Section 8. Whenever land which is valued, assessed and taxed under this chapter no longer meets the definition of recreational use, it shall be subject to additional taxes, in this section called roll-back taxes, in the current tax year in which it is disqualified and in each of the 4 immediately preceding tax years in which the land was so valued, assessed and taxed, but the roll-back taxes shall not apply unless the amount of the taxes, as computed under this section, exceeds the amount, if any, imposed under section 7 and, in that case, the land shall not be subject to the conveyance tax imposed under said section 7. For each tax year, the roll-back tax shall be equal to the difference, if any, between the taxes paid or payable for that tax year in accordance with this chapter and the taxes that would have been paid or payable had the land been valued, assessed and taxed without regard to these provisions. Notwithstanding the foregoing provisions, no roll-back taxes shall be assessed if the land involved, or a lesser interest in the land, is acquired for a natural resource purpose by the city or town in which it is situated, by the commonwealth or by a nonprofit conservation organization, but if any portion of the land is sold or converted to commercial, residential, or industrial use within 5 years after acquisition by a nonprofit conservation organization, roll-back taxes shall be assessed against the nonprofit conservation organization in the amount that would have been assessed at the time of acquisition of the subject parcel by the nonprofit conservation organization had the transaction been subject to a roll-back tax. If, at the time during a tax year when a change in land use has occurred, the land was not then valued, assessed and taxed under the provisions of this chapter, then such land shall be subject to roll-back taxes only for such of the 5 immediately preceding years in which the land was valued, assessed and taxed thereunder. In determining the amount of roll-back taxes on land which has undergone a change in use, the board of assessors shall have ascertained the following for each of the roll-back tax years involved:

(a) the full and fair value of such land under the valuation standard applicable to other land in the city or town;

(b) the amount of the land assessment for the particular tax year;

(c) the amount of the additional assessment on the land for the particular tax year by deducting the amount of the actual assessment on the land for that year from the amount of the land assessment determined under clause (a); and,

(d) the amount of the roll-back tax for that tax year by multiplying the amount of the additional assessment determined under clause (c) by the general property tax rate of the city or town applicable for that tax year.

Interest on roll-back taxes shall be payable and shall be computed as simple interest at 5 per cent per annum. If the board of assessors determines that the total amount of the roll-back taxes to be assessed under this section, before the addition of any interest as provided for in the preceding paragraph, would be less than $10, no tax shall be assessed.

No roll-back tax imposed by this section will be assessed on land that meets the definition of forest land under section 1 of chapter 61, agricultural land under sections 1 and 3 of chapter 61A, or horticultural land under sections 2 and 3 of chapter 61A.

Land retained as open space as required for the mitigation of a development shall be subject to the roll-back taxes imposed by this section.