Although the atmosphere contains a vast reservoir of molecular nitrogen (N2) that is about 79%of the air, however, it is not directly available for use by higher plants. Before assimilation can occur, it must be converted to a so-called fixed form, either by oxidation to NO3 or by reduction to NH4. As molecular nitrogen is highly inert, due to very stable triple bond (N≡N), these conversions are not easy to bring about. Industrially the chemical fixation of N2requires, high temperature and pressure, and natural gas as source of hydrogen (Haber-Bosch process).

Biological N2fixation (BNF) which is the reduction of atmospheric N2 to ammonia, by N2-fixing microbes, is also very energy intensive. The energy for BNF comes from the oxidation of carbon sources such as glucose. Global contribution of nitrogen by BNF ranges from 100 to 180 million metric tons per year whereas yearly industrial N2fixationamounts to 85 million metric tons. BNF has significant contribution in agriculture and offers an alternative to expensive industrial (fertilizer) nitrogen. However, the high-yielding agricultural systems are difficult to sustain solely on BNF.

After photosynthesis, biological nitrogen fixation (BNF) is the second most important biological process on earth. BNF is mediated exclusively by prokaryotes, including many genera of bacteria, cyanobacteria and actinomycetes. N2-fixing microbes can exist as free-living or in association with other microbes or plants.



  1. Symbiotic

Mutually beneficial relationship between certain microorganisms and plants, e.g.:

Legume —Rhizobium (nodule forming bacteria)

Non legume—Frankia (Actinomycetes)

Lichens (algae —fungi)

  1. Associative symbiotic

In this case the microorganisms may depend on rhizo-deposits as carbon source but do not directly depend on the presence of the higher plant.

Grasses —Azotobacter, Azospirillum

(the bacteria live on root exudates etc.)

Azolla —Anabaena

(association of a fern and blue green algae) biomass incorporated in the soil provides nitrogen to plants. Practicable in rice fields.

  1. Non symbiotic

Free-living microorganisms are involved and the presence of plant is not necessary.

e.g. Azotobacter, Beijerinckia, cyanobacteria (blue green algae), Clostridium (anaerobic).


Written by. Shabbar Naqvi


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