Effects of earthworms on soil structure and physical properties

The effects of earthworms on soil structure and the resulting physical properties in natural or cultivated systems were studied in different sites of the Macrofauna project. Four main results were obtained from these experiments : (1) In kaolinitic soils (Lamto Manaus Yurimaguas) irrespective of clay content andogeic earthworms play a major role in soil structure formation and maintenance while in smectitic soils (Martinique) the effects of earthworms on soil structure formation and maintenance are not as clear. In the Vertisols of Martinique the effect of roots and organic materials seem predominant in soil structure development and earthworms play a secondary role. (2) Endogeic earthworm species have different effects on soil properties. Large earthworms such as #Pontoscolex corethrurus$ or #Millsonia anomala$ egest large and compact casts. They increase the proportion of large aggregates in soil and the bulk density ; they are called "compacting species". Conversely small earthworm species such as eudrilid worms feed at least partly on large compact casts and egest smaller and fragile aggregates. They decrease the proportion of large aggregates in soil and the bulk density ; they are called "decompacting species". The effects of "compacting species" on soil structure formation seem to be linked to the presence of organic residues at the soil surface. In agroecosystems of Yurimaguas (Peru) the intense activity of #P. corethrurus$ and the coalescence of surface casts lead to the formation of a compact surfacecrust with impedes water infiltration in the absence of organic residues but a favourable macroaggregate structure develops in the presence of organic residues and leguminous mulch.

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