nitrifier

from The Century Dictionary.

• noun Anything which serves to bring about or to facilitate the production of niter or nitrates; specifically, in bacteriology, any micro-organism which is capable of oxidizing ammonia to nitrous acid, or nitrous acid to nitric acid; a micro-organism which serves to fix, and accumulate in the soil, the free nitrogen of the air. See nitrogen-fixing.

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

• noun (Chem.) An agent employed in nitrification.

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

• noun chemistry An agent employed in nitrification.