Differences in ionic properties of salts affect saline toxicity to the earthworm Eisenia fetida

Most studies on the effect of salinity on soil organisms are conducted with NaCl but this might not be representative of effect when other salt ions are predominant. To assess the impact of different salt cations and anions on saline toxicity to earthworms, two experiments were conducted with specimens of Eisenia fetida exposed in the laboratory for 28 days using OECD artificial soil. In both experiments, series of concentrations (0-125 mM) of different salts were used. In experiment 1, which focused on cations, soil salinity was adjusted using NaCl, KCl, CaCl2 or MgCl2 while in the second experiment, which focused on anions: NaCl, NaNO3, NaHCO3, Na2HPO4 or Na2SO4 were used to allow partial comparison of the toxic effects of the cations and anions. The changes in electrical conductivity (EC) of substrates under the different salts were assessed at the beginning of the experiments. Mortality and weight change of worms were assessed at days 14 and 28. In experiment 1, the chlorides of the divalent cations (Mg2+ and Ca2+) had similar EC values which were higher than those of the monovalent cations (Na+ and K+) and this could explain their higher effects on all worm parameters than those of the monovalent cations. In experiment 2, among the anions, SO42− had the highest EC value while HCO32− had the lowest. The toxicity of the ions did not correlate perfectly with the EC values; therefore the use of EC in the assessment of saline toxicity to soil organisms should be used with caution. These results suggest that the ionic constitution of salts could define the severity of saline toxicity to earthworms, therefore a site specific assessment of soil salinization in affected areas with respect to type of predominant ions should be considered.