Despite plastics providing great benefits to our daily life, plastics accumulating in the environment, especially microplastics (MPs; defined as particles <5 mm), can lead to a range of problems and potential loss of ecosystem services. Current research has demonstrated the significant impact of MPs on aquatic systems, but little is known about their effects on the terrestrial environment, especially within agroecosystems. Hereby, we investigated the effect of MPs type and amount on plant growth, soil microorganisms, and photoassimilate carbon (C) allocation. MPs had a negative, dose-dependent impact on plant growth affecting both above- and below-ground productivity (−22.9% and −8.4%). MPs also influenced assimilated 14C allocation in soil (+70.6%) and CO2 emission (+43.9%). Although the activity of β-glucosidase was suppressed by MPs, other C- and N-cycling related enzyme activities were not affected. The type and amount of MPs in soil greatly altered C flow through the plant-soil system, highlighting that MPs negatively affect a range of C-dependent soil functions. Moreover, MPs increased the soil microbial biomass (+43.6%; indicated by PLFAs), and changed the structure and metabolic status of the microbial community. The evidence presented here suggests that MPs can have a significant impact on key pools and fluxes within the terrestrial C cycle with the response being both dose-dependent and MPs specific. We conclude that MPs in soil are not benign and therefore every step should be made to minimise their entry into the soil ecosystem and potential to transfer into the food chain.