Microplastics and cadmium contamination effects on plant physiology and soil biodiversity

Abstract

Microplastics (MPs) are tiny plastic particles less than 5 mm in size. The MPs enter the environment due to the breakdown of larger plastic materials. MPs pollution has become the most emerging environmental issue and is almost everywhere in the world. The MPs are divided into two major groups. The primary and secondary MPs. Primary MPs are manufactured in micro sizes, while the breakdown of larger plastic materials forms secondary MPs. Polyvinyl chloride (PVC) is one of the agricultural soil's most abundant and toxic MPs. Agricultural soils are also facing the challenges of heavy metals (loids). For instance, Cadmium (Cd), a heavy metal, is one of the most dangerous pollutants in agricultural soils. Cd is a non-essential, more mobile, and bioavailable heavy metal and thus can be a potential threat to plant growth and soil biota. Biochar (a stable carbon obtained by the pyrolysis of organic wastes under anaerobic conditions) has been investigated and recommended to reduce soil contaminants' bioavailability. In the present study, we investigated the interaction of PVC-MPs (0%, 0.25%, and 0.5%, (w/w)), Cd (600mg/kg), and cotton stalk (Gossypium hirsutum L.) biochar (0%, and 0.5% (w/w)), in two controlled experiments, on plant growth, physiology, antioxidants defense, soil microbial community and abundance using 16S rRNA, 18S rRNA and PLFA biomarkers. Results revealed that the PVC-MPs at the low dose of 0.25% increased the dry shoot mass with no significant effect on the dry root mass. There was no significant effect on plant antioxidant activity. However, MP's addition reduced soil urease, dehydrogenase activity, soil organic and microbial biomass carbon, and bacterial and fungal abundance. The Co application of biochar with PVC-MPs reduced the adverse effects of the MPs and increased xxii Urease and dehydrogenase activity, soil organic and microbial biomass carbon, and bacterial and fungal abundance. PVC-MP + Cd reduced the plant shoot biomass and did not significantly affect the root dry weight. On the other hand, PVC-MP + Cd reduced the soil enzyme activity and increased the Cd concentration in the plants' roots and shoot, whereas PVC-MP + Cd increased gram-positive bacteria, gram-negative bacteria, total PLFA (phospholipids fatty acids) biomarkers. Adding biochar alleviated the hazardous effects of PVC-MP + Cd contamination as indicated by increased soil enzyme activity and reduced Cd uptake in the shoot, root, plant, and soil. Applying biochar and PVC-MP + Cd causes a further increase in gram-positive bacteria, gram-negative bacteria, and total PLFA. To sum up, it is concluded that applying biochar to high-risk areas of MPs or heavy metals contamination can be a strategy to reduce the environmental impacts of emerging pollutants.