A Double-Edged Sword of Biodegradable Microplastics on the Soil Microbial Carbon Pump

Understanding how microplastic pollution alters soil microbial communities and their influence on soil carbon processes is crucial to elucidating the associated pollutant-climate change feedback. However, previous studies have mainly focused on the effects of microplastics on soil carbon loss, and whether and how microplastics mediate microbial anabolism and subsequent contributions to soil carbon are largely unexplored. Here, we evaluated how conventional and biodegradable microplastic exposure simultaneously affects carbon mineralization and the accumulation of microbial necromass carbon in two types of soil with distinct nutrient levels. We found that biodegradable microplastics significantly increased soil CO₂ emissions, likely due to the resulting greater resource availability and increased microbial activity. By contrast, biodegradable microplastics increased microbial carbon use efficiency and promoted microbial community turnover, further increasing the accumulation of microbial necromass in mineral-associated organic matter and bulk soil carbon. Additionally, the microplastic-induced increase in soil CO₂ emissions and necromass carbon accrual was more apparent in infertile soil. Jointly, our results highlight the dual roles of microplastics on soil carbon release and stabilization.