Thiobacillus is a group of bacteria helpful in carrying out

Nitrate present in the soil is also reduced to nitrogen by the process of denitrification. Denitrification is carried by bacteria Pseudomonas and Thiobacillus.

The process of conversion of nitrogen (${\mathrm{N}}_2$) to ammonia is termed as nitrogen fixation. Symbiotic biological nitrogen fixing done by Rhizobium. Free-living nitrogen-fixing aerobic microbes are Azotobacter and Beijerinckia while Rhodospirillum is anaerobic and free-living. In addition, several cyanobacteria such as Anabaena and Nostoc are also free-living nitrogen-fixers.

Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or Nitrococcus. The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter. These steps are called nitrification.