Oracle Text

At the beginning of each end step, each player who tapped a land for mana this turn sacrifices a land. Desolation deals 2 damage to each player who sacrificed a Plains this way.

Card Rulings

10/4/2004 It works even if it was not on the battlefield when the land was tapped for mana.
10/4/2004 Each Desolation only causes up to one land per turn to be sacrificed. If you have more than one, each will do this.