Nitrogen fixation is the reductive conversion of molecular (gaseous) nitrogen  to nitrogenous compounds. Nitrogen is an essential element for plant life. Plants absorb nitrogen in its oxidised and anionic forms, such as nitrites and nitrates, or in its reduced and cationic form as ammonium. Free molecular nitrogen is fixed into these forms and made available to plants by certain microorganisms, called nitrogen fixers
Nitrogen fixers are of two major groups, namely free living and symbiotic .
nitrogen cycle
1. Example of Free Living Bacteria
a) Bacteria:
i) Aerobic – Azotobacter
ii) Anaerobic and non photosynthetic – Clostridium
iii) Anaerobic and photosynthetic - Rhodospirillum
b) Cyanobacteria: Free living species of the genera Nostoc and Anabaena

2. Example of Symbiotic
a) Bacteria
i) Several species of Rhizobium, living in the root nodules of legumes.
ii) Several species of Klebsiella, living in the leaf nodules of some tropical plants such as     Psychotria.
b) cyanobaacteria
i) Symbiotic species of Anabaena, living in the root nodules of Azolla (water fern)
ii) Some species of Nostoc, living in certain fungi.
c) Actinomycetes: Frankia, living in the root nodules of non – legumes, such as Cauarina, Alnus, Purshia, Myrica etc.
d) Fungi and Algae: Forms which are parts of tree lichens (Collema)
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