Ala. Code § 16-19-4

As security for the payment of the principal of and interest on any warrants issued under the provisions of this chapter, the local subdivision issuing such warrants may, in the discretion of its governing body, specially pledge for payment of such principal and interest at their respective maturities the proceeds of any ad valorem, privilege, license, excise or other tax the proceeds of which are subject to the control of such local subdivision and are not required by law to be devoted to some other purpose.