Microplastic Mixture Diversity Destabilizes Mineral-Associated Carbon via Constraining the Accumulation of Microbial Necromass

Microplastics are accumulating in terrestrial ecosystems as complex and diverse mixtures, posing a potential threat to soil carbon stocks. However, previous studies primarily focused on an individual microplastic; the ecological risks of diverse microplastics on soil carbon sequestration are still unclear. Here, soils were exposed to diverse microplastics (microplastic diversity, number of polymer types =1, 2, 4, 8, and 12) to assess the consequences of increasing microplastic diversity on soil stable carbon pools. Our results showed multiple microplastics reduced microbial necromass carbon (MNC) and mineral-associated organic carbon (MAOC) by 3.5-9.2% and 4.2-11.4%, respectively, when compared to individual microplastics. Microplastic diversity decreased the microbial carbon uptake and necromass accumulation coefficient (NAC), both of which revealed the low efficiency of necromass formation. Furthermore, increased microplastic diversity induced serious carbon and nitrogen imbalances and great microbial nitrogen demand, promoting microbial reutilization of necromass as a N source. The structural equation model identified bacterial community and microbial physiological properties as primary factors regulating MNC, thereby decreasing this precursor contribution to MAOC pools. Our results emphasized that increasing microplastic diversity can hinder the accumulation of microbial necromass and destabilize soil stable carbon, which may bring ecological risks in mitigating climate change.