Why direct current changes the composition of ionic solution?

Due to Direct Current (DC) in the electrochemical cell, there will be continuous depositing of electrolyte on one of electrode from the solution as well as release of ion from the other electrode in the solution. Hence, the composition will not be same after some time. 

While using AC, whatever is deposited or released on or from the electrodes will automatically revered with the each cycle of the AC because one complete cycle of AC composed of two half cycles which have equal magnitude but opposite in direction. Hence the composition will remain same in AC.