Thresholds and Key Environmental Drivers of Agricultural Film-Derived Microplastic Effects on Soil CO2 Emissions: Transition from Inhibition to Promotion

Microplastics are emerging pollutants that can disrupt soil structure and key biogeochemical processes, such as carbon and nitrogen cycling. However, critical gaps remain in identifying contamination thresholds and understanding how different microplastics affect CO2 emissions through interactions with soil and microbes. To address these gaps, we conducted a microcosm experiment to evaluate CO2 emissions and their key influencing factors in soil amended with four types of agricultural film-derived microplastics (PE, PP, PVC, and PBAT) at varying doses (0.01, 0.10, 1.00, and 5.00%). Findings reveal that at higher doses, PBAT and PVC significantly enhanced soil CO2 emissions and cumulative mineralization, whereas PE and PP exhibited no clear dose–response relationship. Microplastics exhibited a double-edged effect on CO2 emissions, transitioning from inhibition to promotion over time. The zero-point effect stabilized in a short time, with PVC and PBAT being more prone to disrupt soil processes compared to PE and PP, exhibiting dose threshold values of 0.07, 0.09, 2.38, and 2.16%, respectively. Microplastic characteristics, soil physicochemical properties, and microbial communities varied with the types and doses of microplastics added, contributing 34.7, 39.4, and 25.9%, respectively, to the double-edged effect. Furthermore, the type of microplastics (coefficient = 0.67) had a more substantial effect on the double-edged effect than dosage (coefficient = 0.39), highlighting the critical role of the microplastic type in modulating soil carbon dynamics. This study provides new mechanistic insights into how microplastics affect soil carbon cycling, helping predict their impact on climate change mitigation and sustainable soil management.