Do Soil pH Levels Drive the Responses of Catalase Activity and Bacterial Communities to Microplastics? A Case Study in Mollisols

Prolonged application and low recycling rates of agricultural plastic films have resulted in significant accumulation of microplastics (MPs) in soils, posing a threat to soil health. However, the impacts of MPs on microbial communities and enzyme activities in Mollisols remain poorly understood. To address the key question of whether soil pH drives the responses of catalase (CAT) activity and bacterial communities to MPs-a core focus of this Mollisol-based case study-we investigated the effects of different MP concentrations (1%, 5%, and 10%) on bacterial community structure and CAT activity across three Mollisol farmlands with distinct pH levels. CAT activity was stimulated at low MP concentrations but inhibited at high levels, whereas dynamic and thermodynamic parameters displayed irregular responses. Temperature sensitivity (Q10) of CAT remained stable, whereas Q10 of kinetic parameters varied among soils. Correlation analysis indicated that Ea and Q10 in acidic soil and Vmax/Km in neutral soil and alkaline soil governed CAT activity. MPs altered α-diversity in acidic and neutral soils, changed β-diversity only in acidic soil, and promoted deterministic assembly processes. PICRUSt functional prediction suggested that functional gene shifts were most evident in acidic and neutral soils, with soil organic matter and Vmax/Km as key drivers in acidic soils and CAT in neutral soils. In contrast, responses in alkaline soil were negligible. These findings highlight soil type-specific microbial responses to MPs and their ecological risks in agricultural soils.