The impact of soil organisms on soil functioning under neotropical pastures. A case study of a native anecic earthworm species

The description of main biological, ecological and functional attributes of a native earthworm species in a natural savanna and several introduced pastures from the Eastern Plains of Colombia was the main objective in this study. To achieve this a set of different experiments and explicit field work were designed and carried out at the Carimagua Research Station, 320 km east from Villavicencio (Meta department). Martiodrilus n. sp. (Oligochaeta, Glossoscolecidae) is a large native earthworm from the natural savannas of the eastern plains of Colombia. It inhabits semi-permanent Ushaped burrows at the entrance of which individuals deposit superficial and tower-like casts of very large size. A strongly positive correlation was observed between the number of fresh casts and the number of individuals collected in the first 10 cm of soil during the active period of this species (r= 0.907; P < 0.01). Density and biomass of this species were significantly much higher in the introduced pastures compared with the savanna (ANOVA, P < 0.01). The cocoons are ovoid in shape and rather large (23.6 x 14 mm, and almost 20% of adult weight), from which two individuals emerge. Evidence of vertical migration during the year was observed, while it is active in the topsoil during the beginning of the rainy season, it enters in a true diapause to withstand adverse environmental conditions before the onset of the dry period, being adults the . This species selected food substrates with high organic contents since casts produced in the two systems had significantly higher total C contents and total N than the bulk soil. C contents also increased significantly during cast ageing (+ 100%), possibly because of CO2 fixation processes, dead root accumulation and/or macrofaunal activities in casts. Another experiment was conducted to determine the effects of earthworm activities on soil and cast seed banks. The composition of the above standing vegetation was relatively closer to that of the cast seed bank than that of the soil seed bank. This may be due to the presence of enhanced conditions for germination in surface casts, which may increase the chances for egested living seeds to germinate and contribute to the plant cover. The results obtained in this study support the general knowledge of how earthworms can affect soil fertility and plant growth. Martiodrilus n. sp., through the production of casts affects the availability and nature of both the spatial and trophic resource in soil. This species certainly belongs to the functional group of “ecosystem engineers”, as it affects the availability of some resources for other organisms through the production of physical biostructures. The next step in research should be directed now to test whether Martiodrilus n. sp. is a keystone species within the soil community or not.