Nitrogen Fixing plants & Mycorrhizae

I remember when I was younger, farmers cycling their fields with nitrogen fixing plants (buckwheat or clovers) to replenish the fields.

Does anyone use nitrogen fixing plants in their pots? (clover,rooibos, mountain mahogany, lupines*?

I understand that some plants have a capacity to fix nitrogen from the air, others from soil (with help of mycorrhizae). Has anyone researched this?

Your bonsai pot is not a farmer's field. Bonsai get an adequate amount of nitrogen from fertilizer. There are very few vascular plants that can be grown as companions in a bonsai. It is usually not done, except in tray landscapes.
The job of mycorrhiza is not fixing nitrogen. Mycorrhiza colonize the tree's roots to increase its ability to absorb all fertilizer.
Iris

Mycorrhiza colonize the tree's roots to increase its ability to absorb all fertilizer.

And they're not really needed in a bonsai pot because we supply all the fertilizer the trees need (and then some). But many, many bonsaiests feel better if they're there and that's OK, too. Actually, the phosphate in the fertilizers we furnish the trees often damages the Mycorrhiza.

Certain plants such as legumes have root nodules which house specialised atmospheric nitrogen-fixing bacteria (including some trees, Acacias for example).

Thats an interesting point about the phosphorus Jim. How much is too much?

How much is too much?

I don't know. I think it was Nina that pointed us to this issue a while ago, but I no longer have (or can find) the reference. The main gist of it, however, that you do not NEED the fungi in plants grown in pots.

But again, if it makes you feel better . . .

I prepared a soils talk years ago and I did read references that showed a high level of available phosphorus (couldn't now tell you the actual numbers) did inhibit the growth of added Mycorrhizal colonies. On the other hand, perhaps there is accelerated growth of Mycorrhiza in substrates already deficient in Phosphorus, i.e. in growing conditions that would most benefit from a robust Mycorrhiza population.