Synergistic Effects of Biodegradable Microplastics and Herbicide on Plant Growth and Soil Carbon Processes in Karst Agricultural Ecosystems

The coexistence of biodegradable microplastics (BMPs) and herbicides poses emerging risks in vulnerable karst agroecosystems. This study investigated the individual and combined effects of polyhydroxybutyrate (PHB) BMPs and the herbicide nicosulfuron on maize growth, litter decomposition, and soil carbon cycling in two typical soils (yellow soil and limestone soil) in karst regions. Combined pollution synergistically inhibited plant growth, reducing biomass by 50% in yellow soil and 76% in limestone soil, while inducing greater oxidative damage in yellow soil. In limestone soil, combined treatment accelerated litter decomposition (80.5% mass loss) and significantly upregulated carbon cycle functional genes (e.g., aclB by 21.3 times). This study reveals the synergistic ecological risks of combined pollution of BMPs and herbicides in karst farmland, and emphasizes that soil type is a key factor determining the direction and intensity of the combined effects. Therefore, it is necessary to conduct risk assessments for specific soil types.