Rainfall impact effects on ageing casts of a tropical anecic earthworm

We investigated the erodibility of surface casts produced by an anecic earthworm of the Colombian savannahs by means of indoor rain simulations. The kinetic energy applied to samples, 21.62 J minute (-1) m (-2), was estimated to be equivalent to 41% of the energy of the more intense period of a local storm. The erodibility of casts was assessed at different stages of their ageing along with the effects of repeated wetting-drying cycles. Bare soil cores and soil cores of the same size with a cast on their surface (soil + cast) were used as controls. Saturated hydraulic conductivity (HC) was measured to test whether casts enhance soil water permeability. Fresh, almost liquid, individual casts were completely dispersed by a 2-hour rainfall simulation at a dispersion rate of 0.9 +/- 0.5% of sample minute (-1). After 5 hours drying at 32 degrees C and 79% relative humidity, casts were as moist as fresh casts (non-significant Mann-Whitney U-test) but had a solid appearance and were no longer dispersed by raindrop impact. Under simulated rainfall, dry casts were very slowly fragmented into large aggregates (> 5 mm). The HC of casts was increased by repeated cycles of wetting and drying but unaffected by length of air-drying and natural ageing, and averaged 7 (3, n=6), 20 (3, n= 7) and 32 (1, n = 45) cm hour (-1) for bare soil, soil + cast and all casts, respectively (standard error, sample size). Except for bare soil, these values were greater than the greatest rainfall intensities recorded locally. The consequences for soil erosion, nutrient losses and water infiltration are discussed.