Rainfall effects on erosion of earthworm casts and phosphorus transfers bywater runoff

We investigated whether, under a temperate climate and in a maize crop, ear thworm casts could contribute to soil erosion and further favour the export ation of phosphorus by runoff waters. Recording of casts was made in compac ted (wheel-tracks) and non-compacted inter-rows, for a 2-month period in sp ring. To assess the rainfall impact on cast evolution, half of the observat ion sites were protected against rain splash by a nylon mesh placed above t he soil surface. The water runoff was collected and analysed for sediment c ontents and phosphorus concentration. The mean annual production of surface casts was calculated to be 34 kg (dry weight) year(-1) kg(-1) earthworm (f resh weight). Synchronization between cast erosion and rainfall events was shown under natural conditions (unprotected sites). The erosion rate was 4 times greater over rainy periods than dry ones, reaching 80% of cast number s. It appeared that not the runoff effect but the splash effect, due to the kinetics of the drops,:disrupted casts. Newly formed casts disappeared fir st, with the erosion rate decreasing twofold for casts more than 10 days ol d. Cast erosion and runoff, as well as worm casting activity, were greater under compacted sites than under non-compacted sites, indicating an influen ce of earthworms on soil erosion from compacted soils. The total phosphorus content was similar in casts and uningested soil (0.80 mg phosphorus g(-1) ). Potential phosphorus losses from cast erosion was calculated to reach 25 -49 mg phosphorus m(-2) per rainfall event depending on soil compaction. Th e amounts of particulate phosphorus recovered in water runoff after each ra infall event varied from 1 mg to 11 mg phosphorus. These results are compar ed and discussed.