Sustainable strategies for the management of iron deficiency in agriculture are warranted because of the low use efficiency of commercial iron fertilizer, which confounds global food security and induces negative environmental consequences. The impact of foliar application of differently sized γ-Fe₂O₃ nanomaterials (NMs, 4-15, 8-30, and 40-215 nm) on the growth and physiology of soybean seedlings was investigated at different concentrations (10-100 mg/L). Importantly, the beneficial effects on soybean were size- and concentration-dependent. Foliar application with the smallest size γ-Fe₂O₃ NMs (S-Fe₂O₃ NMs, 4-15 nm, 30 mg/L) yielded the greatest growth promotion, significantly increasing the shoot and nodule biomass by 55.4 and 99.0%, respectively, which is 2.0- and 2.6-fold greater than the commercially available iron fertilizer (EDTA-Fe) with equivalent molar Fe. In addition, S-Fe₂O₃ NMs significantly enhanced soybean nitrogen fixation by 13.2% beyond that of EDTA-Fe. Mechanistically, transcriptomic and metabolomic analyses revealed that (1) S-Fe₂O₃ NMs increased carbon assimilation in nodules to supply more energy for nitrogen fixation; (2) S-Fe₂O₃ NMs activated the antioxidative system in nodules, with subsequent elimination of excess reactive oxygen species; (3) S-Fe₂O₃ NMs up-regulated the synthesis of cytokinin and down-regulated ethylene and jasmonic acid content in nodules, promoting nodule development and delaying nodule senescence. S-Fe₂O₃ NMs also improved 13.7% of the soybean yield and promoted the nutritional quality (e.g., free amino acid content) of the seeds as compared with EDTA-Fe with an equivalent Fe dose. Our findings demonstrate the significant potential of γ-Fe₂O₃ NMs as a high-efficiency and sustainable crop fertilizer strategy.

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