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.