Environmentally aged microplastics alter archaeal and bacterial roles in ammonia oxidation of intertidal soils

Increasing microplastic pollution in intertidal wetlands poses significant risks to global nitrogen cycling. As microplastics inevitably undergo environmental aging, their impacts on soil physicochemical properties and microbial functions differ from those of virgin materials. However, how environmentally aged microplastics interfere with the rate-limiting ammonia oxidation step remains poorly understood. Here, we conducted a 90-day microcosm experiment using intertidal soils amended with virgin and environmentally aged polypropylene (PP) microplastics at 0.5% and 1.0% (w/w) concentrations, respectively, and explored the potential roles of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in ammonia oxidation rates (PAR) based on a selective inhibitor. Our results showed that high-concentration aged PP microplastics significantly suppressed total PAR after 90 days, with an approximately 31.4% reduction compared to the control treatment. In addition, aged PP microplastics increased the ammonia oxidation rate associated with AOA, whereas they decreased that for AOB. These functional shifts may be linked to changes in soil microenvironments and geochemical properties, where aged PP microplastics decreased pH, increased organic carbon content and the Fe²⁺/Fe³⁺ ratio, as well as sulfide accumulation. Overall, our findings highlight the importance of considering environmentally aged microplastics in studies of nitrogen cycling, as they may substantially alter soil microbial processes and related nitrogen dynamics in intertidal ecosystems.