Effects of Soil salinization on the earthworm Eisenia andrei: life cicle traits and histochemical biomarkers

Salinisation is the process that leads to an excessive increase of water-soluble salts in the soil, and one of the most widespread soil degradation processes. Along the Adriatic Coast of the Emilia-Romagna region in Italy, our area of interest, soil salinisation is enhanced by salt water seepage, caused among others by subsidence and drainage. Salt prevents, limits or disturbs the normal metabolism and nutrient uptake of plants and other soil biota. Biological effects on plants, in particular crop species, are widely studied because of the socio-economical consequences of agricultural soil loss. Impacts on soil fauna are less known, in spite of its importance in determining soil fertility. Salinity might cause a decreased resistance of organisms to other stressors, eventually leading to synergistic effects. Here we report the results of a laboratory experiment that is intended as a first step of an ongoing study on the interactive effects of soil salinity and pesticide contamination on the earthworm Eisenia andrei. A natural silty soil was collected in the field, dried, grounded and sieved through a 2 mm mesh, to eliminate skeleton. Glass containers (223x121 mm, h: 77 mm) were used as experimental chambers; in each container 500 g of dry soil were mixed with 150 ml of spiking solution (NaCl dissolved in distilled water). A control treatment (distilled water) and five NaCl concentrations ranging from 0.1 to 12 g/L were prepared. Each treatment was replicated in two containers. Ten adult worms for each container were incubated in the experimental soils at controlled environmental conditions (20°C, 400-800 lux, 16 h light/8 h dark). After 28 days ofexposure, three life cycle endpoints were assessed: survival, growth, and reproduction (number of laid cocoons). In addition, three histochemical biomarker were determined: lysosomal membrane stability (neutral red retention assay), lipofuscin and neutral lipid accumulation. Soil salinity had significant adverse effects on all the life cycle endpoints, and it was possible to obtain preliminary estimates of effective median concentration (survival: 10±1 g/L, growth: 8±2 g/L, reproduction: 4 g/L). Soil salinisation did not alter the response of the biomarkers analysed in the present work. Although the biomarker sensitivity to pollutants is reported higher that that of life cycle endpoints, it appears they are not impaired by salinity and may be used for future investigations on polluted salinised soils. As an overall, our study indicates that adverse effects on earthworms are possible at levels of soil salinisation still compatible with agricultural land use, even in the absence of other stressors.