On completion and acceptance of the improvement, the county shall issue and sell at not less than par bonds of the county in an amount sufficient to pay the balance of the costs of the improvements, taking into account the amounts collected on special assessments and any funds contributed to the district. The bonds shall mature in not to exceed ten years from their date and bear interest payable annually or semiannually. The bonds shall constitute a general obligation of the county, but all special assessments, special taxes, or contributions made to the district shall constitute a sinking fund for the payment of the bonds. The county shall collect all special assessments and special taxes and levy and collect annually a tax on all taxable property in the county sufficient in rate and amount to pay any deficiency on the amount required to pay both principal and interest on the bonds as the same fall due. The bonds and tax authorized in this section shall be in addition to all other bonds and taxes authorized by law and shall not be included in computing any statutory limitation on the amount of bonds or tax which may be issued or levied by the county.