Effects of biodegradable microplastics on soil microbial communities and activities: Insight from an ecological mesocosm experiment

Microplastics (MP) are being released into the environment at an increasing rate, causing extensive pollution in soils and affecting biota and processes. Although the use of biodegradable plastic has increased, its effects on the soil microbial community are not yet well understood. A controlled mesocosm experiment was conducted to investigate the response of soil microbial communities to increasing amounts of starch-polybutylene adipate terephthalate MPs (PBAT-BD-MPs) added to the soil. The experiment included microbes, earthworms, springtails, and plants. The PBAT-BD-MPs were added to the soil column at doses ranging from 0 to 0.8 % w/w of soil dry mass, and the columns were incubated for 11 weeks under controlled climatic conditions. Bacterial and fungal amplicon sequencing was used to investigate the dose-dependent response of the soil microbial communities' alpha and beta diversity. The alpha diversity indices of the bacterial and fungal communities increased with increasing PBAT-BD-MP concentration. Bacterial richness was highest at the highest MP concentration (0.8 %). A similar trend was observed in the fungal community, with a significant increase in fungal richness as PBAT-BD-MP concentration increased. The alpha diversity of both bacterial and fungal communities significantly increased in MP treatments compared to the control treatment. At the highest MP concentration (0.8 %), the abundance of the bacterial phylum Planctomycetes showed a significant increase, while Firmicutes showed a significant decrease. The abundance of the fungal phyla Ascomycota and Mortierellomycota also significantly increased at the highest PBAT-BD-MP concentration compared to the control group. Alongside changes in the soil microbial community, we observed a rise in soil respiration as the concentration of PBAT-BD-MPs increased. Our three-month mesocosm study demonstrates that the introduction of biodegradable microplastics into the natural standard soil environment in realistic concentrations (0-0.025-0.05-0.2-0.8 %) and particle size distribution alters the soil bacterial and fungal community.