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

Silica nanoparticles (SiO-NPs) are promising in nanoenabled agriculture due to their large surface area and biocompatible properties. Understanding the fundamental interaction between SiO-NPs and plants is important for their sustainable use. Here, 3 week-old pakchoi ( L.) plants were sprayed with SiO-NPs every 3 days for 15 days (5 mg of SiO-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 SiO-NPs were characterized by high-throughput bacterial 16S rRNA and fungal internal transcribed spacer (ITS) gene sequencing. The results showed that the SiO-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 SiO-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 ) and fungal () genera in the rhizosphere involved in carbon and nitrogen cycles were increased. Our results provide novel insights into the environmental effects of SiO-NPs and point out that foliar application of NPs can alter the soil metabolite profile.