Fig. 6
a Development of soybean root in 4 weeks post rhizobium inoculation with/without 10 mg L−1 MF-NPs exposure and the nodule microsection with toluidine blue staining. b Viability of symbiotic rhizobia evaluated by SYTO9. Green indicates all bacteria, and red after PI staining denotes dead bacteria. c Normalized nitrogenase activity (n = 5). d Individual plant nitrogen fixation efficiency (n = 5). *P < 0.05, **P < 0.01, and ***P < 0.001 indicate three different significance levels