The widespread use of agricultural plastic films has made micro- and nanoplastics (MNPs) and phthalate esters (PAEs) contaminants of emerging concern in agroecosystems. However, the interactive mechanisms underlying their combined pollution in soil–plant systems remain elusive. To fill this gap, this study investigated the interaction between submicron plastics (SMPs, 0.01% and 0.1% w/w) and di(2-ethylhexyl) phthalate (DEHP) in soil–lettuce systems. Contrary to the anticipated synergistic toxicity, DEHP significantly reduced SMP uptake into and by cracked surface cells of lettuce roots (with root concentration factors decreasing by 19%–64%), i.e., DEHP alleviated SMP-induced oxidative stress, as evidenced by reduced levels of reactive oxygen species (−26.8% and −66.7%) and antioxidant enzyme activities (−118% and −128%). Metabolomic profiling revealed that SMP exposure significantly dysregulated multiple metabolic pathways (amino acid, carbohydrate, energy, glycan, lipid, and nucleotide metabolism), while SMP + DEHP co-exposure selectively attenuated these metabolic disturbances, showing enrichment only in glycan biosynthesis/metabolism and suppressing SMP-induced perturbations in other pathways (biosynthesis of secondary metabolites, energy metabolism, and signal transduction). Microbial community analysis showed that high-level SMP exposure significantly diminished bacterial α-diversity and amplicon sequence variant (ASV) richness, whereas DEHP supplementation enhanced those of Myxococcota in the soil, potentially counterbalancing SMP-induced microbial dysbiosis. These findings collectively demonstrate that co-contamination by MNPs and plastic additives may produce antagonistic interactions rather than uniformly synergistic effects, and provide a more comprehensive evaluation of the risks of PAEs and MNPs to food security, human health, and ecological environment.
