Polyethylene microplastics can attenuate soil carbon sequestration by reducing plant photosynthetic carbon assimilation and transfer: evidence from a 13C-labeling mesocosm study

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.