Free living nitrogen fixing aerobic microbes are Azobacter and Beijerinckia while Rhodospirillum is anaerobic and free living. Cyanobacteria such as Nostoc are also free living nitrogen fixers. Rhizobium has a symbiotic relationship with roots of legumes like, pea, lentils, alfalfa etc. They produce nodules on the roots. Similar to this is Frankia which also produces nitrogen fixing nodules on the roots of non-leguminous plant Alnus. 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.