I hoped you might get a more explicit answer from a pro who knew more about these specific plants, but that hasn't happened. Here's what I know.

'Nitrogen fixing plants' cover a *huge* spectrum, and their ability to do more than somewhat augment their own growth with the assist of their symbiotes also varies wildly.

In the research you have done, you may be conflating a couple of different processes/problems. Nitrogen fixing plants will contribute some to the soil nitrogen, depending on growing conditions. "Excess nitrogen", on the other hand, rarely has *anything* to do with neighboring nitrogen fixers. Their effect is just not that strong. "Excess nitrogen" generally has to do with too much added fertilizer.

You're thinking too hard...

Nitrogen fixing plants may make a difference in a vegetable bed where you are rapidly succession-planting, or planning on same-season rotating crops. These are very short-term pluses and minuses.

When thinking of planting shrubs and trees, or berry bushes, or fruiting vines, any nitrogen fixers are most likely to be a 'little blip' at very most. Their mutual compatibility and health will depend *far* more on sun/shade conditions, soil quality and water availability.

Don't worry about nitrogen fixing in these conditions. Just don't throw a lot of commercial fertilizer on your lawn adjacent to those trees and shrubs.....

My Goumis are getting quite large, over my head. I am trying some native legumes this year for potential nitrogen fixing plants, like Amorpha, Dalea, and Psoralea. Some are hard to germinate. Runner beans might be nice growing up into a tree, especially with the red flowers that attract hummingbirds.

excess nitrogen can cause fruiting plants to grow at the expense of fruiting. I have Autumn Olive and Goumi next to a bunch of trees and I cannot comment on the effect on fruiting because all the plants next to the Eleagnus sp plants are much younger. I can tell you that my nectarine next to the goumi seems to have grown much better than either of the nectarines or peaches not next to a goumi.

~Chills

I just planted two autumn olives under two of my dunstan chestnut trees, and I'm trying to root more autumn olives and goumi plants as we speak. I'll probably plant one near my hybrid chestnut/chinkapin trees when some of these take root.

I read somewhere that nitrogen fixers, Autumn Olive specifically, are commonly planted in nut orchards. Evidently they help with both growth and production. I can't say for sure with fruit trees, but they are supposed to help with nut trees.

I wish there were more comments in this thread. I spent hours researching (looking up online) this topic and I was hoping to find my answer in this thread. I like what Linda wrote: I hope it's valid. I would think that, perhaps, that answer is correct for native nitrogen-fixing shrubs like Shepherdia (buffaloberry) spp. Yet, I might imagine that shrubs like Hippophae spp. (sea-buckthorn, H. rhamnoides) might produce way more nitrogen then is beneficial for the neighboring fruit tree - in some cases: i.e. pear, mountain-ash, etc. I think that would promote fire-blight for these two crops, in particular.

Another concern I'm considering is: will the excess nitrogen delay winter dormancy in some marginally hardy plants (i.e. Black mulberry, hardy selections of pomegranate, etc.) Again, I think you might be ok with native Shepherdia spp., but I think some of the non-invasive and invasive exotics would enrich the soil enough, if not to harm fruit trees - still not sure on that one, but definitely to encourage the recruitment of exotic invasive weeds. Autumn-olive, it is believed - or fact[?], is responsible for rendering soils which are less fertile - to which native flowers and graminoids are well-suited - and, after enriching the soils with nitrogen, make them ideal for the recruitment of alien invasive weeds: where I see Autumn-olive I see many weeds and few native herbs; not sure, though, which: chicken or egg, in these cases.

Steve

still no replies lol?

Southern peas are a good plant & you can pick the peas, before cutting the vines for mulch.

Have you read Toby Hemenway's book "Gaia's Garden"? He has a website that might answer many of your questions. Don't have the link handy, but you can google him and find it. The name is something about patterns.
Your local library might have the book, and it's well worth buying as well.

The fixed N is stored in root nodules. You would have to destroy plant to have these nodules to degrade and release the stored N the fixing plant has not used. That is how legumes are used in a rotation in an annual vegetable garden. As Linda mentions if used as a long term succession planting the N fixers would improve poor soil but growing next to another plant comes to no or very little good. The fixed N is simply not released by fixing plant into surrounding soil.

Mike

Following along Mike's comment - in your climate, you might be interested in growing some peanuts if the area gets enough sun. They will die every winter, releasing the stored N, but will also plant the seeds for next year by themselves! I have some peanuts scattered in my raised beds. Sometimes I pull some to harvest, other times I don't. Even when I've harvested all the plants, a few still come up the following year from pods that broke off underground.