If the governing body annexes the area, it shall do so by ordinance. When a copy of the ordinance and an accurate map of the annexed area, certified by the executive officer of the city, are filed and recorded with the county recorder, the annexation becomes effective. An annexation is effective for the purpose of general taxation on and after the first day of the next January. However, the city shall continue to classify as agricultural lands for tax purposes all lands in the annexed area which were classified as agricultural lands immediately before the annexation proceedings until those lands are put to another use. If the governing body determines to exclude the area petitioned for, it may do so by ordinance adopted and recorded as in the case of annexation.