I've come up with an herbal "ley" (salad bar pasture mix) of seed to use in reseeding our predominantly cheatgrass/goatgrass field to a diverse, nitrogen fixing, mineral mining, insect attracting, goat/chicken/human forage source. Before I knew about herbal pastures and about the importance of nitrogen fixing bacteria on alfalfa/clover rootlets, I planted an acre or so of the field with a mix of cool season grasses, alfalfa and clover. I didn't innoculate the alfalfa/clover seed with rhizobium meliloti at the time, figuring that the bacteria would show up somehow on its own. It established well, but this spring I dug some alfalfa up in the field and couldn't find any of the nitrogen fixing nodules. I did find them on alfalfa that was growing in my garden though, not sure why (I inoculated black bean seed planted there last year, but that is a different Rhizobium species than R. meliloti.
Would any of you know of a way to introduce Rhizobium meliloti bacteria on already planted Alfalfa and clovers?
I know this might be more work than you planned, but if you have access to a no-till drill you could make a pass over the already planted acre with a very light seeding rate to put in inoculated seed. That would get the bacteria into the ground and it should tend to migrate to the existing plants over time.
My only other thought would be to make a drench of the inoculant and use a sprayer to place on the existing planted acre. Preferably this would be performed during a rain so that the inoculant would be washed into the soil. This is not something I have tried so others may have better options.
consider trying a small-scale soil tranfer experiment from your inoculated garden soils out to the pasture. When I was in grad school there were people workingto develop mycorrhizal and rhizobium innoculant and they have had some success. But it is noteworthy that many of the scientists (not students) believed that soil transfers were still a good way to go. Get a bucket of the good stuff and trowel it in around a marked area of alfalfa - probably springtime is best - and see how that goes. You could scale up from there. Or it might take off if the plants are spaced tightly and a light till would move the topsoil around a bit.
I also learned that plants which are normally mycorrhizal, eg nursery grown natives ornamentals, tend Not to form the mycorrhizal symbiosis when they are fertilized. We all assumed the plants were thinking "why bother to spend the energy?". There was similar thinking about the n-fixing bacteria, though I don't know that I ever saw a reference. So if your pasture is rich in N from former fertilization that might explain the scarcity of nodules.
Somewhere in my mass of printed knowledge (but not in my gray matter) I must have some serious refs. I will bookmark this thread and come back to it. I studied mycorrhizae on natives in grad school. I learned (from the best) that the vast majority of wild plants form mycorrhizae. Weeds often do not, which is thought to be an adaptive strategy for success in degraded sites. There are academic citations for that. Small scale soil transfers are definitely a good idea for getting natives established in new or damaged sites. Do some little experiments, take pictures and keep notes!