Dosages of Biodegradable Poly(butylene adipate-co-terephthalate) Microplastics Affect Soil Microbial Community, Function, and Metabolome in Plant–Soil System

As a substitute for conventional plastic mulch, biodegradable mulch film (BDM) has been popular in agricultural systems in recent years. However, studies focusing on the systematic effect of BDM residues on the soil microbiome and metabolome remain obscure. Thus, a mesocosm experiment was established, and it aimed to investigate the effects of concentrations of poly(butylene adipate-co-terephthalate) (PBAT) microplastics (MPs) on soil microbial ecology and plant (Lactuca sativa) fitness. Metagenomics and metabolomics analyses were deployed to explore the response of soil microbial communities, functional shifts, and metabolites under different dosages of PBAT MPs (CK, 0.1%, 1%, and 5% w/w). The results showed that PBAT MPs did not significantly affect the morphological traits (shoot length and leaf dry weight) of the plant. Regarding plant biochemical indicators, the highest concentration of PBAT could increase the proline and soluble protein contents compared to low- and medium-dosage PBAT treatments with high malonaldehyde (MDA) or soluble sugar contents. Soil physicochemical properties like the available phosphorus and potassium, ammonium N and nitrate N contents were decreased in a dose-dependent manner. Metagenomics analysis revealed that only a high concentration of PBAT had more profound effects on the soil microbial community composition, diversity, and function when compared to the control (CK). In particular, a 5% PBAT treatment could result in the development of some microbial biomarkers, such as Paraburkholderia and Rhizobium, which had beneficial functions. Moreover, metabolomics analysis showed that 5% PBAT differentially affected the soil metabolites, with a high abundance of bioactives like peptides, organic acid, and nepetaside. This work underscores that soil could recruit certain microbes and bioactive substances to resist external high-PBAT stress. PBAT might pose little threat to the soil ecosystem, and its application is beneficial for soil health management.