[Effects of Polylactic Acid Microplastics （PLA-MPs） on Physicochemical Properties and Microbial Communities of Wheat Rhizosphere Soil]

Abstract

Microplastics, as a new type of pollutant, have significant effects on the soil ecological environment. In order to investigate the effects of polylactic acid microplastics （PLA-MPs） on the physical and chemical properties of wheat rhizosphere soil and its microbial community structure, three contents （0.1, 0.5, and 1 g·kg^-1） and particle sizes （150, 1000, and 4000 μm） were set up for pot experiments. The results showed that the addition of PLA-MPs significantly increased the contents of ammonium nitrogen （NH₄⁺）, nitrate nitrogen （NO₃^-）, total phosphorus （TP）, and organic matter （OM） in rhizosphere soil and decreased the content of total nitrogen （TN） and pH value. For example, TP increased 19.046% and 21.075% at medium contents （0.5 g·kg^-1） and medium particle size （1 000 μm）, respectively. Compared with those of the control group （CK）, the richness and diversity of rhizosphere soil microorganisms were significantly decreased, and the decreases were greatest at high contents （1 g·kg^-1） and high particle size （4 000 μm）, respectively. The addition of PLA-MPs significantly increased the relative abundance of Actinobacteriota and Proteobacteria, while decreasing the relative abundance of Firmicutes, Gemmatimonadota, Myxococcota, and Bdellovibrionota. For example, the relative abundance of Actinobacteriota increased by 5.554% and 6.456% at medium contents （0.5 g·kg^-1） and low particle size （150 μm）, and the relative abundance of Firmicutes decreased by 2.721% and 3.727% at high contents （1 g·kg^-1） and low particle size （150 μm）, respectively. Compared with CK, PLA-MPs with different contents and particle sizes significantly reduced the number of biomarkers, and the reduction of biomarkers under low content （0.1 g·kg^-1） and medium particle size （1 000 μm） was the largest. A significant negative correlation was observed between pH and the dominant microorganisms at the phylum level and genus level of the microbial community and a significant positive correlation between NO₃^- and Bacillus, Firmicutes, and Gemmatimonadota at the P < 0.001 level. The research results can provide certain data and theoretical basis for evaluating the effects of PLA-MPs on the soil microbial environment.

Keywords: microbial community; physicochemical properties; polylactic acid microplastics （PLA-MPs）; reciprocal influence; rhizosphere soil.