The impact of soil organisms on soil functioning under neotropical pastures: a case study of a tropical anecic earthworm species

Martiodrilus n. sp. (Oligochaeta, Glossoscolecidae) is a large native earthworm from the natural savannas of the eastern plains of Colombia. The description of the main biological, ecological and functional attributes of this species in a natural savanna and several introduced pastures at the Carimagua Research Station (320 km east from Villavicencio) was the main objective in this study. Density and biomass of this species were significantly much higher in the introduced pastures compared with the savanna (ANOVA, P<0.01). 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 last to enter into this phase (after reproduction period). Martiodrilus n. sp. seemed to select food substrates with high organic contents since casts produced in the two systems had significantly higher total C and total N contents than the bulk soil. Besides, C content also increased significantly during ageing of casts (+100%), possibly because of CO2 fixation processes, accumulation of dead roots and/or macrofaunal activities. The effects of earthworm activities on soil and cast seed banks were revealed in another experiment. The composition of the above standing vegetation was relatively closer to that of the cast seed bank than that of the soil seed bank. 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.