Integrated microbial activities and isotope analysis unveil the effects of zinc oxide nanoparticles on straw decomposition in agricultural soil

Zinc oxide nanoparticles (ZnONPs) are widely applied across multiple industries and ultimately accumulate in water and soil environments, raising significant concern about their toxicity to organisms in various ecosystems. While the effects of ZnONPs on microflora have been reported, their ecotoxicity to specific biogeochemical process and microbial activities and metabolic functions remains relatively unclear. In this study, a 56-day microcosmic experiment was conducted to explore the toxicity mechanism of ZnONPs (1000 mg kg^-1 soil) on straw decomposition, soil organic carbon (SOC) mineralization, and changes in microbial activities and functions in agricultural soil with general wheat straw incorporation using the ¹³C isotope tracer technique. The results demonstrated that straw incorporation increased the rate of CO₂ emission and promoted the straw decomposition. However, the presence of ZnONPs reduced the CO₂ release rates during incubation period although the rates were still higher than those under the control due to straw incorporation. CO₂ emissions from straw decomposition were dominant before the 7th day of incubation. After day 7, CO₂ emissions from the mineralization of original SOC became dominant with their contribution increasing from 17.52 % on day 7 to 60.20 % on day 56 under straw incorporation. ZnONPs affected soil carbon composition and straw decomposition by inhibiting enzyme activity and reducing the abundance of functional genes, indirectly impacting CO₂ release. Community Level Physiological Profiles (CLPP) showed ZnONPs reduced functional richness indices, including Shannon-Weiner index (H) and McIntosh index (U), and altered C substrate utilization patterns. This may be due to the direct toxicity of zinc ion (Zn²⁺) released by ZnONPs to the soil bacterial community. The findings provide insights into the toxicity effects of emerging contaminants on carbon transformation from straw and SOC. Further investigations involving metabolomics are required to reveal the essential effects of ZnONPs on biogeochemical cycle of elements in agricultural soil.