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 Sphingomonas, Lysobacter, and Bryobacter, 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.