Section 7. Whenever land which is valued, assessed and taxed under this chapter no longer meets the definition of forest land, it shall be subject to additional taxes, in this section called roll-back taxes, in the 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 these roll-back taxes shall not apply unless the amount of the taxes, as computed under this section, exceeds the amount, imposed under section 6 and, in that case, the land shall not be subject to the conveyance tax imposed under said section 6. For each tax year, the roll-back tax shall be an amount 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 in that tax year had the land been valued, assessed and taxed without regard to these provisions. Notwithstanding this paragraph, 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; provided, however, that 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 is not valued, assessed and taxed under this chapter, then the land shall be subject to roll-back taxes only for those years of the 5 immediately preceding years in which the land was valued, assessed and taxed under this chapter.

In determining the amount of roll-back taxes on land which has undergone a change in use, the board of assessors shall ascertain the following for each of the roll-back tax years involved:—

(a) the full and fair value of the 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 subsection (a); and

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

Roll-back taxes will be subject to simple interest at a rate of 5 per cent per annum. If the board of assessors determines that the total amount of 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 land in agricultural use under sections 1 and 3 of chapter 61A or the definition of land in horticultural use under sections 2 and 3 of said chapter 61A or the definition of recreational land under section 1 of chapter 61B.

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.