Joshua Myrvaagnes wrote: I wonder if it's a matter of giving them a rating on a scale from 1 to 10--mildly nitrogen-fixing to really nitrogen-fixing. It would be nice to compare all plants this way in a per-square-foot or per-year basis, so as to have apples-to-apples.
This would be great if achievable, but the variables are so huge from climatic and soil factors through to the existence or otherwise of strains of N-fixing microbes in every soil that unfortunately it would only be achievable on a very small scale. Example is the strains of Frankia that fix N in association with Alders, the levels of N-fixation even in the same species can vary hugely due to the effectiveness of the microbial association. Another example that annoys me, Mimosa scabrella occurs in South America in climates similar to mine, but they just absolutely refuse to grow here. Germination is easy, and they grow to about 4" tall easily, then basically stop. I have found the same problems with a lot of South American tree legumes. By contrast Australian legumes nodulate in my soils extremely easily, but even then the associated Bradyrhizobia associations in New Zealand vary in their N-fixation effectiveness by over 1000%, according to one study. I'm sure these issues would be universal to all soils when non-native N-fixers are contemplated.
Personally I would always prefer N-fixing pioneer trees than willows or birches! The only reason I would plant willows is because they get badly infested by Giant Willow Aphid, which are a great feed source and bring in a lot of predatory insects. But oh boy those willows are easy to grow, any bit of stick or branch stuck into the ground anywhere grows readily. Some spread wildly by seeds too, always better to plant sterile (single sex) clones if possible.