Interactive effects of microplastics and cadmium on soil properties, microbial communities and bok choy growth

The simultaneous presence of microplastics (MPs) and cadmium (Cd) in soil environments has raised concerns regarding their potential interactive effects on soil-plant ecosystems. This study explores how polyethylene (PE) at concentrations of 0.5 % (w/w), 1 % (w/w), and 2 % (w/w), and Cd at concentrations of 3 mg kg^-1 and 12 mg kg^-1, either alone or combined, impact soil physicochemical properties, microbial community structures, and bok choy growth through a 40-day pot experiment. Our findings reveal that the addition of 2 % (w/w) PE significantly increased soil organic carbon (SOC). However, when 2 % PE coexisted with Cd, SOC levels decreased, potentially due to a reduction in enzyme activity (β-1,4-glucosidase). PE increased the proportion of 1-2 mm soil aggregates, while the coexistence of 2 % PE and Cd significantly increased the content of soil aggregates larger than 2 mm. The coexistence of PE and Cd increased available potassium (AK) in the soil by approximately 13 % to 41 %. Regarding bok choy growth, the aboveground biomass under 2 % PE was approximately 210 % of that under 0.5 % PE, possibly because of the enhancement in soil nutrients. The presence of Cd, however, reduced the chlorophyll content of bok choy by approximately 18 % to 34 %. Notably, the coexistence of high PE concentration (2 % w/w) and low Cd concentration (3 mg kg^-1) resulted in the highest aboveground biomass among all coexistence treatments. Furthermore, the addition of PE and Cd significantly altered the structure of soil bacterial and fungal communities, with fungi showing a greater response. Bacteria were significantly associated with soil inorganic N content and plant growth. This study provides new insights into the interactions of microplastics and Cd within microbial-soil-plant systems.