Communities differences between surface and deep soils change
through time
The beta-diversity between surface and deep layers was particularly high
soon after the retreat of glaciers, and then decreased with time. As
seen for the alpha diversity, this pattern was consistent across nearly
all taxa. Collembola and Oligochaeta are the only taxa for which this
was not evident, but these animals were nearly absent from recently
deglaciated soils (and particularly from the deep layers; Fig. 2),
probably because many of them require well developed soils, with
abundant organic matter to find resources (Phillips et al., 2019).
Therefore, for Collembola and Oligochaeta, many plots at early
development stages were excluded from this analysis, reducing
statistical power. In principle, the variation of beta diversity between
surface and deep layer can be attributable to both species gain/loss
(nestedness) and replacement (Baselga, 2010). Turnover was more
important than nestedness for invertebrates (Collembola, Insecta,
Oligochaeta), while for microorganisms (Bacteria and Mycota) turnover
and nestedness showed a similar importance (Supplementary fig. S2), and
the relevance of these two components of beta-diversity remained similar
through time (Supplementary Fig. S2; Table S5).
The decrease in beta-diversity between surface and deep layers through
time confirms the hypothesis of homogenization of communities (Rime et
al., 2015), and extends it to the whole soil biota, as bacteria,
microeukaryotes and animals responded the same way (Fig. 3). Community
homogenization is probably related to the structural modifications
observed during the development of soil horizons (e.g. Schaetzl &
Anderson, 2005). The study of sites at different stages of soil
formation has shown a differentiation of organic horizon immediately
after glacial retreat (O), followed by the development of an
organo-mineral horizon (A) during the first 150 years (Crocker & Major,
1955; Mavris, Egli, Plotze, Blum, Mirabella and Giaccai, 2010). The
strong vertical variation of physical, chemical and structural features
(e.g. light, temperature, pH; Moradi et al., 2020; Mundra et al., 2021)
clearly affects communities, which show a particularly strong response
to fine-scale environmental heterogeneity (Rime et al., 2015; Moradi et
al., 2020; Mundra et al., 2021). For example, immediately after glacier
retreat, the amount of fine sediments is the highest at the surface
(Rime et al., 2015). This can determine differences in humidity between
the surface and the deeper layers, that in turn affect communities (Rime
et al., 2015). The decrease of beta-diversity can be explained by the
progressive deepening of the organo-mineral horizon (Mavris et al.,
2010), where abundant resources favor the establishment of complex
communities. Plant richness and cover quickly increase during the first
decades after glacier retreat, and 40 years after glacier retreat plants
cover generally rises above 50% (Rime et al., 2015). Plant roots
generally influence the first 20 cm of soils and more, and could have
determined the homogenization of superficial and deep samples of our
study (Rime et al., 2015). Differences between surface and deep layers
would probably be stronger if a larger vertical gradient is analyzed
(e.g. from surface to 50 cm deep; Moradi et al., 2020), and this is
certainly an important aspect that deserves future studies. However, in
glacier forelands the study of deep layers by eDNA analysis is sometimes
problematic because rock outcrops are frequent a few centimeters below
the surface. In any case, for all the taxa considered, time since
glacier retreat remained the main determinant of community variation, as
it explained much more variation in community composition compared to
depth (Table 1). This confirms the idea that, even though fine-scale
heterogeneity certainly has a role, time since glacier retreat remains
the main determinant of community evolution after glacier retreat
(Ficetola et al., 2021; Rime et al., 2015).
For microorganisms, the significant community differences between soil
layers (Table 1) likely are determined by taxa that are specialists of
given environmental features. This idea is supported by the observation
that all MOTUs identified as indicators of surface or deep soils are
bacteria or fungi (supplementary Table S6). Conversely, for
invertebrates, soil depth explained a very limited amount of variation
in community composition (Table 1). This could be due to the lower
richness of these taxa (which limits statistical power), or to the fact
that a broader vertical profile would be required to identify
specialists (Moradi et al., 2020). Several taxa identified as indicators
in Rime et al. (2015) showed similar patterns across the different
locations of our study, confirming the strong functional variation of
communities through time. Taxa identified as clear indicators both here
and by Rime et al. (2015) include the Clostridium bacteria, known
as anaerobic, which were consistently found as indicators of the
earliest stage of soil development. Similarly, several fungal
saprotrophs were indicators across the different stages of soil
development, while Lachnum was a microfungus consistently
associated with the most developed soils (Nguyen et al., 2016).Gemmatimonas tend to be copiotrophic bacteria (Ho, Di Lonardo, &
Bodelier, 2017) and include multiple MOTUs that were found as indicators
of different stages. Interestingly, fungi such as Laccaria orHygrophorus (i.e., potential ectomycorrhizal taxa; Nguyen et al.,
2016) were also indicators of later stages of development. Contrary to
Rime et al. (2015), arbuscular mycorrhizal fungi (i.e., Glomeromycetes)
tended to be associated with oldest forelands, confirming the growing
importance of plant-associated fungi along community development (Davey
et al., 2015).