Non symbiotic bacteria such as azotobacter,clostridium also help in fixing nitrogen present in atmosphere. If environmentalconditions are favourble then Frankia (nodulating bacterial symbionts) can alsofix N2 in non-symbiotic association 1. Frankia can bepresent in rhizosphere of non-host plants and help in nitrogen fixation. Recordhas shown Frankia is present in rhizosphere of Betula Pendula2 andin soil with no actinorhizal plants3 .
N2 fixationcannot be estimated by free living bacteria. There are different methods ofinoculating bacteria in green house experiment such as leaf spray, seed soakingand side dressing. The growth of cucumber and barley plants was stimulated byinoculation of Beijerinckia mobilis and clostridium sp.4 .However, it remained unclear about the mechanism of growth.
The growth is notonly stimulated by inoculating bacteria but it can also be due to plant growthhormones.2.1.1 Symbiotic nitrogen-fixingbacteriaRhizobiaRhizobia family (Rhizobium, Bradyrhizobium,Sinorhizobium, Azorhizobium)5 is the best known and most exploitedsymbiotic nitrogen-fixing bacteria. This kind of bacteria cause infection inlegumes.
We should consider rhizobia-host compatibility while selecting biofertilizer. For N2 fixing, the capability of rhizobia varies up to450 Kg N.ha-1 among strains of bacteria and host plant species. Rhizobialstrains that are considered as practical applications as bio fertilizers musthave some additional characteristics like they must have fixation rate ofNitrogen and can compete with naturally occurring rhizobia. Survival and easyproduction of inoculum is necessary in field application when target seeds areinoculated. Inoculation is of prime importance when rhizobial populations ofresident soil are absent.
For example, acidic and basic soil contain lowpopulation density of alfalfa rhizobial symbiont (Sinorhizonium meliloti) andlow inoculum potential of Brady rhizobium sp. respectively 6.It is highly advantageous to inoculate compatible rhizobia under suchconditions.
Evaluating the need for inoculation and performing cost benefitanalysis is necessary before initiating massive scale inoculation program withrhizobia. Inoculation is useful for productivity of crops if rhizobialpopulation density is less than 100 rhizobia/gram of soil 7.Inoculation would prove cost effective in presence of low population densitiesin spite of indigenous rhizobia nitrogen fixation efficiency.CyanobacteriaCyanobacteria find its importance inecology e.g. an aquatic cyanobacterium (Trichodesmium) contributes 36% ofglobal nitrogen fixation according to estimate. 8 Nitrogenfixation using cyanobacteria has proved beneficial in rice cultivation.
Anothernitrogen source for rice cultivation in China was Azolla Anabaena until end of1970s. Cyanobacteria still have key role in rice fields in various parts ofAsia. Furthermore we can remediate arid soils with cyanobacteria and nitrogenfixation. However the production and utilization is yet poorly developed9.
For supporting sustainable agriculture activities in differentenvironments there is serious need to consider cyanobacteria as biofertilizer.
