Earthworms act on the spatial organisation and thus influence soil functioning by the production of biogenic structure: burrows and cast. Aims of our study are to describe, quantify soil bioturbation, and study spatial distribution of earthworm features in situ.
Field experiments were conduced in a pasture located in woodlands of Brittany (France). In this pasture, the topographic position of the hedge has permitted the formation of gradient of thickening of the solum. It is advisable to know how evolve the earthworm bioturbation within this gradient so as to assess the share of the earthworm activities in of historical processes of the woodland soils.
To quantify earthworm bioturbations, we set up an original methodology of cartography of bioturbation facies in soil profile. This methods take account the whole of the observable characters of the bioturbation that is register in typology. In parallel, we studied the spatial distribution of the earthworm communities in order to establish relation between community and activity of earthworms.
In the pasture, the facies of bioturbation represent large shape of the soil profile to 22% at 42% (between 0 and -60 cm). Bioturbation intensity is dependent of the topographic position of the hedge and increase near the hedge. However, we observed a great variability in a same topographical position. The settlements of earthworms reveals that the soils thickening supports the species Nicodrilus nocturna (Anecic strict), but not Lumbricus terrestris (Epianecics), the both species represented more that 90% of the earthworm population. The increase in the total bioturbation would be ascribable with anecic strict, knowing that the species of the Nicodrilus kind are more bioturbator that L. terrestris (Jégou and al, 2001; Bastardie et al, 2003). The thickening of the solum upstream of the hedge offers living conditions more favourable to anecic strict to the detriment of epi-anecic.
At profile scale, the cartography shows that the distribution of the facies is influenced by the succession of the various pedological horizons like by cows trampling. The features of earthworms activities are observable until -120cm, depth maximum of our cartography.
Our study suggests the major role of the earthworm bioturbation in soils pedogenesis processes. The spatial variability of bioturbation could be ascribable with the modifications of the functional structure of the settlements constraint. Our methodology suggested the necessity to evaluate the functional impact of the earthworm bioturbation. Future investigation aims to characterize the various facies in physical term (micromorphology), chemical (Organic matter status) and microbiological (Microbial diversity and activity).