Nitrogen is absorbed by the roots of the plants in the form of amino acids, nitrate or nitrite ions.
The nitrogen absorbed from the soil by roots is then transported to the various parts of the plants. Plants do not get nitrogen they need directly from the air.
This is because the organic nitrogen that is available in the atmosphere is not usable for plants. Plants need non-organic nitrogen which is obtained by changing organic nitrogen into non-organic forms by the bacteria in the soil.