As you know, Azotobacter is an aerobic microorganism, while Azospirillum is a microaerophilic, so in the submerged paddy fields it would be better to use Azospirillum.

For your information, Azospirillum works as a plant growth hormones producer more than as a nitrogen fixer.

Good luck 

Thanks Dr. Gomaa for your kind response.

Some results are available where application of Azotobacter chroococcum produced inspiring results in submerged paddy field condition.Does the air sacs produced by the paddy roots in submerged condition have any effect in case of nitrogen fixation or create any conducive environment for Azotobacter Chroococcum to accumulate as well as supply  nitrogen to plants? I invite more discussion and valuable comments  on the basis of field trials and  observations in this subject.

 with regards. 

You are welcome.

Actually, you are stirring a good point where the paddy soils are not completely anaerobic, so the Azotobacter can also work under the paddy conditions. 

 I appreciate your good finding

Thanks Dr. Gomaa for your contribution.

Regards.

You have to consider that many Azospirilla strains in anaerobic conditions are able to act as denitrifiers. In field experiments we were able to measure increased denitrification rates during rainy seasons. In dry seasons in Italy the same strains were able to increase root extension of Gramineae acting as hormone producers. This effect was not observed in rainy seasons.

I think Azotobacter  will be more effective.

Thanks Marina for pointing out a very interesting result in Italy. 

With regards.

Dear  Mukhopadhyay,

Microaerophilic nature of Azospirillum will be more supporting for its activity under the paddy field. However depends upon the soil properties you can use see the activity of both strain simultaneously. 

Good Luck