Abstract

The coexistence of microplastics (MPs) and drought threatens soil health, yet how MP size modulates microbial drought response remains unknown, hindering prediction of their combined impact. This study investigated the individual and combined impacts of drought and MPs of varying polymer types (non-degradable polypropylene, PP vs biodegradable polyhydroxyalkanoates, PHA) and particle sizes on soil bacterial communities. After a 60-day incubation, we analyzed soil properties, enzyme activities, bacterial composition, co-occurrence networks, and metabolomic profiles. Our results demonstrated that drought and MPs synergistically altered soil physicochemical and biological properties. Notably, the effects of MPs were highly dependent on their size, with small-particle-size MPs exerting the most pronounced influence on microbial diversity and network structure, under non-drought conditions, PHA20 treatment reduced the Shannon index by 25%, while under drought conditions, it decreased by 29%. Co-occurrence network analysis revealed that drought and most MPs decreased overall network connectivity, yet the combination of drought and certain MPs (e.g., PHA) enhanced modularity, indicating a potential shift in community stability strategy. Metabolomic profiling further confirmed that small-particle-size MPs induced the most substantial shifts in soil metabolic pathways. Redundancy analysis identified pH, nitrogen components, and specific enzymes as key drivers shaping the microbial community under different treatments. These findings provide critical evidence that MPs particle size is a crucial factor determining the impact of MPs on soil ecosystems under drought conditions, highlighting the need to consider this variability for accurate ecological risk assessment of plastic pollution in a changing climate.