The water repellency of earthworm (Lumbricus terrestris) casts depends on their particle size composition, organic carbon content and calcium carbonate content

At the soil level, earthworms are key indicators of water-related processes which help soils to accept, retain, release and transmit water by influencing the soil structure through their burrowing, casts, and drilosphere. The water repellency is an indicator of soil structure stability both for the natural soil and for the earthworm casts (bioturbated soil). The water repellency of earthworm casts is a complex property that should be approached both at ecological and functional level in earthworm categorization because this cast property influences the transport and flow processes in soil, such as those of water, nutrients, pollutants, and gas diffusion. The earthworms have been described as bioremediators of soil water repellency. Casts of the earthworm species Lumbricus terrestris collected from an urban green space have been analysed for particle size composition (texture), organic carbon (OC) content and CaCO3 content in relation to their water repellency (the hydrophobicity). The findings of this study showed that the main drivers determining the water repellency of the casts of L. terrestris, as indicator of their structural stability, are chemical (the contents of OC and respectively CaCO3) and physical (contents of clay, silt and fine sand). The water repellency (hydrophobicity) of the casts decreased with their content of clay and silt and increased with their content of organic matter and CaCO3. The hydrophobicity of earthworm casts is in direct positive relation with their previous water accumulation, i.e. the current penetration time of water increases or decreases if the previous penetration time of the water increased or decreased, respectively. The water drop penetration times indicated that the analysed earthworm casts were slightly water-repellent. A small to medium degree of water repellency has been mostly reported as appropriate for the aggregate stability of the soil.