Field assessment of sludge metal bioavailability to crops: Sludge rate response

We conducted a field study (1991-1995) of trace metal (Cd, Cu, Ni, Pb, and Zn) concentrations in two crops, corn (Zea mays L.) and lettuce (Lactuca sativa L.) as affected by a one-time application of an anaerobically digested sewage sludge to Miamian silt loam (fine, mixed, mesic Typic Hapludalf) in Columbus, OH to determine the nature of the uptake response over a wide range of sludge application rates (0, 7.5, 15, 30, 60, 90, 120, 150, 188, 225, and 300 Mg/ha dry solids). Cadmium, Cu, Ni, Pb, and Zn sludge concentrations were 46, 433, 67, 185, and 2334 mg/kg, respectively. Soil pH did not vary significantly with sludge application (6.1-7.5), while electrical conductivity, total C and total organic N increased linearly with sludge application and declined over time. Soil metals extracted with 0.005 M EDTA in 0.01 M Ca(NO3)(2) increased linearly with total soil concentration and generally declined over time. Cadmium, Cu and Zn concentrations in corn increased significantly with sludge application, while Ni and Pb levels were low compared to the control. Cadmium, Cu, and Zn concentrations in corn exhibited a plateau-type response that could be modeled with the Mitscherlich equation. Lettuce concentrations increased linearly with sludge application for Cd, Cu, and Zn in all years, and linear regression slopes generally declined and stabilized after the first 2 yr.