Environmental aging alters microplastic effects on soil DNRA in estuarine and coastal wetlands

Increased microplastic pollution in estuarine and coastal wetlands threatens the balance of the global nitrogen cycle. As microplastics (MPs) inevitably undergo environmental aging, their impacts on soil physicochemical properties and microbial functions differ fundamentally from those of virgin materials. However, how environmentally aged MPs interfere with dissimilatory nitrate reduction to ammonium (DNRA), a critical pathway for nitrogen retention, remains poorly understood. Here, a 90-day microcosm experiment was conducted to investigate the response of DNRA in intertidal soils amended with either virgin or environmentally aged polypropylene (PP) MPs at concentrations of 0.5% and 1.0% (w/w). Our results showed that aged PP MPs significantly enhanced DNRA rates compared to the control treatment, with the low concentration (0.5%) boosting DNRA by 60% after 90 days, whereas virgin PP MPs exhibited negligible effects. Notably, this stimulation exhibited a non-linear pattern, as the high concentration (1.0%) aged PP MPs was less effective than the low concentration (0.5%). Sulfide, total organic carbon (TOC), and nitrate (NO₃^-), rather than functional gene abundance, were the primary influencing factors of the DNRA process. Aging-induced changes in soil physicochemical properties profoundly altered the effect of PP MPs on DNRA. Overall, this work provides new insights into the role of aged MPs in shaping nitrogen dynamics within estuarine and coastal ecosystems and underscores the necessity of incorporating aging effects in future ecological risk assessments of long-term MPs pollution.