Effects of different sizes of microplastic particles on soil respiration, enzyme activities, microbial communities, and seed germination

Microplastics (MPs) are emerging pollutants of terrestrial ecosystems. The impacts of MP particle size on terrestrial systems remain unclear. The current study aimed to investigate the effects of six particle sizes (i.e., 4500, 1500, 500, 50, 5, and 0.5 mu m) of polyethylene (PE) and polyvinyl chloride (PVC) on soil respiration, enzyme activity, bacteria, fungi, protists, and seed germination. MPs significantly promoted soil respiration, and the stimulating effects of PE were the strongest for medium and small-sized (0.5-1500 mu m) particles, while those of PVC were the strongest for small particle sizes (0.5-50 mu m). Large-sized (4500 mu m) PE and all sizes of PVC significantly improved soil urease activity, while medium-sized (1500 mu m) PVC significantly improved soil invertase activity. MPs altered the soil microbial community diversity, and the effects were especially pronounced for medium and small-sized (0.5-1500 mu m) particles of PE and PVC on bacteria and fungi and smallsized (0.5 mu m) particles of PE on protists. The impacts of MPs on bacteria and fungi were greater than on protists. The seed germination rate of Brassica chinensis decreased gradually with the decrease in PE MPs particle size. Therefore, to reduce the impact of MPs on soil ecosystems, effective measures should be taken to avoid the transformation of MPs into smaller particles in soil environmental management.