LONG-TERM EFFECTS OF BIOSOLIDS APPLICATIONS ON HEAVY-METAL BIOAVAILABILITY IN AGRICULTURAL SOILS

Concerns exist over the long-term availability of trace metals in bios olids-amended soils. The objective of this study was to quantify extra ctable forms of trace metals in biosolids-amended, continuously cultiv ated soils after cessation of sewage sludge applications and to determ ine their bioavailability to romaine lettuce (lactuca sativa L.). Trac e metals in surface soils from two biosolids-amended sites were specia ted into increasingly stable chemical fractions using a series of sequ ential extractions that were operationally defined as exchangeable (Ex ch), 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 bioavailabilit y. In control and biosolids-amended soils, 60 to 75% of Cd was found i n the more easily extracted Exch and SA forms, but the percentage was significantly greater in the biosolids-amended soils. Biosolids applic ations 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 wer e found in the relatively stable Ox/AR, R-Org, and R-In fractions of c ontrol and biosolids-amended soils. Concentrations of Cd, Ni, and Zn i n aboveground lettuce tissue, and to a lesser extent, Cu and Cr, were significantly increased by biosolids applications, but Pb uptake was n ot 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 sho w that 15 yr after biosolids applications, the relative bioavailabilit y of biosolids-applied heavy metals was Cd much greater than Zn greate r than or equal to Ni > Cu much greater than Cr > Pb.