Conventional and biodegradable microplastics affected arsenic mobility and methylation in paddy soils through distinct chemical-microbial pathways.

The presence of microplastics (MPs) in paddy soil has become a growing concern, yet the influence of MPs on arsenic (As) dynamics in paddy soil remains largely unexplored. A 98-day microcosm experiment was conducted to investigate the impact of MPs on As behavior in As-contaminated paddy soil. The results revealed that conventional microplastics (CMPs) reduced As concentration in porewater by 25-38 %, but substantially increased the percentage of methylated As (% MeAs) in soil by 8-23 times under 5 % dosages after 98-day incubation. In contrast, at the end of incubation, biodegradable microplastics (BMPs) at 5 % dosages notably increased As concentration in porewater and % MeAs in soil by 2-9 times and 11-395 times, respectively. The combination of network analysis and Random-Forest analysis implied that CMPs might inhibit As mobility through enhancing microbial As(III) oxidation and promote As methylation by enriching arsM-carrying microbes. However, BMPs promoted As release mainly accompanying with microbial iron reduction, and enhanced As methylation through enriching fermenting bacteria (i.e., Clostridiaceae) and arsM-carrying organic matter degrading bacteria (i.e., Gemmatimonas and Nocardia). These findings might provide broaden insights into As cycling induced by MPs and contribute to the prevention of combined pollution from As and MPs in paddy soil.