Changes in beta diversity through time
In order to assess the beta diversity between surface and deep soils,
plots having zero MOTUs in at least one depth were removed,
corresponding to 4.28% (Bacteria), 14.28% (Eukaryota), 6.43%
(Mycota), 39.28% (Collembola), 13.57% (Insecta) and 37.85%
(Oligochaeta) of total plots.
GLMMs allowed us to detect changes in the beta-diversity of communities
between surface and deep soil. Differences in community composition
between the two depths decreased with time since glacier retreat for
Bacteria, Eukaryota, Mycota and Insecta, indicating homogenization of
communities, while we did not detect significant changes through time
for the beta-diversity of Collembola and Oligochaeta (Fig. 3;
Supplementary Table S5). Collembola and Oligochaeta were also the taxa
for which the largest number of sites were discarded because of a lack
of MOTUs. Overall, our models did not show significant changes in the
turnover or nestedness components of the beta diversity measures through
time, with the only exception of Oligochaeta, for which nestedness
between surface and deep soils tended to increase through time
(Supplementary Table S5 and Fig. S2).
Within each deglaciated
foreland, the structure of communities was primarily related to time
since glacier retreat (Fig. 4). Time significantly affected community
structure for Bacteria, Mycota and Eukaryota (PERMANOVA: p< 0.05; Table 1); the amount of variance explained by time
ranged from 2.4% to 5.7%. For Bacteria, Mycota, Eukaryota, as well as
for Insecta, community structure also differed significantly between
soil depths, but the explained variance was smaller (< 1%;
Table 1). For none of the groups, we detected a significant interaction
between time and soil depth (Table 1), suggesting that the effect of
time was consistent between surface and deep soils. Differences in
multivariate dispersions were never significant between soil depths, but
were significant across time except for Collembola (Table 1). Bacterial
community structure was the most strongly related to time and depth
(R 2 = 5%; Table 1). Differences among
deglaciated forelands were marked but tended to follow similar trends
across the taxonomic groups (Fig. 4).
Based on the specificity and fidelity of each MOTU, 86 were identified
as indicators (47 Bacteria, 34 Mycota and five Eukaryota; Table S6). For
Bacteria, 22 taxa were strongly associated with young foreland soils,
including members of the genera Roseiflexus ,Herbaspirillum , Novosphingobium that exhibited
particularly high IndVal, while no one was strictly associated to the
intermediate ages. Seventeen taxa of Bacteria were indicators of both
surface and deep soil layers in older forelands, including members from
the genera Actinoallomurus and Ferrimicrobium that showed
the highest IndVal. Six taxa were indicators of the deep soil layers at
both intermediate and old age. For Eukaryota, five taxa were considered
as indicators; three were fungi related to old soils, while one mite
(genus Gamasina ) was associated with the intermediate age class.
For Mycota, 18 taxa were indicators of both surface and deep layers in
older forelands, including members of the genus Cladophialophoraand the family Glomeraceae. Ten Mycota taxa were indicators of
both surface and deep layers in young forelands while intermediate
forelands contained less indicators, with only five taxa. Only one
Mycota taxon was representative of a specific soil layer (the MOTU
identified as Golovinomyces sordidus , associated to the surface
layer of young forelands).