Effect of flumetsulam alone and coexistence with polyethylene microplastics on soil microbial carbon and nitrogen cycles: Elucidation of bacterial community structure, functional gene expression, and enzyme activity.

Flumetsulam (FLU) is a new class of broad-spectrum herbicides. With the widespread use of plastic products, polyethylene (PE) microplastics (MPs) may remain in the soil. It is possible for these two novel contaminants to co-exist in the soil environment. In the present study, we used brown soil as the test soil and determined the toxicity of FLU at 0.05, 0.5 and 2.5 mg kg-1 alone and in combination with PE MPs (1%) on soil microorganisms. The obtained results demonstrated that the exposure of FLU and FLU+MPs had an inhibitory effect on the numbers of bacteria and fungi. In addition, FLU and FLU+MPs caused changes in the relevant functional bacterial genera, favored nitrogen fixation and denitrification, and promoted soil carbon fixation, but inhibited nitrification. Compared to FLU exposure alone, exposure to FLU+MPs gave rise to significant differences in soil bacterial community composition, but did not affect carbon and nitrogen cycling. The integrated biomarker response results indicated that the toxicity of FLU and FLU+MPs to soil microorganisms increased with increasing concentrations of FLU. The present experiment clarified the toxicological effects of co-exposure of FLU and MPs on microorganisms and filled the toxicological data gap of FLU.