Biogeochemichal cycle// Nitrogen cycle Part 1//biological nitrogen fixation

 

Nitrogen cycle

Nitrogen cycle is a biogeochemical cycle through which nitrogen element is circulated continuously through the living and non-living components of the biosphere.

 Air contains about 78% of nitrogen in the earth’s atmosphere.

Nitrogen is an essential component of life but the abundant nitrogen in the atmosphere can’t be used directly by the plant or animals.

Nitrogen gas exists in both organic and inorganic forms.

Organic from of nitrogen exists in living organisms and passed through the food chain by the consumption of the other living organisms.

Inorganic form of nitrogen found in the atmosphere. This nitrogen is available to plant in the form of nitrites and nitrates.

Nitrogen cycle has the following steps:

A.  Nitrogen fixation

Nitrogen fixation is the process of converting nitrogen of the atmosphere into nitrogen compounds is called nitrogen fixation.

It takes place in to following ways:

1.     Atmospheric nitrogen fixation

·        In this way of nitrogen fixation, during lightning in the sky when a high temperatures and pressure is created in the air nitrogen gas react with oxygen to produce oxides of nitrogen.

·        These oxides of nitrogen get dissolved in the rainwater forming dilute nitric acid and nitrous acids and fall on land with rain water.

·        These nitric and nitrous acids react with the alkalis of the soil (like limestone) to turn into nitrates, which is used by various life forms.

2.     Industrial nitrogen fixation-

It is a man -made way to fixation of nitrogen by the use of Ammonia.

Ammonia is produced by the direct combination of nitrogen and hydrogen and later it is converted into various fertilizers such as urea.

 

3.     Biological  nitrogen fixation –

It is the conversion of atmospheric nitrogen into nitrogen compounds by nitrogen-fixing bacteria for example free living bacteria like Azotobactor and Clostridium or  symbiotic like Rhizobium and also blue green algae like Anabaena and Nostoc.

B.  Nitrogen assimilation-

·        Process by which conversion of inorganic nitrogen compounds into organic compounds that becomes a part of living organism is called Nitrogen assimilation.

·        Primary producers – plant absorb nitrogen compounds like nitrates and nitrites from the soil and water and convert them into protein.

·        These proteins are consumed by primary consumers.

 

C.  Ammonification-

·        The process of conversion of complex organic compounds like proteins into ammonia is called Ammonification.

·        When plants or animals die, the dead remains plants or animals are converted into ammonia, carbon dioxide and water by the action of bacteria and fungi (i.e., decomposer) present in the soil.

    

Organic compounds   --------------->   NH₄⁺

 

D.  Nitrification-

·        The process of conversion of ammonia into nitrites and nitrates is called Nitrification.

·        This conversion is very important because ammonia gas is toxic for plants.

·        Nitrification is done by some nitrifying bacteria present in the soil.

·        Ammonia is converted into nitrites by nitrosomonas or nitrococcus  bacteria present in the soil.

·        Nitrites are converted into nitrates by nitrobactor bacteria.

·        Nitrates from the soil is absorbed by the plants.

                             

------------------->                     NH₄⁺         nitosomonas    NO2^-

                                                

                      NO₂       nitrobactor >     NO3^-

E.   Denitrification-

·        The conversion of nitrate and nitrite salts to elemental nitrogen is called denitrification.

·       This conversion is done by free living bacteria example- Pseudomonas which present in the soil.

·        This elemental nitrogen is then back again into simple nitrogen molecules in the atmosphere this is known as nitrogen cycle.

 

             2NO₃-    pseudomonas  ----------->      N₂

Nitrogen cycle helps in converting nitrogen gas into a usable form for the plant through biochemical process.