The board of trustees of the road improvement district shall, in addition to its other powers, levy a special assessment to the extent of special benefits conferred the cost of such portion of such improvements as are local improvements upon property found specially benefited thereby which shall be a lien as provided by section 39-1614 when properly levied and certified as required by sections 39-1601 to 39-1636. The board of trustees of such district may find the remainder of the cost of such improvements made are of general benefit to the district and the costs thereof shall be paid from taxes levied against all the property in the district in the manner provided for by subsection (1) of section 39-1621.