2O^2----->HNO^2+ H^2O+165kcal--------->H+NO^2

d.) Assimilation: In this process nitrogen fixed by plants is converted into protoplasmic organic molecules such as proteins,DNA& RNA. These molecules make the plant and animal tissue.

e.) Ammonification: In this process amino acids are broken down into ammonia with the release of energy by the decomposers. These waste products as well as dead remains of organisms are converted back into inorganic ammonia by the ammonifying bacteria. These process is called ammonification.

f.) Denitrification: Conversion of nitrates back into gaseous nitrogen is called denitrification.