Plant community responses to polypropylene microplastic and cadmium co-exposure: Implications for mycorrhizal strategies in a coastal wetland

The co-occurrence of microplastics and heavy metals, particularly cadmium (Cd), in terrestrial ecosystems poses a growing ecological risk, yet their combined effects on plant community functioning remain unclear. We conducted a full-factorial mesocosm experiment with four polypropylene microplastic levels (0%, 0.1%, 0.5%, and 1% w/w) and two Cd treatments (0 and 10 mg·kg⁻¹) to assess species-specific and community-level responses. Measurements of soil properties, community composition, root traits, and productivity revealed that microplastic-Cd co-exposure consistently reduced community productivity, primarily through suppression of arbuscular mycorrhizal (AM) plant dominance. Root trait analyses indicated diminished intrinsic nutrient acquisition capacity, leading to greater dependence on AM symbiosis and narrowing the Levins' ecological niche breadth of AM-associated species. Structural equation modeling identified community mycorrhization as the key mediator of productivity loss, while random forest analysis ranked the mycorrhizal index (determined by community-level mycorrhization) as the strongest predictor. Altered soil C:N:P stoichiometry and ionic conditions further emerged as critical environmental drivers constraining AM plants under co-exposure. Collectively, these findings demonstrate that microplastic-Cd interactions destabilize plant-soil symbioses and weaken community productivity by undermining AM plant dominance, underscoring the vulnerability of AM-dominated communities and the importance of integrating symbiotic strategies into ecological risk assessments.