Impact of Microplastics in Biosolids on Carbon Cycling and Food Systems

Microplastics (MPs) are increasingly recognized as persistent, carbon-based contaminants in biosolids produced during wastewater treatment. As biosolids are widely applied to land or disposed of via landfilling and incineration, the incorporation of microplastic-derived carbon into managed and natural ecosystems raises important questions regarding carbon cycling, organic carbon stability, and long-term environmental implications. This review synthesizes current knowledge on the occurrence, characteristics, and fate of microplastics in biosolids, with particular emphasis on their interactions with native organic matter and their influence on carbon-related processes. This work critically assesses how MPs in biosolids influence carbon dynamics, including their role as a persistent carbon pool, interactions with soil organic matter, effects on microbial activity and decomposition, and implications for carbon sequestration and turnover after land application. The review also considers indirect consequences for food systems and human exposure through carbon-associated pathways. Significant knowledge gaps remain regarding the quantification of microplastic-associated carbon stocks and fluxes, transformation processes during biosolid treatment and soil incorporation, and the long-term persistence of this carbon fraction. Methodological challenges in measuring and reporting MPC are briefly highlighted, alongside their implications for understanding MPs as an emerging component of the terrestrial carbon cycle and for sustainable biosolid management.