Effects of Cu, Ni and Zn on lettuce grown in metal-enriched vermicompost amended soil

A greenhouse experiment was conducted to test the effectiveness of cattle manure vermicompost enriched with Cu, Ni or Zn to increase plant yield. An oxisol amended with the metal-enriched vermicompost at the doses 0, 25, 50, 65, and 80 t ha^−1 was used to grow lettuce (Lactuca sativa L). After 39 days of growth, the plants were weighed for fresh and dry matter weight determinations. The soil amended with the vermicompost was analyzed for determination of DTPA (diethylenetriaminepentaacetic acid)-extractable Cu, Ni and Zn. The distribution of the metals in the plants was determined and the metal concentrations compared with the levels commonly found in plants, as well as with the range of the critical levels of toxicity to plants. Within each dose of 50, 65 and 80 t ha^−1 of the metal-enriched vermicomposts applied there was a decrease in lettuce yield as compared to the correspondent 50, 65 and 80 t ha^−1 doses of the natural vermicompost applied. The Cu concentrations in the lettuce leaves from the pots with vermicompost enriched with this element were in the range commonly found in plants (from 5.9 to 13.9 mg kg^−1), although in the roots they were relatively high (from 76.25 to 244.56 mg kg^−1). The Ni concentrations in the leaves from the pots with the vermicompost contaminated with this element were within the range of critical levels of toxicity to plants, i.e., from 10 to 50 mg kg^−1. In the case of Zn, its concentrations in the leaves from the pots with vermicompost enriched with this element were, in general, above the range of critical levels of toxicity to plants, i.e., from 200 to 500 mg kg^−1. The plants grown in soil amended with vermicomposts enriched with Cu and Ni showed high Cu and Ni concentrations in the roots, while the plants grown in soil amended with vermicompost enriched with Zn showed high Zn concentrations in the leaves. The correlation coefficients between the DTPA-extractable Cu, Ni and Zn and their concentrations in the plants varied from 0.835 to 0.950