Microplastics contamination in the protected agricultural soils and its effects on bacterial community diversity

Mircroplastics (MPs) are defined as plastic particles or fibers of size ˂5 mm and have been of concern in terrestrial ecosystems for several years. Protected agricultural soils have been assumed to be potentially contaminated with MPs because of the use of large amounts of plastic films and annual compost inputs. However, there is little published information on MP contamination of protected agricultural soils and its ecological effects on soil biota remain unknown. Here, a total of 54 surface (0−10 cm depth) soils were sampled from intensively managed protected agricultural soils around Hangzhou Bay, east China. MPs were extracted from the soil samples using continuous air flotation followed by density separation. Stereomicroscopy and micro-Fourier transform infrared spectroscopy (μ-FTIR) were used for identification after the extracted MPs were pre-sorted based on their shapes. In addition, a 29-d soil incubation experiment was conducted to observe the changes in microbial community diversity due to MP contamination. Soils were spiked with 1% and 5% (w/w) low density polyethylene (LDPE) MPs (average size 678 μm) and sampled at an intervals of 1, 15 and29 d in an incubation experiment. The soil bacterial community in these samples was analyzed using high-throughput sequencing technology. A field survey of the protected agricultural soils indicates that soil MP abundance ranged from 20 to 1560 pieces kg−1 with an average of 443 pieces kg−1. Fragments dominated the MP types which comprised films, fragments and fibers, likely originating from compost applications to the soils. LDPE MPs occurred mainly in MP films and were widely distributed in soil samples from mulched soils. Soil MP was also influenced by the type of plastic mulching film used. Relatively high MP abundance was found in soils covered with black mulching film compared with other types of film. Soil MP contamination was found to affect soil microorganisms in the incubation experiment. The Chao1 index (which indicates the richness of the bacterial community) decreased initially and then increased in both PE-spiked and PE-free soils during incubation for 29 d. However, larger decreases and smaller increases in Chao1 were observed in PE-spiked than PE-free soils, indicating that the presence of PE in the soil might inhibit the recovery of the bacterial community richness. The 16S rRNA high-throughput sequencing results indicate that changes in bacterial community diversity were not proportional to the PE microplastic contents of the soils. Larger changes occurred in soils spiked with 1% PE than in those spiked with 5% PE. After addition of PE MPs a significant (P<0.05) increase in the relative abundance of the bacterial orders Sphingomonadales, Xanthomonadales, Propionibacteriales, Chitinophagales, Sphingobacteriales and Flavobacteriales was observed but the relative abundance of the orders unclassified Actinobacteria, Betaproteobacteriales, Myxococcales and Gemmatimonadales decreased significantly (P<0.05). Functional profiling of the bacterial communities was predicted using KEGG pathways. A total of seven KEGG pathways showed significant difference between the 1% PE spiked and PE-free soils. For example, the biosynthesis of secondary metabolites such as isoflavonoids will be impacted and naphthalene degradation will be promoted after addition of PE microplastics. Further studies are planned on the transformation of soil nutrients and xenobiotic pollutants by the selection of functional genes.