Microplastics (MPs) are emerging environmental pollutants that potentially affect soil carbon (C) cycling. However, how MPs influence photosynthesized C flow from plants to soil remains unclear, creating large uncertainties in the projection of C dynamics in plant-soil-atmosphere systems. Herein, a ¹³C-labeling technique was applied to unravel the photosynthesized C fluxes from maize to soil under different levels of simulated polyethylene (PE) MPs pollution (0, 1% and 5%) in soil. Resultantly, the 1% and 5% PE MPs treatments significantly decreased the net fixed ¹³C in the maize–soil system. However, the effect of PE MPs on photosynthate–¹³C transfer in the maize–soil system was highly concentration–specific, that is, the addition of 1% PE MPs reduced the transfer of photosynthate–¹³C to the belowground part (−44.6%) and soil (−36.3%), whereas these effects were not observed for 5% PE MPs. PE MPs addition did not change soil moisture and bulk density. The addition of 1% PE MPs increased soil microbial biomass and decreased soil nitrogen (N) availability (inorganic and organic N). The treatment with 5% PE MPs decreased soil microbial community diversity, and altered the microbial community structure. Soil pH, organic N, and aboveground and belowground plant biomass mainly controlled the soil ¹³C content. These findings suggest that plant–soil systems polluted by PE MPs would sequester less photosynthesized C to belowground parts and soil through reducing plant photosynthetic carbon assimilation and transfer. Thus, potential changes in soil C stocks under MPs pollution should be considered for predicting global C dynamics and sustainable agricultural production.

微塑料 (MP) 是新兴的环境污染物，可能会影响土壤碳 (C) 循环。然而，MPs 如何影响从植物到土壤的光合作用 C 流量仍不清楚，这在植物-土壤-大气系统中的 C 动态预测中产生了很大的不确定性。在此，应用¹³ C 标记技术来揭示在土壤中不同水平的模拟聚乙烯 (PE) MPs 污染（0%、1% 和 5%）下从玉米到土壤的光合作用 C 通量。结果，1% 和 5% PE MPs 处理显着降低了玉米-土壤系统中的净固定¹³ C。然而，PE MPs 对光合产物的影响 – ¹³玉米-土壤系统中的 C 转移具有高度浓度特异性，即添加 1% PE MPs 减少了光合产物 - ¹³ C 向地下部分 (-44.6%) 和土壤 (-36.3%) 的转移，而对于 5% PE MP，未观察到这些影响。添加 PE MPs 不会改变土壤水分和容重。添加 1% PE MP 可增加土壤微生物生物量并降低土壤氮 (N) 可用性（无机和有机 N）。5% PE MPs 处理降低了土壤微生物群落多样性，改变了微生物群落结构。土壤pH值、有机氮、地上和地下植物生物量主要控制土壤¹³C内容。这些发现表明，被 PE MP 污染的植物-土壤系统会通过减少植物光合作用的碳同化和转移，将较少的光合作用 C 螯合到地下部分和土壤。因此，在预测全球碳动态和可持续农业生产时，应考虑 MPs 污染下土壤碳库的潜在变化。