Objectives The response of vegetable to soil cadmium (Cd) and arsenic (As) stress and their enrichment and tolerance varies significantly. The uptake and accumulation characteristics of Cd and As of some leafy vegetables cultured in light to medium Cd and As co-contaminated soils were investigated, aiming to provide reference for the rational and safe use of light and medium contaminated soils.
Methods Twelve Cd & As polluted soils were collected from the plough layers of vegetable fields in Xi’an suburbs, and seven kinds of leafy vegetables were assessed in the pot experiment, which was conducted in the greenhouse of the College of Resources and Environment of Northwest A&F University from March to May of 2015. The tested vegetables were: spinach (Spinacia oleracea L.), cos lettuce (Lactuca sativa L.), cole, Romaine lettuce (Lactuca sativa L. var. longifolia), edible amaranth (Amaranthus mangostanus L.), water spinach (Ipomoea aquatica forsk) and garland chrysanthemum (Chrysanthemum coronarium L.). The vegetable yields, Cd and As contents were determined, the safety thresholds of soil pollution of Cd & As were calculated using linear regression models.
Results The soil Cd polluting level of 0.6–2.4 mg/kg reduced the biomass of most vegetables, medium or low co-contamination of Cd and As (Cd 1.1–2.4 mg/kg, As 24.9–26.8 mg/kg) did not exacerbate the inhibition. On the medium co-contaminated soil, Cd contents in the edible parts of spinach, cole, amaranth leaves, and lettuce exceeded the food safety critical value (0.2 mg/kg), and the Cd contents in spinach and cole were even 4 folds beyond the standards, while those of water spinach stem and garland chrysanthemum did not exceed the standard. Although the arsenic contents of all tested vegetables increased with increasing of soil arsenic concentrations, they did not exceed the standard. The enrichment coefficients of Cd in the seven kinds of vegetables were 0.083–0.491, with descend order of cole, spinach, cos lettuce, edible amaranth leaf > Romaine lettuce, edible amaranth stem, water spinach leaf > water spinach stem and garland chrysanthemum. The safety thresholds of soil Cd for spinach, cole, cos lettuce, Romaine lettuce, edible amaranth, water spinach and garland chrysanthemum were 0.33, 0.38, 0.46, 1.15, 0.59–1.79, 1.49–8.16 and 8.98–17.11 mg/kg, respectively. Among them, the thresholds of spinach, cole and lettuce were similar to the current standards (0.3–0.6 mg/kg), while the Romaine lettuce, edible amaranth, water spinach, and garland chrysanthemum were all greater than the soil heavy metal limitation values. As enrichment coefficient was 0.002–0.006, and the As enrichment coefficient of leaves of water spinach and garland chrysanthemum were significantly higher than those of others. The critical thresholds of soil As for the seven kinds of vegetables were 62.31, 70.35, 70.21, 67.41, 67.86–90.43, 57.21–75.70 and 72.43–105.06 mg/kg, respectively, which all were higher than the current standards (25 mg/kg).
Conclusions The cadmium and arsenic enrichment coefficients of vegetables varied significantly, and so were their responses to light-mild cadmium-arsenic co-contamination soils. Water spinach and garland chrysanthemum had low Cd enrichment coefficients and could be recommended for cultivation on mild polluted soil, while spinach and cos lettuce should not be chosen because of the high enrichment of Cd from soil.
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