Potential role of earthworm casts for the phosphorus enrichment of run-off waters

Surface casts of earthworms (dominantly Allolobophora caliginosa) contained a higher proportion of finer particles than the underlying surface soil. The proportion of the finest fraction (< 4 μm), recovered following water dispersion, increased from 6% in the surface soil to 20% in casts. Casts contained appreciably more inorganic and organic phosphorus (P) than underlying soil materials, particularly in the finer fractions (20-4 and < 4 μm). The release of inorganic P to solution from casts was approximately four times greater than that from surface soil. In contrast, the removal of inorganic P from solution by casts was proportionately lower than that by surface soil. Studies of the rate of release of inorganic P to solution indicated the presence in the casts of a pool of loosely-bound inorganic P which was readily released to solution. This was confirmed by isotopic exchangeability studies, the amount of exchangeable P being three times greater in casts than in surface soil. Although the release of total P to solution from fresh casts did not change with time, the release of inorganic P increased as a function of time of incubation. Data for phosphatase activity in casts indicated that 0.1 m NaCl-extractable organic P was converted to inorganic P and that this was quantitatively released to solution. The results obtained are discussed in terms of the potential contribution of surface casts to the particulate and dissolved P loadings of run-off waters.