Initial stages of soil formation in a riparian zone: Importance of biological agents and lithogenic inheritance in the development of the soil structure

Our aim was to determine the relative importance of lithogenic inheritance and biological processes in the formation of the soil structure in the humu s form. This study was applied to the humus forms of four calcareous alluvi al soils (S1, S2, S3, S4). Three methods were employed: micromorphological analysis of thin sections; structural stability by a wet sieving method; an d abundance and biomass of the populations of earthworms and Enchytraeids. The structures of the humus forms of the two pioneer stages (S1, S2) differ ed both quantitatively and qualitatively, even though they had similar popu lations of earthworm and Enchytraeids. The differences in structure were ma inly due to lithological factors. Despite their longer period of evolution, the structures of the humus forms of S3 and S4 were quite similar to that of S2. This similarity is attributed to their formation from similar parent al materials (same textural and mineralogical compositions). This study dem onstrates that the initial composition of the alluvial deposits may modify the effect of the biological agents responsible for the formation of the so il structure. In the case of immature calcareous alluvial soils, lithologic al factors do not affect the type of humus form, but influence the rate of formation of structure. (C) 2000 Editions scientifiques et medicales Elsevi er SAS.