High-Dose Biochar Hinders Micro/Nanoplastic-Induced Soil Positive Priming by Reducing Substrate Quality and Microbial Activity

Micro/nanoplastics are increasingly introduced into croplands via agricultural inputs such as mulching films and may accelerate soil organic carbon (SOC) turnover through priming effects. However, how long-term soil management practices influence these priming effects, and thus their implications for cropland carbon sequestration, remains unclear. Here, we conducted a 70-day incubation by adding polyethylene micro/nanoplastics at environmentally relevant concentrations (0.1, 0.5, and 1% w/w) to soils that had received biochar or straw amendments for 14 years. Using δ 13 C source partitioning, we found that micro/nanoplastics induced positive priming in control and low-dose biochar soils, driven by dilution from micro/nanoplastic-leached dissolved organic carbon (DOC), which increased bulk DOC and reduced aromaticity. These changes increased microbial biomass, C- and N-acquiring enzyme activities, intensifying nitrogen mining and SOC mineralization. Conversely, negative priming occurred in high-dose biochar soils and in straw-amended soils at the 0.1% micro/nanoplastic rate, where micro/nanoplastic addition reduced bulk DOC and increased aromaticity, likely via preferential sorption of low-aromatic soil DOC onto micro/nanoplastic surfaces. These changes reduced microbial biomass and enzyme activities while promoting the microbial preferential utilization of micro/nanoplastic-leached carbon, thereby favoring SOC preservation. Overall, this study demonstrates that high-dose biochar hinders micro/nanoplastic-induced positive priming by regulating substrate–microbial interactions, with important implications for cropland carbon retention.