Complex sentence: Many scientific sources clearly state that Gleditsia does not fix nitrogen by forming root nodules with symbiotic bacteria but many popular sources, permaculture publications in particular, claim that Gleditsia does fix nitrogen but by some other mechanism.
Split into simpler sentences: Many scientific sources clearly state that Gleditsia does not fix nitrogen. In contrast, many popular sources, permaculture publications in particular, claim that Gleditsia does fix nitrogen but by some other mechanism.