Space-time dynamics in situ of earthworm casts under temperate cultivated soils

Soil does not always benefit from disturbance by earthworms. We investigated whether (under a temperate climate and in maize growing in rows running down-hill) earthworm casts could contribute to soil erosion and losses of nutrients in runoff water. Observations of casts were made in compacted (wheel-tracks) and non-compacted (untrafficked) inter-rows, for a 2-month period in spring. Estimates of surface-cast production in a temperate maize crop ranged between 2.5 to 3.2 kg (d.w soil) m⁻² y⁻¹. The mean life-time of casts was shown to vary from 4 d during wet periods to 14 d during dry periods. The oldest casts recovered in situ were at least 2-months old. The relative loss of casts was 70% and 20% during the wet and dry periods, respectively. Splash and runoff effects of rainfall were the main causes of deterioration of surface-casts, especially fresh ones. Apart from rainfall events, a prolonged process of erosion took place whereby casts disappeared gradually by collapsing and mixing in the matrix bulk-soil. Cast production was found to be 50% higher in compacted soil. Particularly, an increase by 20% of soil compactness led to a 2-fold increase of casting activity per worm. This suggests that risks of soil erosion could be increased by earthworm surface-casting in compacted soil. Spatial distributions of both casts and earthworms across inter-rows were shown to be similar under non-compacted inter-rows but different in compacted areas. We estimated that annual soil erosion from surface-casts would range from between 1.2 to 1.5 kg (d.w) m⁻² y⁻¹.