Metabarcoding data reveal vertical multitaxa variation in topsoil communities during the colonization of deglaciated forelands

Ice-free areas are increasing worldwide due to the dramatic glacier
shrinkage and are undergoing rapid colonization by multiple lifeforms,
thus representing key environments to study ecosystem development. Soils
have a complex vertical structure. However, we know little about how
microbial and animal communities differ across soil depths and development
stages during the colonization of deglaciated terrains, how these
differences evolve through time, and whether patterns are consistent among
different taxonomic groups. Here, we used environmental DNA metabarcoding
to describe how community diversity and composition of six groups
(Eukaryota, Bacteria, Mycota, Collembola, Insecta, Oligochaeta) differ
between surface (0-5 cm) and relatively deep (7.5-20 cm) soils at
different stages of development across five Alpine glaciers. Taxonomic
diversity increased with time since glacier retreat and with soil
evolution; the pattern was consistent across different groups and soil
depths. For Eukaryota, and particularly Mycota, alpha-diversity was
generally the highest in soils close to the surface. Time since glacier
retreat was a more important driver of community composition compared to
soil depth; for nearly all the taxa, differences in community composition
between surface and deep soils decreased with time since glacier retreat,
suggesting that the development of soil and/or of vegetation tends to
homogenize the first 20 cm of soil through time. Within both Bacteria and
Mycota, several molecular operational taxonomic units were significant
indicators of specific depths and/or soil development stages, confirming
the strong functional variation of microbial communities through time and
depth. The complexity of community patterns highlights the importance of
integrating information from multiple taxonomic groups to unravel
community variation in response to ongoing global changes.