Effect of Potentially Toxic Elements on Plant and Soil Biodiversity

Abstract

Microplastic (MPs, plastic particles <5 mm) contamination in terrestrial ecosystems has raised serious concerns due to their consistently increasing production with minimal recycling worldwide. Evidence suggests that the non-recycled plastics end up in landfills, passing through weathering and physical changes reach to micro sizes. MPs can easily leak into water bodies, sewage water/sludge, and agricultural soils. In addition, industrial effluents can also be the hotspots of MPs and other contaminants like heavy metals. Biochar, a carbon-negative technology has been proposed as a soil conditioner to improve soil properties and crop yield in less fertile soils. However, little is known about how biochar can interact in MPs and heavy metals (like chromium (vi)) contaminated soils, and secondly, if the presence of biochar can influence the effects of MPs and heavy metals on plant production and soil biology. Therefore, we investigated the effects of polyvinyl chloride (PVC) MPs, and Cr on the soil enzymatic activities and microbial community structure in a soil-plant system in the presence/absence of cotton stalk biochar. A 60- day microcosm experiment was conducted by the application of PVC-MPs (0, 0.5% (w/w)), Cr (50 mg/kg), and biochar (0, 0.5% (w/w)). Mash Bean (Vigna mungo) was taken as a test crop. Plant growth, physiology, soil enzyme activity, and microbial community structure were investigated using PLFA biomarkers analysis. The soil enzymatic activities (acid phosphatase (- 41.30% and -42.50%), ꞵ-glucosidase (-36.53% and -44.04%), urease (-28.03% and -16.58%), and dehydrogenase (-16.83% and -43.56%)) were negatively affected in response to PVC-MPs and Cr contamination. A strong shift was also observed in microbial community structure (-8.93% and - 36.43; indicated by PLFAs) in PVC-MPs and Cr treatments. However, the addition of biochar to PVC-MPs and Cr-contaminated soil significantly enhanced soil enzymatic activities and microbial community structure (0.24% and -25.87%). PVC-MPs (especially with Cr) reduced the microbial biomass carbon and nitrogen (-71.8% and -47.70%) however, biochar addition enhanced microbial biomass carbon and nitrogen (+15.69% and +9.48%). The findings presented here suggest that PVC-MPs and Cr have a significant impact on key pools and fluxes within the agroecosystem while the addition of biochar can be used as a soil amendment to improve the overall soil quality of MPs and Cr-contaminated soil. We conclude that PVC-MPs and Cr in the soil are not beneficial and therefore biochar should be added to minimize their entry into the agroecosystem and potential to transfer into the food chain.