Effects of different microplastic types on soil physicochemical properties, enzyme activities, and microbial communities

Global concern has been raised regarding the accumulation of microplastics (MPs) in soil ecosystems. However, it remains unclear how various types of MPs affect the properties of soil ecosystems. In this study, we investigated the effects of six different types of MPs, including low-density polyethylene (LDPE), polyamide (PA), polystyrene (PS), polyhydroxy-alkanoates (PHA), polybutadiene styrene (PBS), and polylactide (PLA), on soil physicochemical properties, enzyme activities, and microbial communities. The findings revealed that, with the exception of the PS treatment, all types of MPs significantly altered the physicochemical properties of the soil. All types of MPs had an impact on the activities of urease and fluorescein diacetate hydrolase (FDAse), while LDPE, PA, and PS consistently affected the activities of four soil enzymes. The results of high-throughput 16S rRNA gene amplicon sequencing demonstrated that MPs altered the composition of the soil microbial community, but only the PHA treatment influenced the soil bacterial diversity and richness. Specifically, PHA significantly increased the relative abundance of Firmicutes while reducing the relative abundance of Actinobacteriota, Proteobacteria (especially the nitrogen-fixing rhizobia), and Acidobacteriota. Redundancy analysis (RDA) suggested that acid phosphatase and pH are the two major environmental factors influencing the structure of soil bacterial community structure at the phylum and order levels. Furthermore, based on the Tax4Fun2 tool, MPs contamination disrupted bacterial metabolic pathways. Our findings indicate that different types of MPs can affect soil fertility, bacterial diversity, and function in various ways, and biodegradable MPs may induce greater differences in soil bacterial communities compared to conventional MPs.