Discrepant responses of bacterial community and enzyme activities to conventional and biodegradable microplastics in paddy soil

The prevalence of microplastics in soil ecosystems has raised concerns about their potential effects on soil properties. As promising alternatives to conventional plastics, biodegradable plastics have been increasingly applied in agricultural activities, which may release microplastics into the soil due to incomplete degradation. Compared to conventional microplastics, biodegradable microplastics in soil may induce different impacts on soil microbial properties, which have yet to be well studied. Through a 41-day microcosm experiment, we evaluated the differential effects of conventional (polypropylene, PP) and biodegradable (polylactic acid, PLA) microplastics on the geochemical properties, enzyme activities, and microbial community structure in paddy soil. Adding PLA or PP microplastics into soil significantly increased pH values and altered the contents of carbon and nitrogen nutrients. Exposure to microplastics significantly increased the activity of fluorescein diacetate hydrolase, but had varying effects on the activities of urease, sucrase, and alkaline phosphatase depending on microplastic types and doses. The addition of microplastics also influenced the structure of soil bacterial community, with Proteobacteria, Actinobacteriota, and Acidobacteriota being the dominant phyla. Significant differences in the genera of Pseudarthrobacter, Acidothermus, Bacillus, Aquisphaera, and Massilia were observed between treatments. Results of structural equation modeling (SEM) demonstrated that changes in soil carbon and nitrogen nutrients and pH values positively affected the bacterial community, while soil bacterial community as a whole exerted a negative impact on enzyme activities. FAPRPTAX analysis showed that the addition of microplastics altered the relative abundances of functional genes related to the metabolism of cellulose decomposition and ureolysis in paddy soil. Findings of this study clearly suggest that microplastic impacts on soil geochemical and microbial properties should be an integral part of future risk assessment and that to evaluate microplastic impacts, both the concentration and polymer type must be taken into account.