Impact of Heavy Metals and Polystyrene Microplastics on the Bacterial Communities in Rhizosphere and Bulk Soil and the Physiological Health of Allium fistulosum

Microplastics (MPs) are emerging as significant global pollutants due to their persistence, versatility, and resilience across diverse ecosystems. However, research focusing on their combined impact with heavy metals (HMs), their relations with soil microbial communities, particularly saprophytes and pathogenic species, and their influence on terrestrial plants and soil physicochemical properties remains scarce. This study aimed to examine the combined effects of HMs (copper, arsenic, zinc, cadmium, and lead) in contaminated soils from Tongling City and polystyrene microplastics (PS-MP) of varying sizes (13 µm, 50 µm, and 106 µm). The research evaluated changes in soil physiochemical properties, bacterial diversity and composition, plant and animal pathogens, saprotrophs in both bulk and rhizosphere soils, as well as the impact on antioxidant activity and the growth of Allium fistulosum . The blend of PS-MPs and heavy metals was lethal for the spring onion. The smallest MP treatment significantly increased soil organic matter, pH, total carbon, electric conductivity, zinc, copper, and cadmium and significantly reduced total nitrogen, ammonia, and nitrate, and also brutally impeded the growth indicators of spring onion including plant height, leaves length, fresh weight of root, dry weight of leaves and root. However, MP did not affect the length, fresh, and dry weight of stem and root length. Small MP treatment also reduced relative water content, and increased antioxidant activity and electrolytic leakage of the spring onion. Treatment with smaller MP at the middle point (at day 20) increased the bacterial diversity as compared to the final point (at day 40). Microplastic also played a crucial role in the reduction of saprotrophs and increased plant and animal pathogens especially in the small MP treatment and in rhizosphere soil. Our findings revealed that the interaction of heavy metals (HMs) with smaller-sized microplastics (MPs) posed greater harm to soil bacterial communities and the growth of spring onion. This study also highlights critical knowledge gaps and underscores the need for further research into the ecological risks associated with PS-MPs and HMs.