Warming Modulates Microplastic Impacts on Coastal Nitrogen Cycling by Synergistically Amplifying Sediment Hypoxia and Restructuring the Denitrifying Microbiome

Global warming and microplastics (MPs) pollution are emerging stressors that threaten coastal ecosystems, yet their combined impacts on biogeochemical cycles remain poorly resolved. Here, we integrated a factorial microcosm experiment with stable isotope tracing and molecular techniques to disentangle how warming and MPs jointly regulate nitrogen (N) cycling in coastal sediments. We demonstrate that warming and MPs interacted nonadditively to reshape nitrification, denitrification, and associated nitrous oxide (N₂O) production dynamics. Warming reversed the stimulatory effect of polyethylene (PE) on nitrification, turning it inhibitory, and amplified the suppressive impact of poly(butylene adipate-<i>co</i>-terephthalate) (PBAT), primarily through synergistic intensification of anoxic stress. In contrast, warming strengthened PE-driven stimulation of denitrification and mitigated PBAT-induced inhibition, likely due to the selective enrichment of <i>nirS</i>- and <i>nosZ</i>-harboring denitrifiers. Moreover, warming overturned the stimulatory effects of both PE and PBAT on N₂O production, shifting toward inhibition through nitrifier denitrification, as substantiated by dual-isotope (¹⁵N-¹⁸O) tracing and genomic evidence. Collectively, these findings provide novel mechanistic insights into how warming interacts with MPs to reconfigure sedimentary N cycling, with broad implications for predicting the responses and evolution of coastal ecosystems under accelerating global change.