From Fox News:
Alabama became the third state to allow the execution of prisoners by nitrogen gas Thursday after Republican Gov. Kay Ivey signed legislation permitting the practice as an alternative to lethal injection.
The new law would allow execution by nitrogen hypoxia if lethal injection drugs are unavailable or ruled unconstitutional. Breathing the inert gas causes oxygen depletion in the bloodstream.
According to AL.com, inmates on death row also could choose to be electrocuted, though none have done so since Alabama adopted lethal injection in 2002.