Changes in alpha-diversity with soil age and the impact of
depth
Alpha-diversity increased through time, as previously observed in
successional studies of microorganisms, plants and soil invertebrates
(Erschbamer & Caccianiga, 2016; Ficetola et al., 2021; Matthews, 1992),
with a similar pattern across all the study groups. For the whole
Eukaryota and, within them, for Mycota, the highest alpha-diversity was
found in the surface soils, supporting our hypothesis that the richness
of communities decreases toward deep soils. This observation agrees with
the idea that the highest soil biodiversity is hosted close to the
surface, as already observed for fungi, bacteria and some faunal groups
(Carteron et al., 2021; Chen et al., 2020; Chu et al., 2016; Jiao et
al., 2018; Moradi et al., 2020; Mundra et al., 2021; Rime et al., 2015).
In glacier forelands, soils tend to have higher water holding capacity,
more exchangeable cations, carbon and nutrient contents toward the
surface (Rime et al., 2015). These properties are vital for most
belowground organisms, especially in those resource-limited ecosystems,
resulting in higher bacterial activity, DNA concentration, fungal and
root biomass in the first centimeters of soil (Rime et al., 2015). We
highlight that, in our sampling design, the surface sampling covered a
thinner layer compared to the deep one (from 0 to 5 cm vs. from 7.5 to
20 cm of depth). In principle, the deep layer might hold larger
environmental heterogeneity, given that it is the thickest one. Thus,
alpha-diversity between layers might be even larger, had we sampled
layers with similar thicknesses.
For springtails only, the interaction between soil depth and development
stage had a significant effect on alpha-diversity, indicating that for
this group taxonomic richness increased at different rates between the
two soil depths. Springtails were nearly absent in soils aged less than
30 years (Fig. 2). Then, the alpha-diversity increased, but the increase
was faster in the surface layer compared to the deep layer, probably
because the fast accumulation of organic matter in surface soils (Herold
et al., 2014; Moradi et al., 2020) allows the establishment of these
organisms, which have multiple trophic roles, from detritivore to
herbivorous. For the other taxa, we did not detect significant
interaction between soil depth and soil development stage, suggesting
that alpha-diversity increases through time with a similar pattern
between surface and deep layers, except for Collembola and perhaps in
very young soils (see Fig. 2).
In glacier forelands, the amount of organic matter consistently
increases through time (Khedim et al., 2021). By repeating the analyses
of alpha-diversity, considering the organic carbon content as
independent variable instead of time since glacier retreat, we confirmed
that our conclusions are not biased by the issues of using different
sites as substitutes of time (issues of space-for-time substitution in
successional studies; Johnson & Miyanishi, 2008). Soil carbon content
is a major driver of soil biodiversity changes (Chu et al., 2016);
consistently with this idea, alpha-diversity tended to increase with
organic carbon. Nevertheless, models with time showed slightly higherR 2 values than the ones with soil organic
carbon (Fig. 2, supplementary Figure S1), suggesting that time since
glacier retreat is a better predictor of alpha-diversity than organic
carbon, even though these parameters are strongly correlated (Rime et
al., 2015; Zumsteg et al., 2011). Further studies are needed in order to
disentangle the role of both time and soil features as drivers of
primary succession.