denitrify

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To remove nitrogen or nitrogen groups from (a compound).
• transitive verb To reduce (nitrates or nitrites) to nitrogen-containing gases, as by bacterial action on soil.

from The Century Dictionary.

• To remove or destroy nitrates.
• In bacteriology, to break down nitrates into nitrites, and nitrites into ammonia compounds, or to liberate nitrogen from any of these compounds by bacterial action.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To deprive of, or free from, nitrogen.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb transitive, chemistry to remove nitrogen, often through the breakdown of nitrogenous compounds and the release of nitrogen gas.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb remove nitrogen from