Microplastics reduce soil CO₂ emissions and enhance soil organic carbon preservation by promoting aggregate formation...

Microplastics (MPs) have been shown to produce a significant effect on soil ecology. The effects of MPs on soil organic carbon (SOC) mineralization and preservation, however, remain poorly understood, especially in systems that also contain biochar. We conducted a 100-day indoor soil incubation experiment to address this knowledge gap. Our results exhibit that the addition of reed straw biochar (RBC) alone led to a significant increase in soil CO₂ emissions (4.36%) relative to the control. In comparison, co-adding either polylactic acid (PLA) or polyethylene (PE) MPs with RBC pronouncedly inhibited CO₂ emissions (43.05% or 50.98%, respectively), with PE-MPs exhibiting the stronger inhibitory effect. When compared to RBC alone, the co-addition of PLA-MPs or PE-MPs improved the physical protection of SOC in larger soil aggregates. These treatments, especially with PE-MPs, also inhibited organic nitrogen conversion and reduced both bacterial diversity and the activity of enzymes related to SOC decomposition. Structural equation modeling and correlation analysis confirmed that these processes are the likely primary mechanisms for the reduced SOC mineralization observed in the MPs co-addition treatments. The significantly higher CO₂ emissions from PLA-MPs compared to PE-MPs are consistent with their degradability and positive effect on r-strategist microbes. Finally, our results clearly demonstrate that the co-addition of both types of MP with RBC significantly impacted on SOC stabilization pathways, e.g., increasing the proportion of SOC protected within soil aggregates while decreasing the amount preserved in mineral-organic associated matters. This shift may alter the long-term stability of the SOC stock, as a smaller fraction was stored in the more persistent mineral-associated form. The weakened stability of SOC may lead to a reduced resilience of SOC in response to changes in the external environment, which in turn may result in an acceleration of carbon loss in some cases.