Invasion of Microplastic-Derived DOM Disturbs Soil–Microbe–Plant System through Component-Driven Effects

Microplastic-derived dissolved organic matter poses an emerging threat to terrestrial ecosystems. Here, we investigated the effects of PP-DOM and PS-DOM (at concentrations of 0.1, 1, and 10 mg/L) on Chinese cabbage ( Brassica rapa ), planted soils, and soil microbial communities. PP-DOM at 1 mg/L increased soil TOC, TN, and phosphorus, enriched opportunistic pathogens ( Scedosporium ), and affected iron outer membrane receptor proteins, while 1 mg/L PS-DOM enhanced available nitrogen and nitrogen-transforming bacteria ( Rhodobacter, Luteimonas, and Nitrosospira ). Both of them increased the abundance of three carbon metabolism-related genes ( galR, atoB, fabG ). PP-DOM stimulated plant growth at 0.1 mg/L but inhibited growth at higher concentrations; PS-DOM exhibited the opposite trend. PP-DOM at higher concentrations reduced chlorophyll and suppressed CAT/superoxide dismutase activities, while high-level PS-DOM increased chlorophyll and carotenoids with weaker oxidative damage. Transcriptomic analysis revealed that MP-DOM modulated the plant photosynthetic and amino acid metabolism pathways. Collectively, MP-DOM chemical composition critically shapes soil–microbe–plant ecological outcomes.