Controlled-Release Fertilizer Microplastics Can Dynamically Affect the Available State of Cd by Controlling Fe Morphology and Doc Content in Soil

Microplastics (MPs) affect the accumulation of heavy metals by regulating soil environment. However, studies on the dynamic effects of microplastics on the available states of heavy metals in soil are lacking. In order to clarify the dynamic effect of controlled release fertilizer MPs on the effective state of heavy metals and its influencing mechanism. The dynamic effect of polyurethane (PU) MPs on the effective state of cadmium (Cd; DGT-Cd) at different particle sizes (0.5mm, 1mm, 2mm) and different doses (0.1%, 0.5%, 1%) was studied in situ by diffusive gradient in thin-films (DGT) technique for the first time. And the synergistic changes of soil physical and chemical properties (pH, Zeta, DOC, Fe, Mn) and microorganisms. The bioavailability, soil chemical properties, and microbial effects of PU MPs on Cd depend on PU particle size and concentration; only high-concentration (1%) PU MPs cause a significant increase in DGT-Cd concentration. Throughout the culture stage, the changes in DGT-Cd over the first 25 days were drastic, decreasing rapidly, and there were differences in the changes in soils with different levels of Cd contamination after 25 days. Multiple regression analysis combined with path analysis showed that PU MPs directly affected the effective state of Cd by changing soil properties, among which Fe(II) content and DOC were important factors controlling the activation of Cd. Meanwhile, changes in soil properties and heavy metal availability are significantly correlated with microbial community composition, suggesting that PU MPs may indirectly impact heavy metal activity by affecting microorganisms and functional genes associated with C and Fe cycling, thus posing potential risks to the multifunctionality of soil ecosystems. Thus, ecological risk prevention, and measures to control combined MPs-heavy metal pollution in controlled-release fertilizers must be strengthened.