Microplastics pollution amplifies nitrogen enrichment risk in lakes across submerged macrophytes' survival status

Nitrogen removal mediated by microbial processes in lake sediments is critical for mitigating eutrophication. Submerged macrophytes regulate the nitrogen cycle, but how their survival status affects sediment nitrogen dynamics under microplastics (MPs) pollution remains unclear. Here, we established a series of mesocosms to investigates the synergistic effects of macrophytes' survival status (phytomass vs. necromass) and MPs concentration on nitrogen transformation and removal. Mesocosm experiments revealed that necromass significantly increased porewater TN, NH₄⁺ and NO₃⁻ via mineralization, yet paradoxically enhanced all nitrogen-removal potential rates (Anammox, Denitrification, N-DAMO), highlighting its dual role as both a nitrogen source and a removal promoter. MPs further elevated TN loading in necromass systems, but had a limited effect in phytomass systems. Notably, all nitrogen-removal potential rates (Anammox, Denitrification, N-DAMO) were significantly higher in the necromass systems. However, MPs inhibited these nitrogen-removal potential rates to varying degrees, primarily by impairing biochemical processes rather than altering microbial abundance, a direct mechanistic insight beyond previous community perspectives. In summary, although necromass decomposition elevated internal nitrogen loads, it also enhanced the potential for nitrogen removal-a process highly vulnerable to inhibition by MPs. We propose that strategic management of submerged macrophyte distribution and timely harvesting of senescent biomass before decay is essential to minimize nitrogen release, especially in MPs-polluted lakes. This study elucidates a synergistic mechanism between macrophyte's survival status and MPs pollution in regulating sedimentary nitrogen cycling, offering novel insights into the interactive impacts of plant viability and emerging pollutants on lake nitrogen cycling.