Chemical fractionation of trace elements in biosolid-amended soils and correlation with trace elements in crop tissue

A previous study indicated that agricultural biosolid applications increased the concentration of EPA3050-digestible trace elements in soils on Pennsylvania production farms but could not indicate potential trace-element environmental availability. This study was conducted to determine if biosolid application had altered the distribution of trace-elements among operationally defined soil fractions and the relationship of trace element concentrations in soil and crop tissues. Biosolid-amended and unamended soils from production farms in Pennsylvania were extracted using a modified Bureau Communautaire de Référence (BCR) sequential fractionation technique and analyzed for chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). Trace-element concentrations in crop tissues (soybean silage, sudangrass, corn grain, alfalfa hay, and orchardgrass hay) from the same farms were also determined. Fractionation results indicated that the proportion of Cr, Cu, Ni, Pb, and Zn that is potentially bioavailable is quite small in unamended soils. Biosolid applications significantly (P ≤ 0.1) increased concentrations of Cu in all soil fractions (average increase over unamended soil=1.14, 8.27, 6.04, and 5.84 mg kg^-1 for the exchangeable, reducible, oxidizable, and residual fractions, respectively), Ni (0.41, 1.65 mg kg^-1 for the reducible and residual fractions, respectively), Pb (5.12 and 1.49 mg kg ^-1 for the reducible and residual fractions, respectively), and Zn (8.28, 7.12, 4.44, and 8.98 mg kg^-1 for the exchangeable, reducible, oxidizable, and residual fractions, respectively) but did not significantly increase Cr in any soil fraction. Concentrations of Cu in all soil fractions were significantly (P ≤ 0.01) correlated with concentrations of Cu in orchardgrass tissue (r = 0.70, 0.66, 0.76, and 0.69 for the exchangeable, reducible, oxidizable, and residual soil fractions, respectively). Concentrations of exchangeable and reducible Zn were significantly correlated with Zn in sudangrass tissue (r = 0.81 and 0.67), and reducible Zn was significantly correlated with Zn concentrations in orchardgrass tissue (r = 0.65). Application of biosolids had little effect on bioavailability of Cr, Ni, or Pb, whereas higher loadings of Cu and Zn led to a shift toward the more labile soil fractions. Loadings of Cu and Zn were much smaller than cumulative loadings permitted under U.S. Environmental Protection Agency (USEPA) Part 503 regulations. Chemical soil fractionation was able to detect increases in labile soil Cu and Zn that relate to increased phytoavailability.