Polymer type more strongly than concentration drives root responses to microplastics: root biomass-efficiency...

Coastal wetlands, critical for global carbon sequestration and nitrogen removal, face escalating threats from microplastics (MPs) pollution. Yet, whether MPs effects are governed primarily by concentration or polymer type remains unresolved, impeding risk assessment accuracy. Here, through a mesocosm experiment with Scirpus mariqueter, we demonstrate that polymer type more strongly than concentration shapes root morphological and stoichiometric responses to four globally prevalent MP polymers (PP, PET, PS, PE), whereas effects on soil biogeochemistry are more complex and often interactive with concentration. MPs induced a morphological coping strategy characterized by a biomass-efficiency trade-off in roots: despite significant reductions in root biomass (-22.4 % to -35.0 %) and root-to-shoot ratio (-11.1 % to -36.1 %), plants dramatically increased root efficiency traits, including root length (+31.3-43.7 %), root surface area (+30.3 %), specific root length (+67.6-186 %), and specific root surface area (+79.8 %). Concurrently, root nitrogen (-13.5-29.7 %) and phosphorus (-35.9 %) contents declined, elevating C:N (up to +35.8 %) and C:P ratios (up to +105.3 %). Crucially, microplastic polymer types generated antagonistic soil effects: PP elevated soil total carbon (+7.5 %), whereas PE amplified root carbon (+10.0 %); all polymers depleted soil total nitrogen (-29.5 to -36.9 % at 1 %) and tended to shift inorganic N toward nitrate accumulation, particularly under PET, PE, and PS (+8.3-12.3 %). Random forest models showed that root responses were primarily associated with the water-salt balance-soil water content, electrical conductivity, salinity, and pH-and with inorganic nitrogen availability (NO₃^--N and NH₄⁺-N), indicating that key edaphic conditions mediate the effects of microplastics on root traits. Our findings refine microplastic risk paradigms: polymer specificity, rather than dosage alone, primarily controls wetland plant adaptation, while biogeochemical functions respond to polymer identity through more complex and often dose-dependent pathways, demanding polymer-specific management frameworks for preserving these vital ecosystems.