Trophic Transfer, Bioaccumulation, and Detoxification of Lead and Zinc via Sewage Sludge Applied Soil-Barley-Aphid-Ladybird Food Chain

Assessment of heavy metal (HM) transfer across the food chain has attracted much interest. This study investigated the trophic transfer and toxicokinetics of lead (Pb) and zinc (Zn) in the soil-barley-aphid-ladybird system. At the following rates of 0, 5, 10, 20, and 40% (w/w), sewage sludge from the wastewater treatment plant was applied to the soil. Our study revealed that Pb and Zn uptake elevated with increasing amendment rates for ≥ 10% but showed a relatively restricted root-shoot translocation. The transfer coefficient (TC) of Pb was found to be < 1 for both second and third trophic levels, indicating Pb biominimized at both these levels. However, Zn biomagnified in the shoot to aphid (TC > 1) while biominimized in aphid to ladybird (TC < 1). Elimination of Pb and Zn from honeydew (excrement) and pupal exuviae during metamorphosis raises the possibility of a detoxifying mechanism that operates and regulates the bioaccumulation of Pb and Zn at two different trophic levels throughout the studied system. The most efficient Pb elimination was reported by honeydew because the proportion of Pb in honeydew to aphid (extending from 0.18–0.25) was more than that of Zn (0.07–0.10). Thus, it can be concluded from our study that Zn is bioretained and shows minimal Zn removal by honeydew, and Pb is bioeliminated and shows efficient Pb removal by honeydew. Both metals showed effective removal by pupal exuviae. Thus, such toxicokinetics studies need to be researched more to elucidate probable mechanisms, prospective components, and physiological routes related to the bioaccumulation of Pb and Zn along food chains of the environment. Graphical abstract: Graphical representation of heavy metal migration pathways showing biotransfer (bold arrows) and detoxification (dashed arrows) in the studied food chain. The range of lead and zinc content (mg kg⁻¹ dry weight) in the components of the soil-barley-aphid-ladybird food chain exposed to varying rates of sewage sludge (0 to 40%). [Figure not available: see fulltext.].