Impact of repeated organic amendments on trace element exposure and health risks via the soil-vegetable-human pathway

Applying organic fertilizers enriches soil with essential nutrients and organic matter. However, the trace elements (TE) often present in organic fertilizers—such as chromium (Cr), cadmium (Cd), mercury (Hg) and arsenic (As)—can pose food safety risks. Continuous use of organic amendments can lead to the accumulation of TE in the soil, increasing their uptake by plants and potential risks to human health. To investigate this, we conducted a study across two production cycles to examine the presence of TE along the soil-plant-human contamination pathway. Our focus was on assessing the effects of repeated fertilizer applications on vegetable production and TE contamination, particularly in leafy (lettuce) and root (radish) vegetables. The results showed that using sewage sludge (SS), swine manure (SM), and chemical fertilizer (CF) led to higher vegetable yields compared to compost derived from municipal solid waste (CP). Soil amendments had limit impact on the overall TE concentrations in the soil. However, TE were more concentrated in vegetables grown with organic fertilizers than in those grown with chemical fertilizers. TE levels in the edible parts of vegetables increased significantly following repeated applications. In the first productive cycle of both vegetables, Zn and Cd exhibited moderate bioaccumulation, with BCF values ranging from 0.1 to 1.0. In the second cycle, bioaccumulation generally increased, with Zn reaching high bioaccumulation levels in lettuce under SS and SM treatments (BCF >1.0). Cr in both vegetables and Cu in lettuce maintained moderate bioaccumulation levels. Conversely, both vegetables demonstrated strong defense mechanisms against Pb and As accumulation, as indicated by consistently low BCF values. According to the hazard quotient (HQ), repeated fertilization of lettuce with CF, CP, SS, and SM resulted in a 1.4–1.8-fold increase in the total hazard quotient (THQ) for adults and a 1.3–1.9-fold increase in radish bulbs, with a maximum THQ of 0.23 in the twice-CP fertilization treatment. Overall, the consumption of lettuce and radish grown in amended soils poses no significant health risk, as the THQ values remained consistently below 1.