Residual Effects of Sewage Sludge on Soybean: I. Accumulation of Heavy Metals

The potential for excessive crop uptake of heavy metals from land previously amended with sludge is of concern because of the persistence of heavy metals. Field studies were therefore conducted at two locations in 1983 and 1984 to examine the residual effects of sewage sludge on heavy metal uptake by soybean [Glycine max (L.) Merr.]. The Fairland plots (Typic Hapludults) had been amended with two rates of anaerobically degested sludge in 1975. The Beltsville plots (Typic Paleudults) had been established in 1976 with limed-anaerobically digested, limed-raw, limed-compost, or heat-treated sludge and in 1978 with Nu-Earth (Chicago, IL) anaerobically digested sludge. Treatments consisted of various sludge rates and pH regimes. The concentrations of metals in soybean shoots at the R4 growth stage varied according to sludge type, application rate, and soil pH. Metal uptake from low metal, limed sludges was similar to that of controls. Metal uptake was, however, increased by unlimed sludges that were high in metal content. Digested sludge linearly increased shoot metal concentrations of Zn, Cd, Cu, and Ni. Compared to the unamended control, soybean Zn increased from 24 to 165 mg kg⁻¹ and Cd increased from 0.06 to 0.25 mg kg⁻¹ at the 112 Mg ha⁻¹ rate. The shoot Cd concentration was over seven times higher from Cd rich Nu-Earth sludge as compared to the other sludges. Shoot Zn and Cd concentrations exhibited significant linear and quadratic relationships with rates of heat-treated and Nu-Earth sludge. There was a greater response of shoot Zn and Cd accumulation to sludge rate at low soil pH. At the 100 Mg ha⁻¹ rate of Nu-Earth sludge, soybean Zn and Cd concentrations, respectively, were 59 and 1.75 mg kg⁻¹ at soil pH 6.4 and 84 and 2.54 mg kg⁻¹ at soil pH 5.8. These results indicate that sludge composition and soil pH can have a substantial influence on soybean metal uptake for at least 9 yr after the initial sludge application.