The bacterium Nitrosomonas converts
The bacterium Nitrosomonas converts ammonia to nitrite. Nitrobacter converts nitrite to nitrate. Nitrogen is converted to ammonia by Rhizobia, Azotobacter, Beijerinckia, Rhodospirillum, Frankia etc. Nitrate is converted to nitrogen by denitrifying bacteria such as Pseudomonas and Thiobacillus.