In a symbiotic relationship with the soil bacteria known as 'rhizobia', legumes form nodules on their roots (or stems, see figure below) to 'fix' nitrogen into a form usable by plants (and animals). ... Plants cannot fix nitrogen on their own, but need it in one form or another to make amino acids and proteins.
The plants absorb the nitrogen from the atmosphere and it transports down into the soil where the nitrogen-fixed bactera changes the form of the nitogen:)
