Effects of Conventional and Biodegradable Microplastics on Soil Physicochemical Properties and Microorganisms

Abstract

With the widespread use of plastic products in production and daily life, microplastic pollution has become a global concern. As an important sink for microplastics in terrestrial ecosystems, soil is significantly impacted by microplastics, making research on this subject of great importance. This study investigated the effects of conventional plastic (polyethylene; PE) and biodegradable plastic (polylactic acid; PLA) microplastics on the physicochemical properties and microbial communities in soil. Through a 4-month in-situ soil incubation experiment, we analyzed the effects of PE and PLA microplastics at varying concentrations (0.5%, 1.0%, 1.5%, 2.0%, 2.5% w/w) on soil physicochemical properties and microorganisms. Results showed the following: (1) PE and PLA microplastics increased soil organic carbon (SOC), and PLA microplastics had a significant impact on SOC. The stimulating effect of PLA degradation accelerates the breakdown and release of existing SOC, further boosting the total SOC levels. PE microplastics lowered ammonium nitrogen (AN) levels, with the effect lessening at higher concentrations. Low PLA concentrations increased AN, but high concentrations reduced it. (2) PE and PLA microplastics inhibited β-1,4-N-acetylglucosaminidase activity, stimulate β-xylosidase and alkaline phosphatase activities, but had no significant effect on β-1,4-glucosidase and leucine aminopeptidase activities. Moreover, PE and PLA microplastics respectively inhibited and activated cellobiohydrolase activity. (3) Low concentrations of PE microplastics with increased the abundance of fungi, bacteria, actinomycetes, gram-positive and gram-negative bacteria, and total microbial biomass, while high concentrations reduced them. PLA microplastics also increased these microorganisms, but the effect weakened with higher concentrations. In conclusion, both conventional and biodegradable microplastics significantly impact soil properties, enzyme activities, and microbial communities, with effects varying with type and concentration. The structural and degradation differences between PE and PLA resulted in different impacts on soil environment. This study provides important insights into the potential risks microplastics pose to soil ecosystems.