Long-term effects of biosolids applications on heavy metal bioavailability in agricultural soils

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 sativa 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-Mu 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 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 much greater than Zn greater than or equal to Ni > Cu much greater than Cr > Pb.