Influence of biodegradable microplastics on soil carbon cycling: Insights from soil respiration, enzyme activity, carbon use efficiency and microbial community

The rising prevalence of biodegradable microplastics (BMPs) in soils has raised concerns about their impacts on soil ecosystems and carbon cycling. This study investigates the effects of different BMPs on soil carbon cycling, focusing on soil respiration, enzyme activities, and carbon use efficiency (CUE) from 13C-labeled dissolved organic carbon (DOC) in an upland soil. The BMPs tested were polybutylene adipate terephthalate (PBAT), polyhydroxyalkanoates (PHA), and polylactic acid (PLA), at high (H, 1% w/w) and low (L, 0.1% w/w) concentrations. Over a 64-day incubation, cumulative CO2 emissions increased in the PHA_L, PHA_H, and PLA_H treatments, with the highest rise of 665% PHA_H treatment. Microbial biomass carbon (MBC) ranged from 97.73 ± 3.03 mg C kg⁻1 in the control to 223.09 ± 7.91 mg C kg⁻1 in PHA_H, with microbial CUE peaking at 0.26 in PHA_H. Enzymatic activities were notably affected: β-glucosidase (BG) increased by 50% in PLA_H, while cellobiohydrolase (CBH) activity decreased by up to 62% in PBAT_H and PLA_L. N-acetylglucosaminidase (NAG) and phosphatase (AP) activities were highest in PHA_H, indicating enhanced nutrient cycling. Microbial community structure based on PLFAs was significantly altered, with total PLFA content increasing by 191% in PHA_H. Correlation analysis and partial least squares path modeling (PLS-PM) revealed that BMP concentration, DOC content, and microbial diversity were positively correlated with microbial CUE. This study highlights the significant role of BMPs in influencing soil carbon cycling, primarily through their effects on microbial diversity and soil enzyme activities. •Polyhydroxyalkanoates increase soil CO₂ emissions and boost microbial biomass.•Higher biodegradable microplastic concentrations improve carbon use efficiency.•Polylactic acid and polyhydroxyalkanoate enhance soil nutrient cycling and enzyme activity.