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

Soil does not always benefit from disturbance by earthworms. We investigate d whether (under a temperate climate and in maize growing in rows running d own-hill) earthworm casts could contribute to soil erosion and losses of nu trients in runoff water. Observations of casts were made in compacted (whee l-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 peri ods. The oldest casts recovered in situ were at least 2-months old. The rel ative loss of casts was 70% and 20% during the wet and dry periods, respect ively. Splash and runoff effects of rainfall were the main causes of deteri oration of surface-casts, especially fresh ones. Apart from rainfall events , a prolonged process of erosion took place whereby casts disappeared gradu ally 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 su rface-casting in compacted soil. Spatial distributions of both casts and ea rthworms across inter-rows were shown to be similar under non-compacted int er-rows but different in compacted areas. We estimated that annual soil ero sion from surface-casts would range from between 1.2 to 1.5 kg (d.w) m(-2) y(-1). (C) 1998 Elsevier Science Ltd. All rights reserved.