Explanation:

 Justification: Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins.
• The atmospheric nitrogen cannot be taken directly by plants and animals.
Statement 1: Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into compounds of nitrogen
Statement 2: Lightning also fixes atmospheric nitrogen .
Statement 3 and 4: Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins.
• The atmospheric nitrogen cannot be taken directly by plants and animals. Once nitrogen is converted into these usable compounds, it can be utilised 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.
• When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again. Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere.
• As a result, the percentage of nitrogen in the atmosphere remains more or less constant.