What is the ultimate source of the nitrogen that living things use?

Nitrates can be use by plants because atmospheric nitrogen can not use by living beings directly.

Explanation:

Nitrogen fixation is necessary for the plants to change atmospheric nitrogen to the nitrates. Atmospheric nitrogen cannot be used by plants so they can not directly use nitrogen gas present in the atmosphere for producing organic matter for themselves and other living beings.

Nitrogen fixation is or converting atmospheric nitrogen into useful nitrate compounds by the bacteria, algae and lightening.

hope it mark me as brainlieast.