Long‐Term Effects of Biosolids Applications on Heavy Metal Bioavailability in Agricultural Soils

Abstract

Concerns exist over the long-term availability of trace metals in biosolids-amended soils. The objective of this study was to quantify extractable forms of trace metals in biosolids-amended, continuously cultivated soils after cessation of sewage sludge applications and to determine their bioavailability to romaine lettuce (Lactuca saliva L.). Trace metals in surface soils from two biosolids-amended sites were speciated into increasingly stable chemical fractions using a series of sequential extractions that were operationally defined as exchangeable (Exch), specifically adsorbed (SA), Fe-Mn oxide and acid replaceable (Ox/AR), residual organic (R-Org), and residual inorganic (R-In). Romaine lettuce was grown on the soils to determine heavy metal bioavailability. In control and biosolids-amended soils, 60 to 75% of Cd was found in the more easily extracted Exch and SA forms, but the percentage was significantly greater in the biosolids-amended soils. Biosolids applications also increased the percentages of Ni and Zn in the Exch and SA fractions. Biosolids applications had little effect on the Exch and SA fractions of Cr and Pb. Greater than 75% of Cr, Cu, Ni, Pb, and Zn were found in the relatively stable Ox/AR, R-Org, and R-In fractions of control and biosolids-amended soils. Concentrations of Cd, Ni, and Zn in aboveground lettuce tissue, and to a lesser extent, Cu and Cr, were significantly increased by biosolids applications, but Pb uptake was not affected. Cadmium, Cr, and Cu concentrations in lettuce were highly correlated (P < 0.001) to metal concentrations in one or more of the Exch, SA, and Ox/AR soil chemical fractions. Results of this study show that 15 yr after biosolids applications, the relative bioavailability of biosolids-applied heavy metals was Cd >> Zn ≥ Ni > Cu >> Cr > Pb.