The effects of exposure time on the growth of earthworm Eisenia fetida feded by livestock manure in contaminated soil

Soil pollution by heavy metals in the world, especially in countries that require soil to produce food, has become a serious concern. In the context of environmental risk assessment, earthworms are an important component of soil, and ecologically considered as a biochemical indicator (bio Index) for soil health and quality. Earthworms are part of the food chain and the soil decomposition cycle, that growth characteristics are negatively affected by soil contamination. Due to the sensitivity of dirt worms to soil contamination, they can be used as an indicator of contaminated soils. This study was therefore carried out in order to investigate the effect of contact time and soil sampling points on the growth traits of earthworms in contaminated soils. The study was conducted on a contaminated soil that was sampled from the Ahangaran mine and its surrounding land, located at the 26 kilometer far from Malayer to Arak, in the Hamedan province, which lies between longitudes and latitudes 44° 59´ 44´´ and 34° 10´ 20´´ respectively. A factorial experiment was conducted in a completely randomized design with three replications. Animal fertilizers were used as nutrition for soil worms. In this experiment, 12 earthworms with an average weight of 0.3-0.6 mg were selected for each soil sample and during the experiment, no food was added to the dishes. Then, the worms were exposed to soil contamination in seven different time intervals of 3, 7, 14, 21, 28, 32 and 42 days. The viability, weight, cocoon production and reproduction by hand counting in unit weight of the substrate were investigated. The results of analysis of variance showed that lead and cadmium contaminated soils can somewhat affect growth parameters of Eisenia fetida. Despite the high concentrations of lead and cadmium in point S3, the highest percentage of survival, number of coccons and larvae (reproduction) of earthworms was observed, which could be due to the high resistance of earthworms to the toxic effects of cadmium and lead due to poison detoxification by metallothionein proteins in dorsal canals. Usually, the worse the earthworms had in contact with this element longer, the more severe the decline in population, weight and cocoon production. But when earthworms were fed to heavy metals such as lead and cadmium with animal fertilizer, population decline, weight and number of produced cocoons were less than that, increasing the number of cocoons and reproduction in the end of the trial period.