The bridging role of soil organic carbon in regulating bacterial community by microplastic pollution: Evidence from different microplastic additions

The serious threat posed by microplastics pollution to soil ecosystems and human health has attracted worldwide attention. Microplastics of different types are present in the soil environment, whereas research about the effects of different microplastics on soil ecology are limited. This study sought to determine how three common microplastics (polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC)) affect soil physicochemical characteristics, enzyme activities, bacterial community, and their metabolic pathways at a 1 % w/w concentration. All three microplastic treatments significantly increased soil organic carbon (SOC), labile organic C fractions, and enhanced soil carbon to nitrogen ratio. PE, PS, and PVC microplastics exhibited promotion of α-glucosidase (α-GC), sucrase (SC), and alkaline phosphatase (AKP) activities. PS microplastics caused a significant increase in N-acetyl-β-D-glucosidase (NAG) and leucine aminopeptidase (LAP) activities, while PVC microplastics significantly decreased β-glucosidase (β-GC) activity. Microplastic treatments increased the bacterial community diversity while altering its composition. Proteobacteria and Chloroflexi were the dominant bacterial phyla in the soil, with microplastic treatments increasing the relative abundance of Chloroflexi and decreasing that of Proteobacteria. Functional prediction analysis indicated that microplastic treatments enriched genes involved in carbohydrate and amino acid metabolism, while reducing the abundance of genes related to signal transduction and cell motility. Correlation and pathway analyses revealed that microplastics affect bacterial community diversity and composition through direct and indirect effects (by acting on SOC or its key labile fractions), thereby influencing soil enzyme activities. In conclusion, the work emphasizes the impacts of different microplastics on soil ecosystems in terms of commonalities and dissimilarities, with the innovative finding of indirect regulation of bacterial community by SOC under microplastics contamination. This provides new perspectives for subsequent studies.