How do plants absorb the nitrogen fixed in the soil?

Plants absorb the nitrogen fixed in the soil through their roots. Nitrogen in the form of nitrates (NO₃^-) and ammonium ions (NH₄⁺) can be taken up by plant roots and used to synthesize amino acids, nucleic acids, and other nitrogen-containing compounds that are essential for plant growth and development. The process of nitrogen uptake by plants is facilitated by specialized transport proteins in the root cells, which help to move the nitrogen from the soil into the plant. 

Leaves, shoots, and flowers are not directly involved in the absorption of nitrogen from the soil.