Revisiting the environmental impacts of microplastics in soils: Insights from a meta-analysis

Soil microplastic (MP) pollution is rising, posing increasing environmental risks. Although meta-analyses have quantified MP effects in soils, most have treated MPs as a single group and relied on high-concentration experiments, limiting ecological relevance and obscuring type-specific responses. This study synthesized data from 111 studies covering nine soil indicators and developed a type-based meta-analysis framework to evaluate MP effects under environmentally relevant concentrations (≤1% w/w, based on literature). Results show that when MPs are not separated from soil, they can bias soil indicator measurements through both dilution effects and analytical interference, where MPs may be counted as soil constituents. This is particularly important for soil organic carbon (SOC), where a 1% increase in MP input may lead to an overestimation of approximately 0.889 g kg^-1. Under environmentally relevant concentrations, pooled effect sizes differ substantially from those including high-concentration data (-62.25% to 93.56%) and may even reverse in direction. After correcting for dilution effects, overall variability in pooled estimates ranges from -48.91% to 46.30%, with -24.21% to 67.74% across MP types. Distinct MP types exhibit different effects on soil indicators, highlighting the importance of type-stratified analysis. MP characteristics such as shape and size further modulate these effects, while plant presence generally mitigates MP impacts. These findings highlight the necessity of jointly considering MP type and environmental relevance when evaluating their impacts. The framework provides a more robust approach for assessing the effects of MPs in soils and offers a transferable methodological reference for investigating MP impacts in other environmental systems.