Here is a list of nitrogen fixing biofertilizers used for enhancing the productivity of soil.
1. Rhizobium Biofertilizer:
Rhizobium biofertilizer is a carrier based powdered formulation of live Rhizobium Cells. On seed treatment these bacteria multiply rapidly in soil along the developing roots in due course of time they develop large number of nodules. These nodules are the actual site of the biological nitrogen fixation, and the yield and growth of the plant are directly dependent upon the nodules. More the nodules more shall be the yield. In this system it is only the right partnership between legumes and Rhizobium which results into the nitrogen fixation.
Both the partners i.e. Rhizobium and legumes while living alone cannot fix the atmospheric nitrogen. Also in Rhizobium and legume partnership require a very high degree of specificity. A particular species of legume is capable of forming nodules only when it is infected with specific species or strain of Rhizobium. A Rhizobium capable of infecting and forming nodules in moong crop cannot infect and nodulate soybean plant.
Similarly, Rhizobium capable of infecting soybean cannot infect moong or groundnut or clover. Depending upon their host specificity Rhizobium has been grouped into eight cross inoculation groups distributed under 3 generic names. For each group there has to be separate crop specific Rhizobium. Keeping in the tune with the host specificity different packets of Rhizobium biofertilizer are available for different crops.
Azotobacter is a free living nitrogen fixing bacteria and when it is applied to the crop either as seed treatment or seedling root tip or as soil treatment, large number of Azotobacter cells sticks to the seed or roots and form a thick sheath of bacterial population around roots. Azotobacter prefers to thrive mainly in soil close to roots. For their multiplication the food is derived either from dead organic matter present in the soil or from root exudates excreted by the developing roots.
Here during routine course of activity they fix atmospheric nitrogen. Initially it is fixed for their own build up but very soon when the population growth stabilizes, the rate of multiplication and rate of death become at par and fixed nitrogen is released in the soil after decay of dead cells. As all this happening in close vicinity of roots, the released nitrogen is quickly absorbed by the plants.
Azotobacter besides fixing nitrogen are also known to synthesize and secret auxins, vitamins, growth promoting agents and anti-fungal substances. These extra cellular secretions confer additional advantage of Azotobacter use.
The plant response to Azotobacter biofertilizer application in different crops is normally reported in terms of increased vegetative growth. Various trials / experiments conducted in combination with graded levels of chemical N compared the yield attributes in terms of equivalent of chemical N applications.
Azospirillum a curved rod shaped, heteromorphic micro-aerophillic bacterium is also believed to be widely predominant in most of the tropical and sub-tropical Indian soils having pH from 4.5 to 7.5.
Out of four known species of Azospirillum, two species, viz., A. lipoferum and A. brasilense are widely predominant in nature. Both the species are known to colonize the rhizosphere and rhizoplane of a wide variety of plants.
Similar to Azotobacter when Azospirillum is applied to crops through seed, seedling or soil treatment, large number of Azospirillum attached to the seeds and seedling roots, rapidly multiply along the developing roots. Very soon a thick sheath of Azospirillum population is formed around the roots. Unlike, Azotobacter, which prefers to proliferate in soils adjacent to roots, the Azospirillum prefers to proliferate on root surface.
In some cases where Azospirillum strain and host root matches to each other, then a few Azospirillum cells also enter into the roots and survive in root cortex. For their growth and multiplication Azospirillum require constant supply of carbon source in the forms of salt of organic acids is best available in root exudates. That is why the Azospirillum maintains a much more intimate associate-ship with host root compared to Azotobacter.
Experiments conducted on the efficiency of this biofertilizer indicated 15-35 % increase in the yield over control when all parameters kept constant.
Acetobacter biofertilizer is one of the recently added microbial inoculant for nitrogen mobilization in sugar cane crop. This bacteria responsible for a very high rate of plant associated biological N fixation. It has unique physiological properties for a dizotroph, such as tolerance to low pH and high sugar and salt concentration. Lack of nitrate reductase activity and unique nitrogenous activity which tolerates short term exposure to ammonium ions.
For the optimum results, the Acetobacter diazotrophicus inoculant is applied to freshly cut seed sets as dip treatment. The bacterium can enter into the system through the damaged tissue of cut pieces. Once the bacterium is found to be properly established in those cultivars further propagation is done through planting materials. Specifically developed VAM inoculant containing Acetobacter diazotrophicus can also be used for inoculating the micro propagated seedlings in the first stage and then further propagation is done through planting material.
Frankia is a genus of nitrogen fixing filamentous bacteria that live in symbiosis with actinorhizal plants, similar to Rhizobia. Bacteria of this genus form root nodules. Frankia has symbiotic relationships with numerous dicot plants and is said to be responsible for 15% of the biologically fixed nitrogen in the world.
One type of symbiotic relationship including plant, mycorrhiza, and Frankia is called a tripartite relationship and is a complex, multi-layered community of organisms that protect and support each other. Frankia helps in Nitrogen fixation. The process requires high energy. Actinorhizae having symbiotic associations, the microbe obtains the requisite energy by way of carbon compounds from the host plant like Casuarina, Myrica and Elaeagnus.
Cyanobacteria or Blue Green Algae (BGA) have the ability to carry out both photosynthesis as well as nitrogen fixation. The members of order Nostocales and Stigonematales consist of heterocysts which are specialized nitrogen-fixing cells of Nostoc punctiforme, Cylindrospermum stagnale and Anabaena sperica. The process occurs during nitrogen starvation. They fix nitrogen from dinitrogen (N2) in the air using the enzyme nitrogenase, in order to provide the cells in the filament with nitrogen for biosynthesis. Nitrogenase is inactivated by oxygen, so the heterocyst must create a micro-anaerobic environment.
Azolla is a floating fern which harbours blue green algae in its leaf cavities. The fern multiplies very fast with the symbiotic association of the algae and this rapid multiplication creates a huge amount of biomass on the surface of the water. It is then harvested, dried and used as biofertilizer.