Soil Texture Mediates the Toxicity of ZnO and Fe₃O₄ Nanoparticles to Microbial Activity

The widespread use of metal oxide nanoparticles (NPs) in industrial and household products has raised concerns about their potential soil contamination and its ecological consequences. The purpose of this study was to examine and compare the effects of iron oxide nanoparticles (FeONPs) and zinc oxide nanoparticles (ZnONPs) on the microbial activity and biochemical properties of differently textured soils. A mesocosm experiment was conducted using three soil types-clay loam (CL), sandy clay loam (SCL), and sandy loam (SL) amended with farmyard manure (FYM), ZnONPs and/or FeONPs. The results revealed significant differences in microbial colony-forming units (CFUs) and carbon dioxide (CO₂) emissions in the order of SL > SCL > CL. Compared with those from the unfertilized control, the CO₂ emissions from the FYM increased by 112%, 184% and 221% for CL, SCL and SL, respectively. The addition of ZnONPs and FeONPs notably increased the microbial biomass Zn/Fe, which reflected their consumption by the soil microbes. As a result, microbial CFUs were considerably reduced, which led to a 24%, 8% and 12% reduction in cumulative CO₂ emissions after the addition of ZnONPs to the CL, SCL and SL soils, respectively. The respective decrements in the case of FeONPs were 19%, 2% and 12%. The temporal dynamics of CO₂ emissions revealed that the CO₂ emissions from CL with or without FYM/NPs did not differ much during the first few days and later became pronounced with time. Almost all the studied chemical characteristics of the soils were not strongly affected by the ZnONPs/FeONPs, except EC, which decreased with the addition of these nanomaterials to the manure-amended soils. Principal component analysis revealed that the ZnONPs and FeONPs are negatively corelated with microbial CFUs, and CO₂ emission, with ZnONPs being more toxic to soil microbes than FeONPs, though their toxicity is strongly influenced by soil texture. Hence, these findings suggest that while both these NPs have the potential to impair microbial activity, their effects are mediated by soil texture.