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The common nitrogen-fixer in paddy fields is
A
$Frankia$
B
$Rhizobium$
C
$Azospirillum$
D
$Oscillatoria$
Detailed Solution
$Rhizobium$ is a symbiotic nitrogen fixer of the root nodules of legumes, and $Frankia$ forms nodules on the roots of non-leguminous plants such as $Alnus$; neither is associated with paddy.
$Azospirillum$ is a nitrogen-fixing bacterium that lives in loose association with the roots of grasses and cereals, including rice; it is the answer accepted in the key.
$Oscillatoria$ is a cyanobacterium; cyanobacteria ($Anabaena$, $Nostoc$, $Oscillatoria$) are also important nitrogen fixers of paddy fields.
The answer accepted in the key is $Azospirillum$.
Note: NCERT states that in paddy fields cyanobacteria serve as an important biofertiliser, so $Oscillatoria$ is given as the answer in some books; the key of the source PDF is retained here.
$Azospirillum$ is a nitrogen-fixing bacterium that lives in loose association with the roots of grasses and cereals, including rice; it is the answer accepted in the key.
$Oscillatoria$ is a cyanobacterium; cyanobacteria ($Anabaena$, $Nostoc$, $Oscillatoria$) are also important nitrogen fixers of paddy fields.
The answer accepted in the key is $Azospirillum$.
Note: NCERT states that in paddy fields cyanobacteria serve as an important biofertiliser, so $Oscillatoria$ is given as the answer in some books; the key of the source PDF is retained here.