Drought Amplifies Degradable Microplastic Diversity Effects on Soil Bacterial and Viral Ecology

Microplastic (MP) contamination and drought are pervasive global stressors threatening soil ecosystem stability. Yet, the combined effects of MP diversity and drought on soil microbial and viral ecology remain largely unexplored. Here, we conducted a controlled microcosm experiment to examine how increasing MP diversity (0, 1, 3, and 5 types) influences soil bacterial and viral communities, biogeochemical cycling, and ecological risk under drought stress. Degradable MPs exerted stronger effects than nondegradable MPs, altering microbial composition and functional gene profiles. Compared to adequate moisture, drought significantly altered the composition of bacterial and viral communities, enhanced the abundance of functional genes related to carbon and nitrogen fixation, and elevated the prevalence of antibiotic resistance genes (ARGs) and virulence factor genes (VFGs) as the diversity of degradable MPs increased. In response to the increasing diversity of degradable MPs under drought, viral communities exhibited an increased abundance of auxiliary metabolic genes (AMGs) and a higher prevalence of lysogenic lifestyles as an adaptive strategy to environmental stress. Rhizobacter, a key host lacking annotated antiviral defense systems, carried abundant ARGs and VFGs and showed strong positive associations with viral abundance, which suggests it may serve as a crucial hotspot for horizontal gene transfer. These findings reveal that increasing diversity of degradable MPs under drought altered microbial composition, potentially accelerated nutrient turnover, and amplified ecological risks, emphasizing the need to consider multistressor interactions in environmental risk assessments.