Nitrogen fixation and actinorhizal plants

Could anyone shed any light on the relationship between actinorhizal plants and their relationship with the Frankia genus of bacteria that they interact with. Alders are probably the most well known species that fix nitrogen in this way and if you dig up an alder root you'll almost always find root nodules that are hosting the Frankia bacteria. But everything in the Elaeagnaceae family (which includes seaberries, goumi's, autumn olive, silverberry, etc.) also rely on this bacteria for nitrogen fixation. So my question is, do you need to innoculate your soil with Frankia bacteria in order to get the nitrogen fixing benefits of the Elaeagnaceae's or will they some how find it in the soil? I've had mixed results looking at root systems that are grown in a "sterile" medium. Also, would inoculating these plants with soil from underneath alders be effective in getting the Frankia bacteria to their roots?
Also, while Robert Kourik is here, could he provide some insight on the controversial role of nitrogen release from N2 fixing species, such as when is the best time to chop and drop, the different methods of N2 fixing for woody vs. herbaceous species, or any N-fixers that he finds particularly useful in terms of the fertility they release.
And while we have Mr. Kourik here, could he give his theory on the roots/understory of conifers, advice on growing underneath the crown of conifers, etc.
Thanks for visiting permies Robert!

Hi Sam,
I have spent some time wondering about this topic and the complexity of the Frankia species. Our question too was whether the ubiquitous Alders would supply a suitable source of bacteria for seaberries.

The simple answer is that they will not. Alnus have symbiosis with cluster 1 strains of Frankia, and the plant species you and I are interested in, use cluster 3 strains. Some symbiosis of alder is seen by cluster 3 but this seems to be an artificial laboratory event.

The next question is “does it matter?” Both cluster 1 and cluster 3 are widespread, or as the ecologist say “cosmopolitan” in their distribution. Unlike cluster 2, cluster 1 and 3 are widely distributed in soil and probably grow extensively in the rhizosphere of non actinozhizal plants as free living bacteria. Cluster 1 is found under birch forest for example. The ecologic preferences of the strains is complex, and understanding seems incomplete. Some Alaskan have ecologist suggest that the strains may vary greatly even for alder species within the state and vary geographically. I can’t find a citation for this, which seems a little against the prevailing viewpoint.

We have been using donor plants obtained from outside for the seaberries with an initial closely approximated growth under nursery conditions before out planting of starts. This helps survival but may have nothing to do with Frankia colonization. If you had space and energy, it would be interesting to see what happened with Frankia capture in your region after direct seeding.
Good luck with your plantings and a great resource for all things Frankia is in the link below.

http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm