Trace Metal Chemistry in Arid‐zone Field Soils Amended with Sewage Sludge: I. Fractionation of Ni, Cu, Zn, Cd, and Pb in Solid Phases

Abstract

The surface horizons of two arid‐zone field soils that had received amendments of either liquid or dried, anaerobically digested sewage sludge for 4 years were sampled to determine the forms of selected trace metals in the solid phase. The soils had been amended with sludge twice annually at rates of 0, 22.5, 45.0, or 90.0 tons · ha⁻¹ · year⁻¹. Barley and sorghum had been grown on the soils in randomized experimental plots. The soil samples were analyzed for total Ni, Cu, Zn, Cd, and Pb and were fractionated by sequential extraction to estimate the quantities of these metals in “exchangeable,” “sorbed,” “organic,” “carbonate,” and “sulfide” forms.The total contents of the five metals in the two field soils were governed by the total content of the metals in the sludges applied and by the rate of sludge application. The accumulation of metals in the surface horizons of field plots receiving liquid sludge was less than that in the plots receiving composted sludge, possibly because of a lesser reduction in soil bulk density resulting from sludge applications. The percentage of the total metal content in exchangeable and sorbed forms was very low, averaging between 1.1 and 3.7% for all of the metals regardless of the type of soil, the form of sludge applied, or the sludge application rate. The application of sludge tended to reduce the sulfide fraction and to increase the organic and carbonate fractions of all five trace metals. At the highest rate of sludge application, the predominant forms of the metals were: Ni, sulfide; Cu, organic; and Zn, Cd, and Pb, carbonate.