Research Progress on Toxicity of Microplastics in Soil to Terrestrial Plants and Their Degradation Mechanism

Microplastics(MPs), as a new type of environmental pollutants, have gradually attracted widespread attention since they were introduced by British scientists in 2004. Soil is an important accumulation site for microplastics, which can expand the scope of contamination and accumulate with agricultural practices such as irrigation and tillage. Microplastics in soil cause a variety of toxicities to terrestrial plants. The small particle size, difficult degradation, and strong adsorption capacity bring a challenge to the microplastic pollution treatment of soil. In this study, the toxicity of microplastics to terrestrial plants was reviewed in terms of their direct or indirect toxicity and combined effects with other pollutants, mainly in terms of mechanical injury, induction of oxidative stress, and cytotoxicity and genotoxicity to plants, resulting in plant growth and plant tissue metabolism obstruction. In general, the toxicity of microplastics depended on the polymer type, size, and dose; plant tolerance; and exposure conditions. In addition, the production of secondary microplastics and endogenous contaminants during their degradation in soil enhanced the biotoxicity of microplastics. Further, the physical, chemical, and microbial degradation mechanisms of microplastics were introduced in this study based on the current research. At first, the physical and chemical degradation of microplastics mainly occurred by changing the particle size and surface properties of microplastics and producing intermediates. Then, smaller-sized microplastics and their intermediates could eventually be converted to water and carbon dioxide through physical, chemical, and biological functions. Finally, further prospects regarding soil microplastics were introduced, and we provided information for future improvement and pollution control of microplastics.