-9c is chilly!

Europe has very few nitrogen fixing native trees, the Ice ages wiped them out.
But there are species in Asia and the Americas, even Alpine Australia that can be used.

Tagasaste will cope down to -6c, -3c is ok while young.

I was in Kyoto , Japan and they have bush clover. Lespedeza bicolor var japonica. Its a very good shrub legume and very beautiful

I also have used Albizia julibrissin , the Persian silk tree as a support legume.

Black locust (Robinia pseudoacacia) is good but weedy. I probably dont reccomend it.

Acacia alpina and Acacia phlebophylla from the Australian alps are cold hardy to snowfall. If you can find seed.

These are hardy plants that can grow in the mountains on rocky slopes