Eric Toensmeier has just written a superb article on patterns of nitrogen fixation.
Here's a snippet...
When Dave Jacke and I were writing Edible Forest Gardens, we discussed polyculture design extensively. We felt that we were beginning to see the vague outlines of an international pattern language of polycultures – a set of basic templates that might be used regardless of climate or the particular species in question.
Given the huge diversity of crop species and possible combinations, there may be an almost infinite number of polycultures grown in the world today. In my garden I grow many, and I’ve been fortunate to be able to travel and see many more. Reading and research have introduced me to a great number including commercial systems from around the world.
That pattern language is beginning to seem clearer to me. For example, though agroforestry systems including food forests are practiced around the world and in many climates, I only see a few basic patterns representing the majority of the use of nitrogen fixers.