Effects of salinity on partitioning, uptake and toxicity of zinc in the earthworm Eisenia fetida

Salinisation of soil is a problem in many parts of the world especially in agricultural lands that could also be contaminated with metals from pesticide use. This study aimed to derive a quality criterion standard in a defined substrate with the eventual aim of protecting earthworms against salinity, and to assess the influence of salinity on partitioning of, uptake in and toxicity of zinc to earthworms. To achieve this, two experiments were conducted with specimens of Eisenia fetida exposed in the laboratory for 28 days using OECD artificial soil spiked with either NaCl (experiment 1) or combination of Zn and NaCl (experiment 2). In the first experiment, NaCl was added in the following concentrations: 0, 1000, 2000, 4000, 6000 and 8000 mg kg⁻¹ NaCl. Mortality, growth and cocoon production were assessed at day 28. The results showed 28-day LC₅₀ of 5436 mg kg⁻¹ for NaCl. The EC₅₀s for growth and cocoon production were 4985 and 2020 mg kg⁻¹ NaCl, respectively. In the second experiment, Zn, added as ZnCl₂ in a range of sub-lethal concentrations (0, 250, 500, 750 mg kg⁻¹ Zn) was combined with 0, 2000 or 4000 mg kg⁻¹ NaCl. The endpoints: mortality, weight change, and the internal zinc concentration were assessed at day 1, 7, 14 and 28 while cocoon production was assessed only at day 28. Apart from the total zinc concentration in the substrates, DTPA and CaCl₂ extractable Zn concentrations were also determined at day 1 and 28 to assess how salinity influenced the partitioning of this metal in the substrates. There was a significant increase in CaCl₂ and DTPA extractable Zn in the substrates as salinity increased suggesting that salinity influenced the partitioning of Zn in the substrates. Weight and mortality of worms were not significantly affected by NaCl and Zn as individual substances, but in combination both had significant effects on these parameters. In contrast, cocoon production was significantly affected by increased NaCl and Zn administered as individual substances, and the effects were more severe when both substances were present. The apparent synergy between Zn and NaCl could not be fully explained by internal zinc concentrations in the worms. It could, however, be partly explained by Cl⁻ effect from the addition of Zn as ZnCl₂. It is concluded that salinity resulting from increased NaCl had an additive to synergistic effect in combination with Zn, in influencing toxicity to these earthworms.