Section 16. Continuance of land valuation, assessment and taxation under the provisions of this chapter shall depend upon continuance of such land in agricultural or horticultural uses and compliance with other requirements of this chapter and not upon continuance in the same owner of title to such land. Liability to roll-back taxes, determined pursuant to section thirteen, shall attach when such land no longer qualifies as actively devoted to agricultural or horticultural use and shall be the obligation of the then owner of the land. For purposes relating to roll-back taxes such qualification shall depend on the actual use of such land for the previous 5 years, and not on the filing of application under section six for any year.