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

Abstract Our aim was to determine the relative importance of lithogenic inheritance and biological processes in the formation of the soil structure in the humus form. This study was applied to the humus forms of four calcareous alluvial soils (S1, S2, S3, S4). Three methods were employed: micromorphological analysis of thin sections; structural stability by a wet sieving method; and abundance and biomass of the populations of earthworms and Enchytraeids. The structures of the humus forms of the two pioneer stages (S1, S2) differed both quantitatively and qualitatively, even though they had similar populations of earthworm and Enchytraeids. The differences in structure were mainly 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 parental materials (same textural and mineralogical compositions). This study demonstrates that the initial composition of the alluvial deposits may modify the effect of the biological agents responsible for the formation of the soil structure. In the case of immature calcareous alluvial soils, lithological factors do not affect the type of humus form, but influence the rate of formation of structure.