Divine Metamagic( Complete Divine, p. 80)
You can channel energy into some of your divine spells to make them more powerful.
Ability to turn undead or rebuke undead,
When you take this feat, choose a metamagic feat that you have. This feat applies only to that metamagic feat. As a free action, you can take the energy from turning or rebuking undead and use it to apply a metamagic feat to divine spells that you know. You must spend one turn or rebuke attempt, plus an additional attempt for each level increase in the metamagic feat you're using. For example, Jozan the cleric could sacrifice three turn attempts to empower a holy smite he's casting. Because you're using positive or negative energy to augment your spells, the spell slot for the spell doesn't change.
This feat may be taken multiple times. Each time you take this feat choose a different metamagic feat to which to apply it.