Mechanistic study on the synergistic promotion of γ-Fe₂O₃ nanomaterials and Bacillus methylotrophicus to the growth and yield of soybean (Glycine max)

Objectives This study aimed to investigate the performance and mechanism of the synergistic effect of applying iron oxide nanomaterials (γ-Fe₂O₃ NMs) and Bacillus methylotrophicus (BM) by foliar method on the growth, yield, and seed quality of soybean. Further, the study provides new insight and basic theoretical support for the application of γ-Fe₂O₃ NMs in agricultural ecosystems.

Methods The experimental crop was soybean. For the NMs-treatment group, 0, 1, 10, 30, and 50 mg/L γ-Fe₂O₃ NMs were applied to soybean using the foliar method. BM and a series γ-Fe₂O₃ NMs concentrations were co-applied to the leaf of soybean in the co-application treatment group. Photosynthesis parameters of soybean were measured at 40 days of growth, while plant biomass, total sugar content, and yield were measured at the maturity stage. Luria-Bertani medium in vitro experiment and OD₆₀₀ of BM solution were performed to investigate the effect of γ-Fe₂O₃ NMs on BM's growth. A single particle inductively coupled plasma mass spectrometer was used to explore the effect of BM on the γ-Fe₂O₃ NMs bioavailability.

Results Results indicate that the application of γ-Fe₂O₃ NMs to soybean by foliar method improved its photosynthesis and biomass. Foliar spray of γ-Fe₂O₃ NMs in 30 and 50 mg/L significantly increased soybean yield and carbohydrate content in seed. The co-application of BM and γ-Fe₂O₃ NMs enhanced soybean yield by 31.5% and 13.4% compared with the single application of 30 and 50 mg/L γ-Fe₂O₃ NMs. BM treatment significantly increased the number of soybean root tips compared to the control group, while γ-Fe₂O₃ NMs treatment did not affect the number of soybean root tips. However, the number of soybean root tips with the co-application of BM and γ-Fe₂O₃ NMs were (P < 0.05) higher than those treated with BM alone. The aggregation of γ-Fe₂O₃ NMs (P < 0.05) reduced in the fermentation broth of BM. 10, 30, and 50 mg/L γ-Fe₂O₃ NMs promoted the growth of BM, with the attendant increase in indole acetic acid content in the bacterial fermentation broth from 3.8 mg/L to 7.6–8.8 mg/L in the presence of 10, 30, and 50 mg/L γ-Fe₂O₃ NMs. The co-application of γ-Fe₂O₃ NMs and BM enhanced the nutrient uptake of soybean and the elemental nutrient content (e.g., Fe, Mn, S, Mg) in seed.

Conclusions γ-Fe₂O₃ NMs enhance the photosynthesis of soybean—the main mechanism for improving soybean growth and yield. The co-application of γ-Fe₂O₃ NMs and BM exhibites a synergistic effect on the growth, yield, and seed quality of soybean. The mechanisms identified are that: 1) the metabolites of BM effectively decrease the aggregation of γ-Fe₂O₃ NMs, thereby promoting its bioavailability; 2) γ-Fe₂O₃ NMs promote the growth of BM and the content of indole-3-acetic acid in the metabolites of BM.