Review of soil carbon cycling processes involving microplastics

Soil organic carbon (SOC) sequestration is a pivotal process that drives soil fertility evolution and carbon (C) balance in terrestrial ecosystems. Microplastics, due to their persistence and ecological risks in soil ecosystems, are increasingly gaining attention for their impacts on soil properties and processes. However, the influence of microplastics on SOC cycling remains uncertain. Microplastics can indirectly affect SOC sequestration and mineralization by influencing soil physical and chemical properties. Moreover, they can directly participate in the soil C cycle, further complicating our understanding of the SOC cycle. In light of this context, this study outlines the development of SOC sequestration pathways, summarizes the sources of soil microplastics, explains how microplastics affect different soil C pools, and delves into the potential mechanisms through which microplastics regulate the soil C cycle. Finally, we provide an overview and prospects for research on the SOC cycle and microplastics. Our findings revealed that microplastics impact SOC balance by influencing the formation and degradation of soil physical structures, microbial community structure diversity, enzyme activity and functional genes, biofilm formation, animal reproduction and growth, plant growth, and root deposition. Additionally, microplastics directly participate in the SOC cycle as part of the broader soil biogeochemical cycle. However, it is important to note that research in this area is still in its early stages, and differentiating and linking microplastic turnover with the SOC cycle using scientific methods presents challenges. Future research should aim to distinguish the potential contribution of microplastic-derived C in various SOC pools, explore the mechanisms through which microplastics directly and indirectly affect the SOC cycle, and examine the role of microplastics in the SOC cycle under the influence of multiple factors. This will require the integration of advanced soil study methods as well as innovative interdisciplinary approaches.