TRANSFORMATION OF THE SOIL STRUCTURE THROUGH PONTOSCOLEX-CORETHRURUS (OLIGOCHAETA) INTESTINAL-TRACT

Pontoscolex corethrurus is a geophagous tropical earthworm whose population in a Mexican pasture can ingest annually 400 Mg of dry soil ha-1. A comparative study of the soil before ingestion, the gut content and casts of P. corethrurus using transmission or scanning electron microscopy (TEM and SEM) showed that the soil structure was completely destroyed. In the control soil the organo-mineral material was compact, while the soil in the anterior gut was dispersed in a semi-liquid medium with a great amount of free polysaccharides (intestinal mucus) and swollen clays by the increase of water content. At the end of the gut and in the casts the soil had retaken a similar structure than before its ingestion. Although in the gut a great reorganization of the soil occurred, as formation of new aggregates like microbial colonies surrounded by polysaccharides and clays, or activation of some dormant microorganisms. These observations supported with other physico-chemical (pH, water content) and microbial activity measurements showed how P. corethrurus had a dramatic impact on soil macro- and microstructure. This destructuration and restructuration of the soil made that organic elements that were protected were recycled and others, on the contrary, were protected by the formation of new microaggregates.