Microplastics as pathway-selective modulators rewire nitrogen cycling in coastal wetlands: Polymer identity outweighs...

Coastal wetlands are globally important nitrogen sinks that mitigate eutrophication via microbial denitrification and anammox. However, the impact of microplastic (MP) pollution on nitrogen transformation pathways remains poorly understood. Here, we conducted a field-based mesocosm experiment across high, middle, and low tidal marshes, exposing sediments to environmentally relevant concentrations (0.1-1.0% w/w) of four common polymers (PE, PP, PS, and PET). We revealed highly significant three-way interactions among tidal elevation, polymer type, and concentration, demonstrating strong context-dependency. Rather than acting as generic stressors, MPs functioned as pathway-selective modulators, fundamentally shifting nitrate fate from gaseous removal (N₂ via denitrification/anammox) toward ammonium retention (NH₄⁺ via DNRA). Random Forest modeling indicated that polymer type exhibited higher predictive importance than concentration. Notably, in the low marsh, intermediate MP concentrations (0.5%) triggered a non-linear threshold response, elevating DNRA contributions from ∼10% to 65-70% of total nitrate reduction. Concurrently, nitrogen removal multifunctionality declined significantly under PS and PP treatments at concentrations of 0.5-1.0%, particularly in the middle marsh. These findings identify PS and PP as priority polymers for mitigation and underscore the necessity of incorporating polymer-specific characteristics into environmental policies to safeguard the nitrogen buffering capacity of coastal wetlands.