Sludge‐borne Heavy Metal Availability and Uptake by Vegetable Crops Under Field Conditions

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Abstract
Municipal sewage sludge was applied in a single application to an acid (pH 5.6) silt loam soil in a field study at rates of 0, 20, and 60 dry metric tons/ha. Metal concentrations in soil and plant tissue were examined over a 2‐year period (1976–1977). Levels of Cd, Cu, Ni, and Zn were measured in carrots (Daucus carota L., cv. Danvers), radishes (Raphanus sativus L., cv. Cherrybelle), tomatoes (Lycospersicon esculentum L., cv. New Yorker), and lettuce (Lactuca sativa L., cv. crispa, Salad Bowl). Metal uptake in both years was Zn > Cu > Ni > Cd. Lettuce tissues generally accumulated the highest concentrations of Cd, Cu, and Zn, while carrot roots accumulated the most Ni. Applications of sewage sludge generally did not increase concentrations of Cd or Cu in plant tissue; the only exceptions were the significant increases of Cd in lettuce and Cu in tomatoes during 1977. Most of the vegetables exhibited significantly higher concentrations of Ni and Zn as a result of sludge applications during both years. Concentrations of heavy metals in plant tissue were generally higher during the second year of the experiment, suggesting an increase in their availability with time. The Cd/Zn ratio of the sludge was 1.3%, while the averages for the edible and nonedible tissue, and DTPA‐extractable soil were 2.9, 4.3, and 2.2%, respectively. The availability of heavy metals in the soil, as measured by DTPA‐extraction, supported the results of plant tissue analyses. Applications of sewage sludge resulted in significant increases in DTPA‐extractable metals. In addition, there was a general increase in the DTPA‐extractable heavy metals with time, which appeared to be related to the decomposition of the sludge as pH values were not appreciably different during the second year.