DIGESTION OF A VERTISOL BY THE ENDOGEIC EARTHWORM POLYPHERETIMA-ELONGATA, MEGASCOLECIDAE, INCREASES SOIL PHOSPHATE EXTRACTIBILITY

The effect of the endogeic earthworm, Polypheretima elongata, on the p hosphate status of vertisols (South East Martinique, French West Indie s) has been evaluated in laboratory conditions. Fresh surface casts we re collected and compared to the non-ingested soil. Phosphates were ex tracted by a sequencial procedure : anion resin desorption followed by sodium bicarbonate extraction. In some replicates chloroform fumigati on followed the resin desorption to quantify bacterial immobilisation. Total phosphate contents were 43 % higher in casts than in soil, but the total phosphorus contents were not different. Fumigation had no ef fect on the phosphate contents as measured from bicarbonate extracts o f casts. A particle size fractionation of casts and soil indicated tha t worms selectively ingest fine soil particles. However, the observed increase in phosphate extracted by anion resin was mainly attributable to digestive and microbial processes during the gut transit.