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 lo ng-term consequences to crops and soil when loading phytotoxic levels of one or more trace elements to cropland. Municipal sewage sludges co ntaining trace elements, including high concentrations of Cd, Cr, Cu, Pb, Ni, and Zn, were applied to cropland from 1977 to 1986 at three ra tes plus an untreated control. Plant and soil samples were collected b etween 1985 and 1990. These results report on yield and metal uptake o f corn (Zea mays L.), sorghum-sudangrass (Sorghum bicolor L. Moench x S. sudanese P. Stapf.), and soybean (Glycine mau L.). Yields of all cr ops were reduced on one or more sludge treatments because of phytotoxi c concentrations of soil trace elements, probably Zn and Ni. Phytotoxi city has continued since the last sludge application. Chemical fractio nation of surface soils was performed using a sequential extraction te chnique and trace elements were measured in each of eight fractions. C admium, Ni, and Zn occupied soil fractions that were potentially avail able for plant uptake. Copper and Cr loadings increased the environmen tal availability of these two elements to a smaller extent. Loadings o f Pb to the levels seen in this study did not appear to significantly increase its environmental availability. The Toxicity Characteristic L eaching Procedure (TCLP) and guidelines used to characterize hazardous wastes were inappropriate to use when testing soils for potential phy totoxic concentrations of trace elements or uptake of these elements i nto the food chain.