There's a few things that came to my mind when I listened to your very interesting conversation:
Early on you briefly talked about humanure as a very good but sometimes risky nitrogen source,
and I was wondering whether the classical way of dealing with it (
compost toilets) could and should not,
wherever possible, be replaced with what one might call a Permaculture Reed Bed.
Normal reed beds are constructed to allow for denitrification to remove almost all the nitrogen from
human waste before the water is released into the ground or a waterway. But what if one modified it
to recapture much of the nitrogen before the anaerobic bacteria are allowed to clean up the rest ?
Stinging nettles f.e. do tend to invade reed beds and are normally removed - what if one let them have
a "first bite of the cherry" and regularly harvested them as nitrogen-rich mulch ?
This could be a safe way to quickly make use of all the nitrogen from an omnipresent source that would otherwise have to be composted for a long time.
Then, when you talked about locust and the studies of its uses in agriculture I remembered this film:
http://www.dailymotion.com/video/xgy4cg_agroforesterie-enjeux-et-perspectives_tech It has Robinia in it, although the nitrogen from the tree is not used directly on the fields in this system.
There's also books about the subject containing a lot of photos:
http://books.google.de/books?id=Xns4ZxvFtMIC&printsec=frontcover&dq=agroforesterie&hl=de&ei=-bcNTtLrJ83KtAbgppyGDw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CDIQ6AEwAQ#v=onepage&q&f=false http://books.google.de/books?id=z_E6aojn29MC&printsec=frontcover&dq=Fabien+Liagre&hl=de&ei=d7gNTtTfDY2RswaYn43kDg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CC4Q6AEwAA#v=onepage&q&f=false Those should really be translated asap.
And oh, is there a good link to an article on the Rhizobium et al interactions ?