Great question, but I don't know the answer. There are a couple people I can ask, more soon.
I spoke with three colleagues in the Department of Natural Resources (Fahey, Yavitt, Goebel). All were of the same opinion that there was no need to treat the seeds in any special manner before planting because in the moist soils where alder usually grows, the tree roots will find the bacteria they need. The nodulation isn't essential for tree growth, so even if they don't immediately find the bacteria the tree can survive.
If you wanted to plant the alder on drier sites (which may not be a good idea), you could start the seeds in soil from a moist area likely to have the bacteria, and then transplant the seedlings.
I hope this helps. Let me know if you need more.
thanks Peter. So it sounds like I can start them and
1) they would thrive but not fix nitrogen
2) the bacteria Alders host is so widespread it is likely to be in any wet-ish habitat even if recently disturbed
3) I could find some Alders and later introduce nitrogen fixing bacteria with a arbored hole to the root area.
4) All of the above.
Apparently the bacteria are common in soils that might normally hold alder. Also, there doesn't seem to be an effective way to inoculate the seed, though I believe that happens for other plant species.
My take-away was there wasn't a viable way to introduce the bacteria, that the bacteria probably existed in the soils if the soils were suitable for alder, and that the absence of bacteria (if that was the case) wasn't significant.
Thanks for all the good questions.
Thanks for the reminder about transferring some soil from a place where mature plants are thriving & using it to inoculate the new planting.