DISCUSSION
Our work provides new insights on the colonization and primary succession patterns in deglaciated terrains, by integrating soil depth in the primary succession studies and by implementing a multi-taxa approach across multiple forelands. Here, the multi-taxa approach allowed characterizing patterns for a wide range of topsoil organisms involved in colonization and successional processes. In order to cover the largest proportion of biodiversity, we considered both generalist (for Bacteria, Mycota and Eukaryota) and more specific (for Collembola, Insecta and Oligochaeta) markers. Alpha- and beta-diversity variation through time showed a strikingly consistent pattern across these taxa. The considered depths did not strongly affect the alpha diversity of some taxa at any stage of soil development, even though communities inhabiting surface and deep soil layers were not exactly the same. Importantly, beta-diversity between surface and deep soil decreased through time across most of taxa, supporting the hypothesis of homogenization between surface and deep soil along the succession (Rime et al., 2015).