Meta-analysis reveals differential impacts of microplastics on soil biota

Contamination of microplastics (MPs) is a global environmental issue that has received much attention from the scientific and public communities due to ecological concerns in recent decades. Comparing with aquatic ecosystems, soil systems, regardless of the high importance and complexity, have been less studied under widely existing and increasing MP contamination. This review, combined with data assimilation and meta-analysis methods, has summarized current contamination conditions of soil MPs across different sites reported in earlier studies. While performing this meta-analysis, we investigated the effects of MPs on soil biota including their numbers, biomass, diversity, and physiological properties. The results showed that abundance of soil MPs ranged from 0.34 to 410958.9 items kg^-1 and concentration ranged from 0.002 to 67500 mg kg^-1 across sites, with agricultural soils containing significantly lower abundance and concentration of MPs than others. Presence of MPs significantly decreased the individual number of soil biota, operational taxonomic unit, diversity index (Simpson), movement index and reproduction rate, whereas the mortality rate was significantly increased by the soil MPs. Despite these significant effects, MPs did not significantly alter the biomass of soil biota, which could be due to a counteraction of their negative and positive effects on different groups of soil organisms. Moreover, we observed that soil MPs could significantly increase the Chao1 index, suggesting that MPs may act as a food resource for the soil rare biosphere. Based on the existing knowledge, we suggest that future studies should focus on research areas that include but are not limited to methodological improvements, intensive field investigations, risk assessment from the perspective of soil food web and bioaccumulation, MPs induced antibiotic resistance, and restoration strategies to reduce their concentrations in soil.