Microplastics drive both linear and threshold-type shifts in soil multifunctionality along concentration gradients

Microplastics are increasingly recognized as emerging contaminants in terrestrial ecosystems, yet their mechanistic impacts on soil multifunctionality remain poorly understood. Here, we evaluated the influence of two microplastic polymers, polyethylene terephthalate and polypropylene, on soil functioning by subjecting soils to a gradient of concentrations of these microplastics, and measuring six variables representing soil physical, chemical, and biological functions. A statistical framework combining multi-model inference with threshold detection and machine learning was implemented in this study to identify the main pathways of soil multifunctional change. Most significant responses followed nonlinear trends and threshold shifts, primarily in physical properties, indicating that microplastic stress first impacts soil structure before cascading to chemical and biological processes. We identified two system-level thresholds at 0.3% PP and 0.55% PET w/w; while random forest highlighted water-stable aggregates as the dominant predictor of overall soil multifunctionality. Our findings provide new quantitative evidence of complex soil multifunctionality responses to microplastic pollution. Most importantly, physical deterioration emerged as an early-warning signal of microplastic disturbance, thereby advancing our understanding of microplastic pollution on soil systems.