Special assessments for maintaining grass plots or trees or for parking or other improvements for the beautification of the streets of the municipality shall be payable in equal annual installments or in such annual amounts as will permit the annual increase in payment of principal to approximate the annual decrease in the interest on amounts remaining unpaid, extending over a period of not more than ten years as the governing body may fix by ordinance or resolution.
N.D.C.C. § 40-24-08