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Responses of Pb speciation and bioavailability in loessial soil (sierozem) to the presence of polystyrene microplastics with different size and abundance
Summary
A 60-day soil incubation experiment showed that polystyrene microplastics alter soil pH, microbial community structure, and enzyme activity in ways that shift lead (Pb) from a stable residual state into more bioavailable forms. This means microplastics don't just contaminate soil themselves — they amplify the toxicity of co-occurring heavy metals, compounding risks to agriculture and human health.
The interaction between microplastics (MPs) and heavy metals (HMs) has garnered significant attention. However, the mechanism by which MPs influence the physicochemical properties of soil and microbial community structure, thereby driving the transformation of HMs speciation, remains unclear. This study investigated the effects of polystyrene-MPs (PS) on the soil physicochemical properties, enzyme activities, microbial community diversity and structure, and Pb speciation distribution and bioavailability, while the mechanisms was explored. PS with 1 mm (PS1) and 100 μm (PS0.1) size and dose (0.1%, 1%, and 10%, w/w) incubated into a loessial soil (sierozem) contaminated with Pb (1000 mg·kg− 1) for 60 days. Soil properties were examined. Bacterial community changes were assessed using 16 S rRNA amplicon sequencing. pH and CEC decreased, while the SOM increased significantly with increasing PS dose. The presence of PS0.1 reduced ammonium nitrogen and available potassium contents, with a 10% dose resulting in reductions of 12.93% and 74.33%, respectively. PS decreased the activities of sucrase and alkaline phosphatase and enhanced urease activity. PS also increased the Shannon, Simpson, and Chao1 indices of bacterial communities. The presence of PS1 resulted in 1.55% to 7.71% decrease in the speciation of residual state (F4) Pb, increasing bioavailability. F4 fraction was negatively correlated with NH4+-N content and positively correlated with sucrase activity. Structural equation modeling revealed that PS dose, pH and available phosphorus were the primary factors driving the transformation of Pb speciation.