Effects of variations of the organic matter content and pH of soils on the availability and toxicity of zinc to the earthworm Eisenia fetida

Nine toxicity tests were conducted using a procedure based on the OECD (1984) artificial soil earthworm toxicity test. For each test, Eisenia jet ida were exposed to zinc in a range of artificial soils with differing pH and/or organic matter (0 M) content. Earthworm survival and cocoon production, and the concentrations of zinc in two fractions (nitric acid extractable, water extractable) and the exposed earthworms were measured. The proportion of total zinc that was soluble was greater in soil with low pH and low OM content. Zinc burdens were greatest in worms maintained in the most contaminated soils. However, the slopes of the relationships between earthworm zinc concentrations and zinc concentrations in soils (total and soluble) were less than one indicating probable regulation of net assimilation of this essential element by Eisenia jet ida. Toxic effects of zinc on Eisenia fetida as measured by reductions in survival and cocoon production were related more closely to soluble than total metal concentrations in soil. In a previous series of tests, zinc toxicity for Eisenia fetida was found to be at least ten times greater in OECD artificial soil when compared to contaminated soils collected from a polluted field site (Spurgeon & Hopkin, 1995). It was concluded that this was due to the greater bioavailability of the metals in the OECD soil. The results of the present paper support this hypothesis.