Importance of salt ions in ecotoxicological tests with soil arthropods

Abstract

In soil ecotoxicological tests, the toxicity of metals is usually determined by investigating the effects of metal salts without paying attention to the influence of the anionic partner of the investigated metal. In our studies, we evaluated the role of salt anions on the larval development of the hemiedaphic carabid Poecilus (Pterostichus) cupreus Linné 1758 and on the reproduction of the endogeic collembolan Folsomia candida Willem. The experiment with P. cupreus was conducted in a sandy soil treated with CaCl 2, CuCl 2, CuSO 4, and MgSO 4 solutions. MgSO 4 caused no observable effect. Both copper salts and the CaCl 2 treatment caused a high mortality of P. cupreus larvae compared with the control, and the toxic effect produced by copper could be clearly distinguished from a chloride ion effect. Effects on the reproduction of F. candida were investigated using a soil with variable total soil water content and a standard mixed salt solution, containing CaSO 4, MgSO 4, MgCl 2, KCI, and NaCl, applied at different concentrations. At higher salt concentrations, egg development was inhibited. Tests with single salt solutions showed that this was due to the inclusion of 43.5 mmol NaCl/kg d.wt. soil in the mixed salt solution. CaCl 2 tested separately at a concentration of 25.9 mmol/kg d.wt. soil also reduced egg survival. The rate of collembolan reproduction differed with total soil water content, showing that tests with Collembola must be carried out in media with constant water content for results to be comparable. A comparison between a solution of salts and an elutriate of toxic waste containing heavy metals and similar salt ions showed a clear combination of salt effects and heavy metal effects. These studies indicate that chloride ions may interfere with the demonstration of toxic heavy metal effects. Consequently, when chloride salts are used to determine the toxicity of metal cations, additional tests with comparable anion solutions of non-toxic cations are necessary, to clarify the results.