The influence of microplastics (MPs) on the availability of soil heavy metals (HMs) is a current research hotspot, but how MPs regulate HM availability via soil properties and the bacterial community remains unclear. This study investigated the effects of polyethylene (PE) MP concentrations on soil properties, bacterial communities, surface chemistry, and the speciation of cadmium (Cd) and lead (Pb) through soil incubation. Results indicated that as PE MP concentration increased, soil pH and cation exchange capacity declined, while organic carbon concentration increased. Available phosphorus and alkali–hydrolyzable nitrogen concentrations increased at 0.1% and 1% PE MPs, but decreased at 10% PE MPs. Bacterial community indices, including Simpson, ACE, and Chao1, increased at 0.1% and 1% PE MPs but decreased at 10% PE MPs. PE MPs (0.1% and 1%) reduced DTPA–Cd/Pb, promoting their transformation into stable forms and surface complexation with oxygen–containing groups. In contrast, 10% PE MPs disrupted the formation of PbO, PbCO₃, and Cd(OH)₂, producing the opposite effect. The random forest model revealed that soil organic carbon and available phosphorus were the primary factors influencing DTPA–Pb and DTPA–Cd, respectively. Partial least squares path modeling demonstrated that PE MPs altered the physicochemical characteristics of soil and structure of bacterial communities, ultimately impacting transformation of Cd and Pb speciation, with these changes being highly dependent on PE MP concentration.

Although selenium (Se) application is known to reduce arsenic (As) accumulation in rice, the mechanisms by which Se alleviates As toxicity-particularly those involving the regulation of rhizosphere reactive oxygen species and influence of Fe-(hydro)oxide-As dynamics at the root-soil interface-remain unclear. Therefore, to clarify the mechanisms underlying the inhibition of Se-mediated As uptake, this study investigated how different Se dosages (0, 50, 200, and 400 μg Na₂SeO₃ kg⁻¹ soil) affect antioxidant enzyme activities and phytochelatin (PC) biosynthesis in rice grown in As-contaminated soil and promote As transformation in the rhizosphere. The results showed that the Se-200 and Se-400 treatments significantly reduced As levels in brown rice by 52.4 % and 35.7 %, respectively, compared with the control. The Se-200 treatment significantly enhanced root superoxide dismutase and catalase activity and PC content (P < 0.05) and increased the •OH and H₂O₂ concentrations in the rhizosphere (P < 0.05). In addition, Se promoted the formation of Fe-(hydro)oxides and oxidation of As(III) to As(V), with the As(III)/As(total) ratio increasing by 18.9 %, which decreased As mobility in the rhizosphere. Microbial analysis further revealed a 91.4 % increase in the abundance of the aioA gene (encoding As(III) oxidase) under the Se-200 treatment, with Bacillus and Pseudomonas identified as the dominant genera. Collectively, the Se-200 treatment mitigated As-induced oxidative stress in rice roots, enhanced As(V) immobilisation via Fe-(hydro)oxides, and altered the rhizosphere microbial community to reduce As bioavailability. These findings provide new insights into Se-based strategies for managing As-contaminated paddy soils.