Effects of polyethylene film debris size and concentration on soil bacterial functions

Polyethylene macro- and microplastic residues have emerged as their potential impacts on the soil ecosystems. However, it remains unclear on the influence mechanisms of the plastic debris on the structure and functions of the soil bacterial community. In this study, a three-year field incubation experiment was conducted to explore the effect of the plastic debris and concentration on the soil bacterial function. Specifically, three sizes of the polyethylene debris: small debris (Small, 0.4×0.4 cm), medium debris (Medium, 4.0×4.0 cm), and large debris (Large, 10.0×10.0 cm). Two levels of the application concentration were set as: low concentration (L, 1350 kg/hm²) and high concentration (H, 2750 kg/hm²). A control (CK) group was set without polyethylene debris. A systematic investigation was also made to explore the impacts of the polyethylene debris on the plastic film-sourced pollutants (e.g., microplastics and phthalic acid esters), bacterial community structure, and soil ecological functions. The results indicated that both fragment size and concentration significantly influenced the abundance of microplastics and phthalic acid esters content. Notably, microplastics abundance in the Small_L and Small_H treatments was significantly higher than in the rest treatments. Compared with the CK, the polyethylene debris were was significantly reduced in the functional diversity of soil bacteria, with the exception of the Large_L treatment. Small and medium debris caused a reduction in the bacterial alpha diversity, whereas the large debris showed a slight increase. Additionally, the polyethylene fragment treatments significantly decreased the relative abundance of the beneficial bacteria in the soil. There was the a more pronounced decline with the increasing fragment size and concentration. Linear discriminant effect size (LEfSe) analysis showed that the eight genera and one class of microbial taxa were identified highly sensitive to polyethylene debris. Notably, the polyethylene fragment pollution was significantly reduced in the relative abundance of functions that related to the soil carbon cycling, compound degradation metabolism, and nitrogen cycling. While tThere was the an increase in the relative abundance of the functions associated with the pathogen metabolism of the soil ecosystem. Furthermore, the regression analysis revealed that there was the a significant correlation between soil carbon cycling functions and plastic film-sourced pollutants. There was no significant correlation between these pollutants and other functions, including pollutant metabolism, nitrogen cycling, or pathogen metabolism. Random forest analysis highlighted that the bacterial functional diversity, soil enzyme activity, pH, and the size and concentration of polyethylene debris were the primary drivers of the bacterial function (R²=0.225). Moreover, the soil enzyme activity, bacterial community diversity, and fragment size and concentration were identified as the key influencing factors on the bacterial functional diversity (R²=0.730). These findings suggest that the controlling thresholds of the fragment size and concentration was were critical to mitigate the adverse effects of polyethylene debris on the soil microbial ecosystems during film-derived pollution. In summary, this finding can provide new scientific evidence for the influencing mechanisms of the plastic pollution on the soil microbial properties and ecological functions. The theoretical guidance can also offer to monitor and mitigate the plastic pollution in agricultural soils..