Microplastics in soil affect the growth and physiological characteristics of Chinese fir and Phoebe bournei seedlings

Pot experiments were conducted using Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) and Phoebe bournei (Hemsl.) Yang) to investigate whether soil microplastics adversely affect the nurturing and renewal of plantations. Microplastics composed of polyethylene and polypropylene with a size of 48 μm were used. The treatments included a control group (without microplastics) and groups treated with microplastic concentrations of 1% and 2% (w/w). The effects of microplastics on the growth, photosynthetic pigments in leaves, antioxidant systems, and osmotic regulation substances of the seedlings were analysed by measuring the seedling height, ground-line diameter growth, chlorophyll (chlorophyll a, chlorophyll b, and total chlorophyll) contents, antioxidant enzyme (superoxide dismutase, peroxidase, catalase) activities, and malondialdehyde, soluble sugar, and soluble protein levels. The results indicated that treatment with 1% polyethylene microplastics increased the chlorophyll a, total chlorophyll, and soluble protein contents in the leaves of both types of seedlings while inhibiting superoxide dismutase and peroxidase activities in P. bournei seedlings. Treatment with 2% polyethylene or polypropylene microplastics suppressed the chlorophyll a, chlorophyll b, and total chlorophyll contents; superoxide dismutase, peroxidase, and catalase activities; and soluble sugar and soluble protein levels in the leaves of both types of seedlings, resulting in reduced growth in terms of height and ground-line diameter. The physiological effects of polyethylene microplastics were more evident than those of polypropylene at the same concentration. The results demonstrated that microplastics can affect photosynthesis, the antioxidant system, and osmotic regulation in Chinese fir and P. bournei seedlings, thereby inhibiting their normal growth and development. Exposure to 1% (w/w) microplastics triggered stress responses in seedlings, whereas 2% (w/w) microplastics impeded seedling growth.