Degradation dynamics of surface earthworm casts in grasslands of the eastern plains of Colombia

Earthworms are generally considered to fit the definition of ecosystem engi neers. The casts they produce are recognised to have a great importance in the regulation of soil processes. Lifetimes and degradation rates of these structures remain poorly known. In this study, the dynamics of disappearanc e and the changes in the physical properties of the surface casts of the an ecic earthworm Martiodrilus capimaguensis were assessed in a native savanna and an intensive pasture. In both systems, casts were composed of superpos ed layers deposited by earthworms over a period of at least a few days. The half-life of casts ranged between 2 months and 11 months in the pastures ( trampled and protected, respectively), and 5 months in the savanna. Some dr y casts remained at the soil surface for more than 1 year after having been excreted. The disappearance of casts was mostly attributed to rain-drop im pact and the effect of animal trampling. The bulk density of fresh casts wa s higher (+17%, P<0.05) or equivalent (-1%, P>0.05) to that of the surround ing soil, in the savanna and the pasture, respectively. Fresh cast aggregat es were of larger size than bulk soil aggregates (about +70% in both system s, P<0.05). Bulk density and the size of cast aggregates decreased with cas t ageing (from -29% to -24% for bulk density, and from -68% to -80% for siz e, in the pasture and the savanna, respectively). Macroinvertebrates were o bserved digging into casts, and were assumed to be partly responsible for t he physical degradation of casts with time.