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Co-occurrence of soil microplastics and heavy metals in low-Cd maize remediation fields: Implications for nutrient and ecological risk of microplastics in farmland
Summary
Farms in China using special low-cadmium corn and soil treatments to clean up heavy metal pollution have an unexpected side effect: tiny plastic bits (microplastics) are building up in the soil, likely from the fertilizers and farming products being used. While most fields showed low-to-moderate risk from these microplastics, some sites were creeping toward higher risk levels—meaning the very techniques designed to make farmland safer could be introducing a different kind of contamination worth watching, since microplastics in soil can end up in crops and eventually our food.
Heavy metal-contaminated farmland is increasingly being remediated through low-accumulation crop varieties and soil amendments, yet the co-occurrence of soil microplastics (MPs) and heavy metals under these management practices remains poorly understood. Here, we investigated 84 field sites within a long-term heavy metal remediation project in Malong County, Yunnan Province (China), where low-Cd maize varieties and amendments of microbial inoculant, organic fertilizer, foliar fertilizer, soil conditioner and biochar have been deployed. Soil MPs were ubiquitous, with abundances ranging from 60 to 560 items kg⁻¹, dominated by fibrous and fragmental particles < 2 mm and polymers such as PE, PP, PET and ABS, most sites showed no or medium MP ecological risk, but Zhuoyu 299 and Zhuoyu 6009 planting area approached high-risk levels. Cd, Hg and As were the main heavy metals exceeding the national farmland quality guideline. The remedial measures have affected the cadC gene related to heavy metal resistance, altering the Hg content; and they have impacted the copA gene, changing the contents of Cd and Zn. Microbial inoculants, organic fertilizers and foliar fertilizers reduced the available levels of Cd and Hg through non-covalent interactions/affinity adsorption, and due to the defects in the production process, MPs entered the soil, whereas highly alkaline biochar and soil conditioner increased the exchangeable pools of Cd and As under acidic soil conditions. This study provides the first field-scale evidence of MPs-heavy metals coexistence in low-Cd maize remediation systems and demonstrates that both crop variety and amendment choice shape soil nutrient–pollutant interactions and MPs risk in industrial crop production landscapes.