A good green manure in rice fields is

Solution :
Aspergillus is a fungus, Mucor is also a fungus. Azolla harbors blue-green algae Anabaena which fixes $${N_2},$$ increasing fertility.
Notes : Azolla is unique because it is one of the fastest growing plants on the planet – yet it does not need any soil to grow. Unlike almost all other plants, Azolla is able to get its nitrogen fertilizer directly from the atmosphere. Azolla is able to do this because it has a unique mutually beneficial ‘symbiotic relationship’ with a cyanobacterium (blue-green alga) called Anabaena.