Study on the Impact of Microplastic Characteristics on Ecological Function, Microbial Community Migration and Reconstruction Mechanisms during Saline-Alkali Soil Remediation

Saline-alkali soils constrain agricultural production due to high salt-alkali stress, and the microplastics introduced during agricultural improvement processes, such as through agricultural film residue and irrigation inputs, may have complex effects on soil ecological functions. As a new type of pollutant, microplastics are widely distributed in soils, water, and the atmosphere. However, the interactions between microplastics in the saline-alkali soil remediation process and the environment remain unclear. This study systematically analyzed the ecological effects of PE, PP, and PBAT microplastics during the remediation of saline-alkali soils with biogas slurry through laboratory simulation experiments. The results showed that microplastics significantly affect carbon-nitrogen cycle-related indicators (such as ammonia nitrogen and DOC) by altering the pH, electrical conductivity, and organic matter decomposition process of saline-alkali soils, and strongly correlate with the microbial community composition and functional pathways. Microplastics triggered the activation of redox enzyme activity and the co-expression of heavy metal resistance/carbon-nitrogen cycle genes, driving the adaptive reconstruction of microbial communities. Conventional microplastics (PE/PP) exhibited slow surface oxidation in saline-alkali soils, with physical adsorption dominating their ecological effects. They inhibited microbial diffusion, induced ecological niche competition, and selectively enriched hydrocarbon-degrading bacteria Alcanivorax and salt-alkali-resistant actinobacteria Nitriliruptoraceae. Their community assembly was mainly driven by random processes, and high microplastic abundance (10 wt%) showed a threshold effect on bacterial composition compared to control samples. In contrast, as the particle size decreased, the degradable PBAT accelerated degradation due to ester bond hydrolysis (C=O functional group decreased from 52.50% to 34.92%), releasing decomposition products that drove deterministic community assembly and reconstructed microbial communities (enriching Proteobacteria, Firmicutes, and Halomonas). The rapid degradation of PBAT may exacerbate short-term ecological disturbances, while the chemical inertness of PE/PP poses a long-term retention risk. This study provides key data for risk management of microplastics in saline-alkali soil remediation. Although microplastic pollution may accelerate soil remediation by promoting microbial metabolic activity, further microplastic safety risk assessment during saline-alkali soil remediation is still needed.