Long-Term Microplastic Accumulation Is Associated with Element-Specific Heavy Metal Mobilization in Agricultural Soils: Field Evidence from the Anning Valley, China

Microplastics (MPs) are increasingly recognized as emerging contaminants, making a systematic understanding of the co-occurrence patterns between MPs and conventional heavy metals (HMs) essential for addressing composite environmental risks. This study investigated MP abundance, morphology, and polymer composition, together with HM concentrations and chemical speciation in greenhouse farmlands across the Anning Valley, China, spanning different cultivation histories (1–8 years) and soil depths (0–40 cm). The results revealed that longer cultivation histories were accompanied by a marked accumulation of MPs, characterized by pronounced topsoil enrichment. MPs abundance increased from 758.53 n/kg in newly cultivated plots to 3373.19 n/kg in long-term plots, with topsoil level being 2.6 times higher than those in deeper layers (2462.25 vs. 951.25 n/kg). Under long-term cultivation, heavy metal contamination escalated, and contamination hotspots shifted from deeper soil to the MP-rich topsoil. In topsoil, HM also exhibited a higher proportion of potentially mobile fractions, reflecting greater potential mobility and bioavailability. Statistical associations between MPs and HMs co-varied with soil pH, organic matter, and coarse soil fractions, indicating complex inter-relationships influenced by shared agricultural management practices. This association weakened with soil depth and varied among elements, with Cd showing the strongest relationship with MPs and becoming the dominant contributor to HM risks. These findings demonstrate that farmland MP–HM co-contamination is spatially heterogeneous and metal-specific, providing field-scale evidence for improving ecological risk assessment and soil quality management under long-term plastic use.