Effects of naturally aged microplastics on arsenic and cadmium accumulation in lettuce: Insights into rhizosphere microecology

Abstract

Naturally aged microplastics (NAMPs) are commonly found in farmland soils contaminated with heavy metals (HMs), such as arsenic (As) and cadmium (Cd); yet their combined effects on soil–plant ecosystems remain poorly understood. In this study, we investigated the toxic effects of NAMPs and As–Cd on lettuce, considering the influence of earthworm activity, and examined changes in As–Cd bioavailability in the rhizosphere. Four experimental systems were established: soil–only, soil–lettuce, soil–earthworms, and soil–lettuce–earthworms systems, with four NAMPs concentrations (0, 0.1, 0.5, 1%). Our results showed that exposure to 0.1% NAMPs reduced As accumulation in lettuce shoots (0.17–0.25 mg kg⁻¹) and roots (1.13–1.72 mg kg⁻¹), while increasing biomass and enhancing root growth by alleviating toxicity. In contrast, the combined stress of higher NAMPs concentration (0.5%/1%) and As–Cd caused a 28.4–58.4% reduction in root activity, which stimulated low-molecular-weight organic acid (LMWOA) secretion in the rhizosphere, increasing the bioavailability of As and Cd and enhancing their absorption by lettuce. Partial least squares path modeling (PLS-PM) revealed that co-exposure altered LMWOA content, soil enzyme activity, and microbial community stability in the rhizosphere, ultimately influencing the bioavailability and uptake of As and Cd by lettuce.