denitrifier

from The Century Dictionary.

• noun That which destroys or prevents the formation of nitric acid or nitrates, as certain ferments in the decomposition of nitrogenous organic matter.
• noun In bacteriology, any micro-organism which is capable of breaking down nitrates into nitrites, and nitrites into ammonia compounds, or of liberating nitrogen from any of these.

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

• noun biology, soil science An organism that engages in denitrification, the removal of nitrates from the environment