Occurrence Characteristics of Microplastics and Influencing Factors in Coastal Salinized Soil.

Microplastics are widespread in terrestrial and marine environments. As a transition zone between land and ocean, coastal soils have unique microplastic pollution characteristics. To reveal the characteristics of microplastic pollution in coastal soils, soils with different salinization levels were collected from Wudi County toward the sea. The distribution characteristics of microplastics and their relationship with soil physical and chemical properties were analyzed through density separation, oxidative digestion, and micro-Raman spectroscopy techniques. The pollutant load index method was used to assess its ecological risk. The results showed that microplastics were detected in 51 sampling points of coastal soil in Wudi County, and the abundance of microplastics ranged from 550 to 3 950 n·kg-1. Polyethylene （PE）, polyethylene terephthalate （PET）, polypropylene （PP）, polystyrene （PS）, and polyvinyl chloride （PVC） accounted for 53.1%, 13.9%, 16.4%, 8.4%, and 8.2%, respectively. The shapes of microplastics mainly included film （accounting for 62.0%）, fiber （accounting for 13.7%）, sphere （accounting for 13.2%）, and sheet （accounting for 11.1%）. Microplastics with grain size less than 1 000 μm accounted for 85.0%. The lowest abundance of microplastics appeared in the bare land with the highest degree of salinization, and the highest abundance appeared in the non-salinized cotton soil. The abundance of microplastics was significantly correlated with soil salinization levels （P< 0.05）. With saline level increasing, the total abundance of microplastics and the abundance of film, PE, and PET microplastics decreased significantly （P< 0.05）. The proportion of microplastics with grain size greater than 1 000 μm decreased significantly （P< 0.05）, but the proportion of microplastics with grain size less than 100 μm increased significantly （P< 0.05）. This may be because of the different soil use types and different sources of microplastics in soils with varied saline levels. Soil organic carbon （SOC） was significantly positively correlated with the abundance of microplastics （P< 0.05）. The risk load index （PLI） values of all soil samples ranged from 1.19 to 2.41, which were low risk level pollution. Among them, the PLI values of wasteland and bare land with high saline level were lower, and the PLI values of soils with low saline level were higher. The results of this study can provide an important basis for understanding the microplastic pollution and exploring the relationship between soil properties and microplastic distribution characteristics in coastal saline soils.