Foliar Application of SiO2 Nanoparticles Alters Soil Metabolite Profiles and Microbial Community Composition in the Pakchoi (Brassica chinensis L.) Rhizosphere Grown in Contaminated Mine Soil

Silica nanoparticles (SiO2-NPs) are promising in nanoenabled agriculture due to their large surface area and biocompatible properties. Understanding the fundamental interaction between SiO2-NPs and plants is important for their sustainable use. Here, 3 week-old pakchoi (Brassica chinensis L.) plants were sprayed with SiO2-NPs every 3 days for 15 days (5 mg of SiO2-NPs per plant), after which the phenotypes, biochemical properties, and molecular responses of the plants were evaluated. The changes in rhizosphere metabolites were characterized by gas chromatography–mass spectrometry (GC–MS)-based metabolomics, and the response of soil microorganisms to the SiO2-NPs were characterized by high-throughput bacterial 16S rRNA and fungal internal transcribed spacer (ITS) gene sequencing. The results showed that the SiO2-NP spray had no adverse effects on photosynthesis of pakchoi plants nor on their biomass. However, the rhizosphere metabolite profile was remarkably altered upon foliar exposure to SiO2-NPs. Significant increases in the relative abundance of several metabolites, including sugars and sugar alcohols (1.3–9.3-fold), fatty acids (1.5–18.0-fold), and small organic acids (1.5–66.9-fold), and significant decreases in the amino acid levels (60–100%) indicated the altered carbon and nitrogen pool in the rhizosphere. Although the community structure was unchanged, several bacterial (Rhodobacteraceae and Paenibacillus) and fungal (Chaetomium) genera in the rhizosphere involved in carbon and nitrogen cycles were increased. Our results provide novel insights into the environmental effects of SiO2-NPs and point out that foliar application of NPs can alter the soil metabolite profile.