On or before the first Monday in May of each year, the board of trustees of the sinking fund shall certify to the legislative authority of the municipal corporation the amount of tax necessary to provide a sinking fund for the future payment of bonds issued by the municipal corporation prior to August 17, 1921, for the payment of interest on such bonded indebtedness, the rents due on perpetual leaseholds of the municipal corporation not payable from a special fund, and the expenses incident to the management of the sinking fund. The legislative authority shall place the several amounts so certified in the tax ordinance before and in preference to any other item, and for the full amount certified. Such taxes shall be in addition to all other taxes authorized by law.