Nitrogen Cycle

• Nitrogen cycle is the constant interaction between biotic and a biotic components of the earth to circulate the nitrogen in atmosphere, lithosphere, hydrosphere and biosphere in different forms.

Occurrence:

• Nitrogen gas makes up 78% of our atmosphere and nitrogen is also a part of many molecules essential to life like proteins, nucleic acids and some vitamins.
• Nitrogen is found in other biologically important compounds such as alkaloids and urea.
• Nitrogen is an essential nutrient for all life-forms but cannot be used directly from the atmosphere as N₂ except few forms of bacteria.
• Thus atmospheric nitrogen is first converted to oxides [nitrites and nitrates] and then transformed into different living forms.

Steps involved in nitrogen cycle:

Nitrogen fixation:

• Atmospheric nitrogen cannot be taken directly so they have to be fixed in the soil before it is being used by the plants.
• Conversion of atmospheric nitrogen into inorganic form by living organisms or physical factors is known as nitrogen fixation.

• Nitrogen fixation is of two types: Biological and Physical

Biological fixation:

• Nitrogen can be fixed by micro organisms growing in root nodules of leguminous plants or those living in soil.
• Rhizobium residing in the root nodules of leguminous plants and certain blue green algae helps in fixing atmospheric nitrogen.

Root nodules of leguminous plants

• Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen.

Assimilation:

• Once nitrogen is converted into these usable compounds, it can be utilized by plants from the soil through their root system.
• Nitrogen is then used for the synthesis of plant proteins and other compounds.
• Animals feeding on plants get these proteins and other nitrogen compounds. This process of utilizing absorbed nitrogen for synthetic purposes is known as assimilation.

Ammonification:

• When plants and animals die and decay bacteria and fungi present in the soil convert the nitrogenous wastes into inorganic nitrogenous compounds (ammonia). Not only this, living organisms excrete nitrogenous waste in the form of urea, uric acid or directly ammonia. This conversion is known as ammonification.

Nitrification:

• Ammonia present in the soil is acted upon by nitrifying bacteria in soil which converts it into nitrites and nitrates. Nitrate is the form that can be used/ absorbed by the roots of the plants. This conversion of ammonia to nitrites and nitrates is known as nitrification.

Denitrification:

• Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere and process is known as denitrification. As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Representative diagram of Nitrogen Cycle: