Enhancing Azotobacter chroococcum with Fe3O4 NPs and n-MoO3: A Promising Strategy for Sustainable Agriculture

(1) Background: Overuse of chemical nitrogen fertilizers drives the need for biological alternatives. Azotobacter chroococcum is a promising free-living nitrogen-fixing bacterium, but its efficiency needs improvement. This study investigated how Fe3O4 nanoparticles (Fe3O4 NPs) and molybdenum trioxide nanoparticles (n-MoO3) affect A. chroococcum growth and nitrogen fixation, and tested the modified inoculants on Glycine max (legume) and Nicotiana benthamiana (non-legume); (2) Methods: In vitro tests measured bacterial growth, viable counts (CFU), nitrogenase activity, and nitrogen metabolites (total N, NO3−-N, NH4+-N) under 0–100 ng·mL−1 Fe3O4 NPs or n-MoO3. Pot experiments then tested modified inoculants on Glycine max and N. benthamiana for biomass and N, P, K uptake; (3) Results: Both nanomaterials showed low-dose stimulation and high-dose inhibition. At 10 ng·mL−1, bacterial growth (OD600 up ~1.2×) and nitrogenase activity (up >90%) rose significantly (p < 0.05–0.001), along with higher total N, NO3−-N, and NH4+-N. In pots, 10 ng·mL−1 modified inoculant improved all Glycine max traits and nutrient uptake (p < 0.05). For N. benthamiana, biomass peaked at 20 ng·mL−1, while stem and root growth did best at 10 ng·mL−1. At 100 ng·mL−1, effects weakened or vanished. A “metabolic remodeling–rhizosphere transformation–systemic response” mechanism is proposed; (4) Conclusions: Low concentrations (10–20 ng·mL−1) of Fe3O4 NPs and n-MoO3 can effectively boost the nitrogen-fixing function and growth-promoting effect of A. chroococcum inoculant, showing good potential for use on both legume and non-legume crops. This study provides a theoretical basis and technical reference for developing efficient, broad-spectrum nanomaterial-microbe composite inoculants.