Index >>Mineral Transformation in Soil >>Various Groups of Microorganisms and Nitrogen Fixation

Blue green bacteria fix nitrogen aerobically specially in flooded soils and constitute another potential group of organisms for nitrogen fixation.They are both terrestrial and aquatic and have not only the ability to fix nitrogen, but are capable of obtaining their carbon requirement through photosynthesis. There are claims that blue green bacteria are mainly responsible for maintaining the fertility and productivity of rice fields of the orient.

These bacteria grow abundantly in flooded rice fields and the bacterial mass undergoes decomposition and provides the nitrogen to the rice crop. In addition to this, these bacteria excrete both ammonia and growth promoting substances which are utilized by the rice crop. It is believed that nitrogen fixation occurs in the heterocysts of blue green bacteria although there is some evidence to the contrary.

One blue green bacterial strain namely Anabena symbioses witht he water fern Azolla. Although this blue green bacterium is capable of photosynthesis and nitrogen fixation, it does best only when it grows in association with Azolla. This symbiotic system is believed to contribute substantially to the nitrogen economy of rice soils. The other anaerobic nitrogen fixing bacteria such as the photosynthetic Chlorobium, Rhodospirillum etc. may have a role to play only under certain environmental conditions.

Among these, the bacterium Azospirillum brasilense earlier called Spirillum lipoferum is best studied for its association with cereal roots. This bacterium is capable of fixing nitrogen both in the free living state as well as under microaerophilic conditions. Other bacteria that are known to fix nitrogen, in association with cereal plants are Pseudomonas azotogensis, Enterobacter, Klebsiella, Bacillus etc.

The potential of this system is appealing because of the association with cereals and grasses which form the major component of the animal food chain. The magnitude of nitrogen fixed by this system, however, does not appear to be appreciable at present and plant response to inoculation with these bacteria has been found only in low fertility soils. These bacteria may have a role in helping the plants make more efficient use of the limited nitrogen available in the soil.