Polyethylene and polyvinyl chloride microplastics promote soil nitrification and alter the composition of key nitrogen functional bacterial groups.

Microplastics (MPs) contamination in soils seriously threatens agroecosystems globally. However, very few studies have been done on the effects of MPs on the soil nitrogen cycle and related functional microorganisms. To assess MP's impact on the soil nitrogen cycle and related functional bacteria, we carried out a one-month soil incubation experiment using typical acidic soil. The soil was amended with alfalfa meal and was spiked with 1% and 5% (mass percentage) of low-density polyethylene (LDPE) and polyvinyl chloride (PVC) MPs. Our results showed that both LDPE and PVC addition significantly increased soil nitrification rate and nitrate reductase activity, which could further promote soil denitrification. The relative abundance of diazotrophs, ammonium oxidizing, and denitrifying bacterial groups were significantly altered with MPs addition. Moreover, the MPs treatments greatly enhanced denitrifying bacteria richness. Redundancy analysis showed that nitrate reductase activity was the most significant factor affecting the soil functional bacterial community. Correlation analysis shows that Nitrosospira genus might be for the improvement of soil nitrification rate. Our results implied that MPs exposure could significantly affect the soil nitrogen cycling in farmland ecosystems by influencing essential nitrogen functional microorganisms and related enzymatic activities.

Response of soil enzyme activities and bacterial communities to the accumulation of microplastics in an acid cropped soil.

The ecological stress of microplastics (MPs) contamination in agroecosystems raise worldwide concerns. However very few studies concentrated on the effects of MPs exposure on soil microbial community. The alterations of enzymatic activities and bacterial communities were assayed by spiking 1% and 5% (w/w) of polyethylene (PE) and polyvinyl chloride (PVC) MPs in an acid soil. The results showed that both PE and PVC addition inhibited fluorescein diacetate hydrolase activity and stimulated urease and acid phosphatase activities, and declined the richness and diversity of the bacterial communities. More severe effects were observed in the PE treated soils compared to the PVC treated soils generally. The relative abundances of families Burkholderiaceae increased significantly (p < .05) after MPs addition, suggesting the bacteria associated with nitrogen fixation stimulated by the MPs input. Meanwhile, significant (p < .05) decline of Sphingomonadaceae and Xanthobacteraceae after addition of 5% PVC and 1% PE MPs, respectively implied that MPs might inhibit the biodegradation of xenobiotics in the soil. Mover, the PICRUSt analysis demonstrated that membrane transporter was a sensitive prediction functional gene of microplastics exposure in the soil. Future studies could be focused on the role of MPs on the regulation of nitrogen cycling and organic compounds degradation in soils.

Microplastics in agricultural soils on the coastal plain of Hangzhou Bay, east China: Multiple sources other than plastic mulching film.

Microplastic contamination in agroecosystems raises great concerns. Here, we investigated the impacts of mulching and irrigation on microplastic accumulation in cropped soils. Sixty soil samples covering mulching and no-mulching farmlands, and forty-five irrigation water samples were collected for analysis. Microplastics were obtained from the soils using continuous air flotation followed by density separation. Stereomicroscopy and micro-Fourier transform infrared spectroscopy (µ-FTIR) were used for identification. Mulching soils contained larger amounts of microplastics than non-mulching soils, with 571 pieces kg-1 and 263 pieces kg-1, respectively, on average. The abundances of films and fibers were significantly (p < 0.05) higher in the mulching soils. Microplastics in the soils and waters were dominated by fragments and fibers, respectively. The particle size of the microplastics in soils mostly ranged from 1 to 3 mm, and primarily from 90 µm to 1 mm in waters. Multiple polymers, e.g. polyethylene, polypropylene, polyester, rayon, acrylic and polyamide, and shapes found in the soil microplastics indicate contributions from irrigation and plastic waste residues other than plastic mulching. Future studies might include the long-term accumulation of microplastics in agroecosystems from multiple sources under intensively managed cropping systems.