Chemistry and micromorphology of aggregation in earthworm casts

The mechanisms by which earthworms produce and stabilize soil aggregates are not well understood yet this information is necessary before management practices that promote the beneficial aspects of their activity can be devised. Therefore, selective chemical pretreatments and micromorphological observations were used to investigate the nature of aggregate formation and stabilization in worm casts. Passage of soil through worms disrupted pre-existing microaggregates due to breakage of some bonds of the water and cation bridge type; however, incorporated organic debris fragments became plasma encrusted and served as nuclei for new aggregates. In excreted pellets, aging and drying facilitated close approach and bonding of plant and microbial polysaccharides and other organic compounds associated with the organic fragments to clay, thereby stabilizing the new microaggregates. In the soil material investigated, these bonds consisted predominantly of clay-polyvalent cation-organic matter (C-P-OM) linkages involving calcium when the worms were provided alfalfa- or corn-leaf diets.