Effect of Salinity on Growth and Survival of Earthworm Eisenia fetida

Salinization of soil has been reported as a problem in many parts of the world. Salinization could occur either as a result of natural processes e.g. high concentrations of salt in parent materials or groundwater and/or anthropogenic actions such as over-irrigation. The salinization probably affects the chemical and physical properties of soil, soil microbiological processes, plant growth, and soil fauna. Both quantity and quality of water, however, are the most important eco-factors needed for earthworm survival and development, and also biodegradation processes.

In order to investigate the effect of irrigation water salinity on the survival and growth of earthworm Eisenia Fetida, an experiment was conducted in a completely randomized design with three replications under environmental conditions of the laboratory of Soil Sciences Department of Bu-Ali Sina University in Hamedan. The different types of water used in this study were: distilled water and saline water made with NaCl salt with electrical conductivity (EC) of 2, 4, 6, and 8 dS m-1. The experiment was carried out using completely randomized design in plastic containers of size 19 × 13 × 8 cm. Ten earthworms per container used in each exposure regime were introduced into the relevant test salinity by placing them on the surface and allowing them to burrow in. The test containers were covered with perforated lids to limit water loss due to evaporation and kept in 16 hours light, 8 hours dark at 25°C in a climate chamber for 42 days. Sampling was done at 3, 15, 21, 27, 33, 39, and 42 days after earthworms were introduced to the substrates to investigate mortality and weight changes of earthworms. The LC50 (concentration at which 50% of the earthworms are killed) and the EC50 (effect concentration at which a 50% reduction in a measured parameter) values for the salts expressed as conductivity (dS m-1) were calculated on day 27 and 42 by using the Probit Analysis.

On day 3, no significant effect of salinity on percentages of survival was found.  The survival rate of Eisenia fetida was significantly affected in the EC range used during 42 days (Table 2). The irrigation with distilled water (EC0) had the highest survival rate while the irrigation water with EC 8 dS m-1 had the lowest value. During the 39 days of exposure, no significant difference was found in survival rate of earthworms between EC 2, EC 4 and EC 6 dS m-1, but at the end of day 42, the salinity levels with EC 8 dS m-1 had a significant effect on percentage of earthworm survival in which 91.68% mortality occurred. The mean weight change of earthworms exposed to water with EC 2 dS m-1 was not significantly different (p < /em> < 0.01) from those exposed to the distilled water during 39 days, but there was a decrease in earthworm weight on day 42. The calculated LC50 for mortality after 27 and 42 days was 7.5 and 4.31 dS m-1, respectively, and EC50 for growth was 7.94 and 6.82 dS m-1, respectively.

Our results showed that increased salinity had harmful effects on the growth and mortality of the earthworms (Eisenia fetida). Salinity can have detrimental effects on earthworms at concentrations considered safe for many plant species. We determined 42 day LC50 for mortality 4.31 dS m-1 (2521 mg lit-1). The EC50 for growth was 6.82 dS dS m-1 (3989 mg lit-1). The weight of earthworms was significantly affected by NaCl and dispersion analysis showed that NaCl concentration had a statistically significant influence on the weight of earthworms. The argument for using NaCl is that it is the predominant salt in most saline environments particularly in wastewaters. Since the salt type is dependent on the source of the contamination, it is, therefore, possible that other salts apart from NaCl could be the main compounds in saline toxicity in a specific area. The results of the current study suggest that the effects of salinity depend on the salt composition. Therefore, it would be important to assess the type of salt ions in soil in risk assessment, as this affects the extent of toxicity to soil organisms.