Looking for a container friendly perennial nitrogen fixer

I currently have 4 half wine barrels each planted with a dwarf columnar apple tree, 2 chive plants, and an herb (2 have rosemary, 1 has thyme and lemon balm, and the other oregano and lavendar). They have been going strong for about 3 years now and produce pretty well, but I would like to add a perennial nitrogen fixer for a bit of soil health. I'm in Zone 8 and have looked at adding licorice (Glycyrrhiza Glabra), Huang Qi (Astralagus Propinquus), or even AlfAlfa, but I am a little concerned that they might not have enough space to grow. Any suggestions? If this is the wrong forum my apologies.

there are not a whole lot of small legumes that are perennials, at least native to the US. lespedeza and desmodium come to mind, but i don't know if they would be considered perennial or not, but the desmodium prevalent in my area is. Plus, has amazing flowers. without knowing your location it's hard to say if it would grow there. in the south there are some very small nitrogen fixing trees like mimosa, another bonus of attractive flowers. does perennial peanut thrive where you are? it's a small plant with yellow flowers that bloom all summer, but unfortunately does not produce peanuts.
the nitrogen-fixing capability of legumes is vastly overstated. but if it were me, i would pop in some much easier to acquire annual legumes, maybe some that trail over the sides of the containers attractively, like runner bean or peas.

Thanks Chrissy for the suggestion, I'm in Portland, OR. Runner beans and peas are pretty prevalent here, so thats probably my best shot.

I would also consider (Chinese) Snow Peas. They do very well here in the PNW.

Territorial Seeds has a good selection, including one or two developed by the famous plant breeder Dr. Jim Baggett, while he was working/teaching at Oregon State.

Snow Peas have small/shallow root systems, so they shouldn't harm the tree's root system.
Remember to innoculate whatever you plant, for maximum N fixation.

I'd get the most dwarf baptisia you can get if not annual veggies. Perhaps just grow what legume vegetables you want and plant rampant legumes guerrilla style everywhere, harvest, compost, and topdress?

I would just grow clover. It is nitrogen fixing but small sized and works as a ground cover. Other stuff like perennial beans could maybe compete for nutrients with the dwarf apple, but that's my feeling. But of course an edible species would be ideal.

I don't know Astralagus, but I heard they are small, could they work?

I can echo the clover (Trifolium pratense) suggestion. Great success with this species, specifically in pots. They love it. I love them.

I'm pretty sure (not 100%) that dutch white clover should perform as a perennial in zone 8.