Polyethylene microplastics-biochar coexistence induces a positive priming effect on native soil organic carbon mineralization while enhancing glucose retention.

The impacts of microplastics (MPs) and biochar on root exudate-induced priming effect on native soil organic carbon mineralization (PE-NSOC) and on root exudate mineralization remain poorly understood. Here, 13C stable isotope labeling combined with biochar-specific biomarkers (benzene polycarboxylic acids, BPCAs) was used to partition CO2 emissions from NSOC, glucose (root exudate representative), and biochar. Polyethylene MPs (PE-MPs) exerted limited effects on glucose-induced PE-NSOC, which remained negative, similar to glucose alone. Conversely, corn straw biochar (CSB) markedly reversed PE-NSOC from negative to positive, likely through early-stage nutrient limitation followed by late-stage microbial stimulation. Importantly, pyrolysis temperature dictated the direction of MP-biochar interactions: PE-MPs further enhanced positive PE-NSOC induced by low-temperature CSB, but weakened it when combined with high-temperature CSB. All treatments suppressed glucose mineralization, with the strongest inhibition observed for PE-MPs combined with high-temperature CSB, probably due to the enhanced adsorption, bacterial diversity, and nitrogen-acquisition enzyme activities (e.g., leucine aminopeptidase). Carbon balance analysis revealed net SOC gains in most treatments, except PE-MPs combined with low-temperature CSB, with the greatest increase occurring under glucose plus PE-MPs. These findings highlight the importance of biochar pyrolysis temperature in regulating MP-biochar interactions and provide guidance for biochar-based carbon management in MP-contaminated soils.