Chemical fractionation and plant uptake of heavy metals in soils amended with co-composted sewage sludge

A sequential extraction method (KNO3, H2O, NaOH, Na2-EDTA, HNO3) was used to determine the soil fractions and plant availability of Cd, Cr, Cu, Ni, Pb, and Zn in three Atlantic coastal plain soils amended with four rates of Co-composted sewage sludge (CCSS:0, 11, 22, and 44 Mg/ha), limed to four pH values (pH 5.3-7.2) and used to grow wheat (Triticum aestivum L.) and soybean (Glycine max L.) under greenhouse conditions. The average (cross soils) increases in total soil Cd, Cr, Cu, Ni, Pb, and Zn, at the highest rate of CCSS were 0.1, 2, 7, 5, 14, and 14 mg/kg, respectively. Distribution of all metals, except Cd, differed in CCSS-amended soils, with greater percentages found in the NaOH and EDTA fractions and less in the HNO3 form than in unamended soils. The more labile fractions (KNO3, H2O) of Cu, Ni Pb, and Zn, but not Cd and Cr, increased slightly (< 1 mg/kg) when CCSS was added. Liming usually resulted in minor (< 5%) changes in the distribution of Cd, Cr, Ni and Pb among soil fractions. However, liming consistently decreased NaOH extractable Cu and (KNO3 + H2O) extractable Zn, and increased the percentage of these metals in the EDTA and HNO3 fractions. Amendment with CCSS reduced wheat dry matter production in all soils, probably from decreased N availability caused by the high C/N ratio (52:1) of the compost, and slightly increased soybean growth (6-12%). The CCSS had little effect on concentrations of Cd, Cr, or Pb in either crop, but consistently increased Cu, Ni and Zn in vegetative tissues of wheat and soybean, and Ni and Zn in soybean grain. Liming decreased Mn and Zn in wheat and soybean, and Ni in soybean grain, but rarely affected plant Cd, Cr, Cu, or Pb. With the exception of Zn, consistent correlations between total soil metal content or individual metal fractions and plant metal concentrations or uptake were not observed. However, significant multiple regression models between soil metal fractions and pH and metal concentrations in wheat and soybean were obtained for Ni(R2 = 0.42 -0.86), Cu (R2 = 0.37 -0.57), and Zn (R2 = 0.62 -0.82) with all three soils.