Nitrogen Fixation

There are number of microorganisms in the nature those are involved in the process of nitrogen fixation. These microorganisms use atmospheric nitrogen and convert it to ammonia. The process of conversion of atmospheric nitrogen to ammonia is called nitrogen fixation. 

On the ease of study, nitrogen fixation bacteria are group into two categories (1) non-symbiotic those live freely in the environment and (2) symbiotic microorganisms those live in the roots of host plant and full fill the nitrogen requirements of plants in the exchange of food and shelter.

Non Symbiotic Nitrogen Fixation

Non symbiotic nitrogen fixation bacteria are those bacteria, those fix nitrogen in the environment while living freely in the soil. These bacteria are abundant in number and can be easily found in soil. Clostridium pasteurianum and Azotobacter are most study microorganisms those are involved in free living nitrogen fixation bacterias. For many years these were the only known to be capable of nitrogen fixation. Cl. Pasteurianum is the anaerobic  bacillus and Azotobacter are aerobic oval to spherical shape bacteria.

The nitrogen fixing capacity of Azotobacter is higher than Cl. Pasteurianum. 

In recent years many other microorganisms have been found to fix nitrogen. It has been estimated that the amount of nitrogen fixed by the non-symbiotic process range between 20 to 50 lb/acre annually. This estimate is no doubt subject to much variation depending upon the conditions peculiar to a particular soil type.

Symbiotic Nitrogen Fixation

Symbiotic nitrogen fixation is accomplished by bacteria of the genus and Rhizobium in association with legumes (plants those were seeds in pods). Before these bacteria can fix nitrogen, they have to establish themselves in the roots of legumes. Infection of the root system by rhizobia bacteria is closely associated with the formation of an “infection thread” that develop in certain root hairs.

The nitrogen fixation bacteria invade in host cells by infection thread. As a result of it some cells of plant are infected. It causes cell enlargement and rapid cell division. This causes abnormal growth (nodules) on the root system.

The legume, the bacteria, and the nodules consist a system for this type of nitrogen fixation. Bacteria living in nodules get food and shelter and the plant get nitrogen, fixed by bacteria. So it is considered as symbiotic relationship.

Not all species of Rhizobium produce nodulation and nitrogen fixation with any legume. There is a degree of specificity among bacteria and legumes.

For the purpose of inoculation with commercial preparations if these bacteria, legumes are divided into seven major categories as follow: alfalfa, clover, peas and vetch, cowpeas, beans, lupines, and soybeans. Rhizobium strains effective for one species may be ineffective or slightly effective against another species. Even within a species some strains may have higher degree of effectiveness.

Because not all agriculture soil have the right kind of bacteria. So, inoculation of seed before germination is a better practice towards optimum symbiotic nitrogen fixation with legumes crops.

Recombinant DNA And Nitrogen Fixation

In the last few years vast amount of work has been done on gene mapping and genetic engineering. Which help scientist to identify the gene responsible for nitrogen fixation. Now researchers are working on the, implanting gene to the plant itself. This will leads to reduce the dependency of plants over microorganisms.

Alternatively it may also possible that scientist develop symbiotic relation among other bacteria and other plants similar to the legume plants.

Reference: Microbiology Prescott