Introduction Microplastics are emerging soil contaminants that can alter soil physicochemical properties and consequently influence the mobility, bioavailability, and chemical speciation of heavy metals. Because the environmental behavior of metals is governed primarily by their chemical speciation rather than their total concentrations, understanding the factors controlling metal speciation is essential for predicting their environmental fate and ecological risk. Among these factors, soil pH and dissolved organic carbon (DOC) are the primary regulators of metal speciation, and both microplastics and plant growth-promoting rhizobacteria (PGPR) can modify these properties, thereby influencing the geochemical behavior of metals. Although increasing attention has been paid to the interactions between microplastics and heavy metals and to the role of PGPR in regulating metal behavior in the rhizosphere, most previous studies have focused on the adsorption of metals onto microplastic surfaces, whereas their effects on metal speciation in soil solution remain poorly understood. Therefore, the present study investigated the interactive effects of low-density polyethylene (LDPE), polylactic acid (PLA), and two PGPR strains on the chemical speciation of nickel and lead in the soil solution of a contaminated calcareous soil.
Materials and Methods A greenhouse experiment was conducted as a factorial arrangement in a completely randomized design with three replications. Artificially contaminated calcareous soil containing Pb, Ni, Cd, and Zn was amended with two types of microplastics, low-density polyethylene (LDPE) and polylactic acid (PLA), each applied at two rates (1 and 5%, w/w), and inoculated with two plant growth-promoting rhizobacterial strains, Pseudomonas fluorescens and Azospirillum lipoferum. Maize (Zea mays L.) seeds were sown in the treated soils and grown for 45 days. After harvest, soil pH, electrical conductivity (EC), dissolved organic carbon (DOC), dissolved Pb and Ni concentrations, and the ionic composition of the soil solution were determined.
The chemical speciation of Pb and Ni was predicted using the geochemical equilibrium model Visual MINTEQ based on soil solution chemistry, and the resulting data were statistically analyzed using SAS software.
Results and Discussion The results demonstrated that microplastics significantly altered the chemical speciation of nickel and lead through changes in soil chemical properties, particularly pH and DOC. LDPE decreased soil pH, shifting the speciation equilibrium toward higher proportions of free Ni²⁺ and Pb²⁺ ions and lower proportions of carbonate and hydroxide species. Under the 5% LDPE treatment, the proportion of Ni²⁺ was approximately 53–62% of total dissolved Ni, whereas Pb²⁺ accounted for approximately 11–21% of total dissolved Pb. In contrast, PLA increased both pH and DOC, promoting the formation of carbonate, hydroxide, and dissolved organic matter complexes (NiDOM and PbDOM). These findings are consistent with the well-established role of pH in regulating metal speciation and the contribution of DOC to metal–organic complexation. Furthermore, inoculation with PGPR, particularly Azospirillum lipoferum, further enhanced the formation of organic complexes under PLA treatments by increasing DOC. Analysis of the relative distribution of aqueous species revealed that Ni²⁺ (26–62%) and PbCO₃ (34–57%) were the predominant species of nickel and lead, respectively. Despite the enhanced formation of organic complexes under PLA treatments, nickel remained predominantly in its free ionic form, whereas lead exhibited a much stronger tendency to form organic complexes. In the PLA treatments inoculated with PGPR, the proportion of Ni²⁺ (34.76%) remained higher than that of NiDOM (21.97%), whereas for lead, the proportion of PbDOM (24.29%) was substantially higher than that of Pb²⁺ (1.23%). Overall, LDPE increased the proportion of free ionic species of both metals, suggesting a potential increase in their bioavailability, whereas PLA altered their geochemical behavior primarily by promoting complexation with dissolved organic matter, particularly for Pb. Conclusion The findings of this study demonstrated that microplastic type significantly altered the chemical speciation of nickel and lead in calcareous soil through changes in soil pH and DOC. By lowering soil pH, LDPE increased the proportion of free metal ions, thereby potentially enhancing their bioavailability. In contrast, PLA promoted the formation of metal–organic complexes, particularly for Pb, through increases in both pH and DOC. Furthermore, PGPR amplified the effect of PLA by stimulating DOC accumulation, resulting in greater metal–organic complexation. Overall, these findings highlight that both microplastic type and microbial activity are key drivers of the geochemical behavior and environmental fate of nickel and lead in the soil solution phase.