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Chemistry and Phytotoxicity of Soil Trace Elements from Repeated Sewage Sludge Applications

Municipal sewage sludges, or biosolids, can be applied to croplands to supply and recycle nutrients and organic C. Trace elements in sludges, however, may be of environmental concern. This study examines the long-term consequences to crops and soil when loading phytotoxic levels of one or more trace elements to cropland. Municipal sewage sludges containing trace elements, including high concentrations of Cd, Cr, Cu, Pb, Ni, and Zn, were applied to cropland from 1977 to 1986 at three rates plus an untreated control. Plant and soil samples were collected between 1985 and 1990. These results report on yield and metal uptake of corn (Zea mays L.), sorghum-sudangrass (Sorghum bicolor L. Moench x S. sudanese P. Stapf.), and soybean (Glycine max L.). Yields of all crops were reduced on one or more sludge treatments because of phytotoxic concentrations of soil trace elements, probably Zn and Ni. Phytotoxicity has continued since the last sludge application. Chemical fractionation of surface soils was performed using a sequential extraction technique and trace elements were measured in each of eight fractions. Cadmium, Ni, and Zn occupied soil fractions that were potentially available for plant uptake. Copper and Cr loadings increased the environmental availability of these two elements to a smaller extent. Loadings of Pb to the levels seen in this study did not appear to significantly increase its environmental availability. The Toxicity Characteristic Leaching Procedure (TCLP) and guidelines used to characterize hazardous wastes were inappropriate to use when testing soils for potential phytotoxic concentrations of trace elements or uptake of these elements into the food chain.