Surfing Wikipedia for Information on the Honey locust I encountered this interesting paragraph on the nitrogen fixing capabilities of the tree:
The ability of Gleditsia to fix nitrogen is disputed. Many scientific sources clearly state that Gleditsia does not fix nitrogen. Some support this statement with the fact that Gleditsia does not form root nodules with symbiotic bacteria, the assumption being that without nodulation, no N-fixation can occur. In contrast, many popular sources, permaculture publications in particular, claim that Gleditsia does fix nitrogen but by some other mechanism.
There are anatomical, ecological and taxonomic indications to indicate nitrogen-fixation in non-nodulating legumes. Both nodulating and non-nodulating species have been observed to grow well in nitrogen-poor soil with non-nodulating legumes even dominating some sites. The litter and seeds of non-nodulating species contains higher nitrogen than non-legumes and sometimes even higher than nodulating legumes growing on the same site. How this happens is not yet well understood but there has been some observations of nitrogenase activity in non-nodulating leguminous plants including honey locust. Electron microscopy indicates the presence of clusters around the inner cortex of roots, just outside the xylem, that resemble colonies of rhizobial bacterioids. These may well constitute the evolutionary precursors in legumes for nitrogen fixation through nodulation. It is not known whether the non-nodulating nitrogen fixation, if it exists, does benefit neighboring plants as is said to be the case with nodulating legumes.
Interesting to see, that Permaculture research comes to diverging results that science and especially interesting that permaculture publications are named here. I would love to see this permaculture view on things in many other articles.
All the best.