Availability of Heavy Metals in Soils and their Uptake by Vegetable Species

Abstract

Head lettuce, bush beans and celery were grown in subsequent years in an experimental field on anthropogenously uncontaminated or heavy metal contaminated soils at a mean pH of 6.3 ± 0.1. The contaminated plots were made up by amending or replacing the upper 20 cm soil layer with heavy metal contaminated alluvial top soil. Contamination includes Cd, Zn, Pb and Cu. Phytoavailable fractions of these elements were extracted with 1 M ammoniumnitrate. Cd, Zn, Pb and Cu were also determined in the leaves of all three species as well as in bean pods and celery bulbs. The results show that plant uptake of Cd and Zn increased with increasing soil contamination while the uptake of Pb was low. No dependence of Cu uptake on total soil Cu content could be seen within the given contamination range. The relationship between Cd, Zn, Pb and Cu in plants and ammonium nitrate soil extracts was determined bytwofactorial linear regression where r was approximately 0.8 for Zn, 0.7 for Cd and 0.5 for Pb whereas no relationship was found for Cu. For the pot experiments carried out in a greenhouse two different soils (loamy sand, silt loam) were amended with 5 and 10 % metallurgical slag and adjusted to pH levels around 7 and 5 while controls did not contain slag. Phytoavailable heavy metal fractions were extracted from soil samples using ammonium nitrate or Calcium chloride + DTPA (CAT). Spinach was grown on these soils and the concentrations of Cd, Zn, Pb and Cu in the shoots correlated with their concentration in the soil extracts. The uptake of these elements by the plants increased with increasing slag amendment and decreasing pH. Strong depression of growth was observed at a pH around 5 in all treatments of the lighter and in the slag treatments of the heavier soil. In the slag treatments this was accompanied by increased endogenous Cd and Zn concentrations. The plant content of Cd, Zn and Pb correlated better with the ammonium nitrate extractable soil fraction of these elements than in the field experiments (r > 0.9). Correlations based on the CAT extractable fraction of Cd and Zn yielded values for r around 0.5 while it was only 0.1 for Pb. As in the field trials correlations for Cu were very poor with both extraction methods.