Any county with a population in excess of eighty-five thousand inhabitants that participates in the Retirement System for Nebraska Counties established by the County Employees Retirement Act shall establish and fund a supplemental retirement plan for the benefit of all present and future commissioned law enforcement personnel employed by such county. The auxiliary benefit plan shall be funded by additional contributions to the county employees retirement plan in excess of the amounts established by sections 23-2307 and 23-2308. The additional contributions made by employees shall be credited to the employee account, and contributions paid by the county shall be credited to the employer account, with each amount to be established at a rate of three percent of compensation. All contributions made pursuant to this section shall be invested and administered according to the County Employees Retirement Act.