Compost Mitigates Metal Toxicity and Human Health Risks and Improves the Growth and Physiology of Lettuce Grown in Acidic and Neutral Loam-Textured Soils Polluted with Copper and Zinc

A pot study was conducted to assess the potential of green waste compost on soil properties, growth, physiology, and metal uptake of lettuce plants grown in acidic and neutral loam-textured soils irrigated with copper- and zinc-polluted wastewater (WW). The experiment consists of sixteen treatments involving two different soils with and without compost addition (compost and NoCompost) and irrigated with unpolluted WW, Cu-polluted WW, Zn-polluted WW, and Cu- plus Zn-polluted WW, arranged following factorial layout with three replications. The results illustrated that Cu- and Zn-polluted WW significantly reduced the growth, chlorophylls, and carotenoid pigments of lettuce plants in both soils under NoCompost conditions. However, the decline in these attributes was more pronounced in acidic soil (45–59%) than in neutral soil (30–38%). In the case of neutral soil, Zn-polluted WW did not negatively affect these attributes compared to control. All the metal-polluted treatments increased total polyphenols, polyphenolic acids, flavonoids, and antiradical activity in lettuce shoots. Alternatively, the compost application consistently increased (8–50%) the growth and physiological attributes of lettuce in both soils. Compost treatment decreased root and shoot metal (Cu, Zn) concentrations and uptake by 25–60% and 16–25%, respectively, in both soils. Likewise, compost decreased the metal health risk index (37%—2.7 folds) in both soils. Copper-polluted WW decreased the dehydrogenase activity of soils more than Zn-polluted WW, but compost significantly increased it in both soils, enhancing the organic matter contents of both soils. Conclusively, the addition of compost at the rate of 2% substantially alleviated the metal toxicity thereafter human health risks in both soils.