I should have said all bacteria that lives in the soil fixes nitrogen .Also all algae can fix nitrogen same like bacteria because the algae are not plants.
Certain bacteria live in simbiosis with plants but i dont believe they give the plants the N for free or that it is really a simbiosis.
I believe the plants use bacteria,basically they are parasitising bacteria or farming bacteria .They fed it sugar then they kill the bacteria to use all that N the bacteria has in them.
The man that wrote that article is 50% clueless about how this happens and my half illiterate comment above with the ,,underground flowers ,,comparison would have teached him a lot.
Bacteria and algae are exactly like meat .They have in their proteins all the soo called ,,essential aminoacids,, wich cant be found in plants.
Those essential aminoacids decompose into amines when the bacteria dies wich are foul smelling substances that give the characteristic corpse smell ( cadaverine,putrescine,etc).
Further the amines decompose ammonia wich plants can use directly .Further into NO2 and then in NO3 and then nitrifying bacteria ( this ones doesnt takes the N from air)decomposes the NO3 into Nitrogen and Oxigen,both gasses that leave the soil.
So any dead bacteria its like a tiny dead animal in the soil wich plants can use as fertiliser.
Even dead nitrosomas,the nitrifying bacteria that doesnt splits the N from air its still fixing the N because when it dies it releases nitrogen just like any otther dead animal matter.
One thing is certain ,that splitting nitrogen requires hard work that bacteria wouldnt give it to plants for free and that the bacteria has to die in order to release the N to be available for plants.
Because the bacteria has to die i dont think of this relationship ,plant-bacteria as simbiosis ,its more like parazitism or ,,farming,, what these plants do.