Microplastics and decabromodiphenyl ethane co-exposure exerts antagonistic effects on microbial communities in a soil-earthworm system.

The co-occurrence of microplastics (MPs) and decabromodiphenyl ethane (DBDPE) is of increasing concern due to their potential combined environmental impacts. This study examined microbial communities in earthworm epidermis, gut, and soil, alongside soil enzyme activities and nitrogenous substances, under separate and combined exposure to acrylonitrile-butadiene-styrene MPs (ABS-MPs) and DBDPE. MPs alone significantly reduced the microbial diversity in earthworms. At the genus level, MPs and DBDPE oppositely regulated Bacillus, Streptomyces, Rubrobacter, and Ammoniphilus, with combined exposure restoring abundances towards the control. Network analysis showed DBDPE enhanced competitive epidermal interactions (positive edges from 82.7% to 66.7%), whereas MPs simplified gut networks. Both pollutants, especially MPs, promoted gut nitrate reduction and denitrification while suppressing chemoheterotrophy. In soil, DBDPE dominated nitrogen transformations, increasing nitrate reductase and urease activities and elevating NO2--N and NH4+-N, with MPs exerting negligible effects. Under combined exposure, antagonistic effects were observed for several microbial community parameters in earthworms (e.g., alpha diversity, co-occurrence network topology, and genus-level abundance of certain taxa), but DBDPE still dominated soil nitrogen transformations. These findings reveal that ABS-MPs primarily affect earthworm-associated microbiota, whereas DBDPE drives soil nitrogen cycling, and their co-exposure partially offsets each other's negative impacts, highlighting the complexity of risk assessment for co-contaminated sites.