Macro- and micro-plastics change soil physical properties: a systematic review

Plastic pollution in terrestrial environments is a global issue due to its adverse effects on soil health, with negative impacts on ecosystem services and food production. However, the enormous heterogeneity of both plastic and soil characteristics complicate the assessment of the impact and overall trends in plastic-induced changes in soil properties beyond experimental conditions. In this work, we have carried out a systematic and in-depth review of the existing literature on the impact of plastics on soil physical properties. To this end, we have quantified the effects of macro- (MaP, >5000 μ m) and micro-plastics (MiP, <5000 μ m) on soil bulk density, soil porosity, water-stable aggregates (WSAs), saturated hydraulic conductivity, and soil moisture at field capacity (FC), based on four characteristics of plastics: polymer types, shapes and sizes of plastic particles, and plastic concentrations in soil. Results showed that MaPs and MiPs significantly modified the values of the analyzed soil physical properties compared to the control without plastic in over 50% of the experimental dataset, albeit with a large variability, from a reduction to an increase in values, depending on the specific experimental conditions and the soil physical property. Depending on the plastic concentration, soil bulk density and porosity decreased moderately (4%–6%) with MiP and MaP. MiP reduced WSA by an average of 20%, ranging from a 40% decrease to a 20% increase depending on the shapes and concentration of MiP. Saturated hydraulic conductivity changed depending on the polymer types, shapes, and concentrations of MaP and MiP, varying from a 70% decrease to a 40% increase. Soil water content at FC varied depending on the soil texture, and concentration and sizes distribution of conventional MiP, decreasing from 10% to 65%. However, biodegradable plastic increased soil water content at FC. The few studies available provide evidence that not enough attention is being paid to soil physical properties influenced by plastic input. It is recommended to consider the wide range of characteristics of MaP and MiP and their effects on soil physical properties in future studies, for an advance understanding of the impact of MiP and MaP on soil health in the medium-long term under different environmental conditions.