Foliar Application of SiO₂ Nanoparticles Altered Rhizosphere Metabolite and Microbe Profiles: Surface Chemistry-Driven Biological Effects

Silica nanoparticles (SiO₂NPs), as emerging foliar nanofertilizers, demonstrate promising potential in agriculture. However, whether foliar application of SiO₂NPs alters belowground soil metabolites and microbe composition and abundance remains largely unknown. In this study, 3-week-old cucumber plants were foliar-sprayed with fumed or Stöber SiO₂ NPs dosing at -4 mg of NPs per plant for 5 days. Results show that fumed SiO₂NPs, enriched with three-membered rings (3MRs) of siloxane on the surface, induced more pronounced rhizosphere metabolite profile alterations compared to Stöber SiO₂NPs (4MRs). Similarly, 16S rRNA sequencing revealed that fumed SiO₂NPs caused more noticeable shifts in soil microbial community composition compared to Stöber SiO₂NPs. Microbes involved in carbon cycle, nitrogen metabolism, and antagonistic pathogens, such as <i>Sphingomonas</i>, <i>Lysobacter</i>, and <i>Bryobacter</i>, were significantly enriched in belowground soil upon fumed SiO₂NPs exposure. The elucidation of the relationship between silica surface structure and belowground biological effects will guide the rational design of silica nanofertilizers.