Microplastic‐Induced Shifts in Soil Leachate Chemistry, Redox Response, Biochemistry, and Biofilm Dynamics

Microplastics (MPs) are increasingly recognized as soil contaminants, yet their time‐dependent associations with dissolved organic matter characteristics, redox behavior, biochemical constituents, and biofilm‐associated responses are often investigated separately. This study evaluated these responses within an integrated soil‐exposure framework using particles derived from three commonly used consumer products—surgical masks (predominantly polypropylene, PP), plastic water bottles (polyethylene terephthalate, PET), and cardboard cups (cellulose‐rich composites containing a polymeric barrier layer)—at three nominal loadings (0.1, 0.5, and 2.0 g per 200 g soil), three exposure periods (1, 3, and 7 weeks), and two extraction media (distilled water and phosphate‐buffered saline, PBS). UV–Vis‐derived aromaticity‐related (AR), humification‐related (HI), and spectral slope factor (SF) indices were assessed alongside DTT‐based redox responses, CUPRAC‐based extractable reducing capacity, extractable carbohydrate and protein contents, and crystal‐violet‐based biofilm‐associated responses. The investigated materials exhibited distinct material‐, loading‐, time‐, and extraction‐medium‐dependent patterns across these endpoints. At week 1, several treatments showed relatively elevated HI and SF ratios compared with matched soil controls. These responses generally declined by week 3, whereas selected PET‐bottle‐ and cardboard‐cup‐derived treatments exhibited renewed increases in aromaticity‐ or humification‐related optical indices at week 7, particularly in PBS extracts. DTT‐based redox responses also varied substantially across conditions, although these measurements did not directly quantify DTT consumption rates or ROS formation. Extractable carbohydrate contents and CV‐stainable biofilm‐associated material showed pronounced responses in selected cardboard‐cup and PET‐bottle treatments, whereas protein responses were generally modest. Overall, the findings demonstrate condition‐dependent changes in soil‐extract optical characteristics, redox‐related analytical responses, and extractable biochemical constituents, without establishing a consistent monotonic relationship with particle loading or a causal link between redox changes and biofilm formation.