Earthworm casting: stabilization or destabilization of soil structure?

The aim of the study was to determine the effect of the gut passage through earthworms on the aggregate stability of soils varying in texture, carbonate and organic matter content. The soil material used originated from the Ap and B horizon of a barn soil (Gleyic Luvisol) and from the Ap and P horizon of a clay soil (Calcaric-Vertic Cambisol). Sieved aggregates of 1-2 mm size were filled into vessels and inoculated separately with two earthworm species: the anecic detritivorous Lumbricus terrestris and the endogeic geophagous Aporrectodea caliginosa. The casts of both species were collected for the determination of chemical (total carbon, organic carbon, CaC03, C-to-N ratio, pH) and physical factors (particle size distribution, water stable aggregation, tensile strength). These data were compared with those of natural and remoulded (physical data only) soil aggregates which were not influenced by earthworms. Remoulded aggregates were artificially formed simulating mechanical processes in the earthworm gut. Differences were detected between the earthworm species in the same soil as well as between casts of the same species but from different soils. The contribution of earthworm activity to the cast stability depends decisively on the original composition of the parent soil. The tensile strength of casts was positively correlated with the clay and carbonate content of the parent soil. For the water stable aggregation the correlation was inverse. Moulding processes in the earthworm gut destabilize the soil structure but at the same time biochemical processes act as an antagonistic stabilizing system. The more sensitive the parent soil is to physical disturbance the more effective is the casting for the water stable aggregation but less effective for the tensile strength.