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Contrasting potential mediating roles of DOC and SOM in microplastic effects on soil heavy metals bioavailability: A meta-analysis and machine learning approach.
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Analyzing data from 52 studies using statistical modeling, researchers found that microplastics in soil increased dissolved organic carbon, soil organic matter, and heavy metal availability. Dissolved organic carbon tended to make heavy metals more available, while soil organic matter had a mixed effect, sometimes increasing and sometimes reducing their availability, suggesting these two factors work in opposing ways.
Microplastics (MPs) and heavy metals (HMs) often co-occur in soil, yet the role of soil organic matter (SOM) fractions in mediating the effects of MPs on HMs bioavailability remains unclear. Here, data from 52 studies were used to investigate whether dissolved organic carbon (DOC) and soil organic matter (SOM) serve as contrasting mediating pathways, by integrating meta-analysis, machine learning, and structural equation modeling (SEM). MPs increased DOC, SOM, and HMs bioavailability by 14.84%, 13.82%, and 5.21%, respectively. DOC changes positively correlated with HMs bioavailability, whereas SOM showed a U-shaped relationship. MPs size, time, and soil pH were identified as common drivers across all responses. Biodegradable MPs and higher water holding capacity promoted DOC increases and HMs mobilization, while higher MPs concentrations enhanced SOM accumulation and HMs immobilization. SEM supported DOC and SOM as two opposing mediating pathways: DOC exerted a positive mobilization effect, while SOM exerted a negative immobilization effect. Their dynamic balance may contribute to the net direction of HMs bioavailability. These findings reveal that MPs influence HMs bioavailability primarily through two contrasting OM pathways, rather than through direct MPs-metal interactions. Distinguishing DOC from SOM is therefore critical for assessing the ecological risks of MPs-HMs co-contamination in soil.
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