Soil activity and microbial community response to nanometal oxides were not due exclusively to a particle size effect

Abstract

Studies supporting assumptions that engineered nanomaterials (ENMs) are more toxic than their bulk counterparts are sparse. We have previously shown that soil response to ENM lithium oxide (nanoLi₂O) was not different than bulkLi₂O. Here we investigated how soil microbiota function responds to exposure to molybdenum oxide (MoO₃), nickel oxide (NiO), and zinc oxide (ZnO), either as bulk or the ENM equivalent, and to nanoLi₂O. We evaluated the solubility of bulk vs. ENM metal oxides and their influence on pH of saline solution and a nutritionally rich medium to understand their behavior in aqueous solution, as a simulation of soil pore water. Metal oxides more drastically affected pH of saline solution than aqueous media. Both forms of MoO₃ decreased soil acid phosphatase activity, pH, and total DNA. Soil exposure to highly soluble bulkMoO₃ showed an increase in bacterial and fungal biomass and relative abundance of Acidobacteria, Nitrospira, and Proteobacteria. Exposure to nanoMoO₃ increased prokaryotic alpha diversity. Both forms of relatively insoluble NiO decreased soil pH and microbial biomass. Both forms of ZnO (highly soluble in LB) increased soil pH while decreasing basal respiration and prokaryotic alpha diversity. BulkZnO decreased microbial biomass while nanoZnO decreased soil β-glucosidase and β-N-acetylglucosamidase activity and increased the relative abundance of Firmicutes. Soil exposure to nanoLi₂O showed the most significant change in soil pH (+2.83 units) and response. This study provides evidence that ENMs influenced soil function and microbial diversity and composition. However, we found no evidence that changes were caused exclusively by a nano-size effect.