How is alpha-diversity related to soil depth, time since
deglaciation and soil features?
Overall, the alpha-diversity
was highest for the generalist markers (Euka02, Bact02, Fung02) compared
to the specialist ones (Coll01, Inse01, Olig01). Estimates of
alpha-diversity obtained with different Hill numbers (q = 0 and q = 1)
were strongly correlated (for all taxonomic groups, r >
0.78; Table S2).
When we used q = 1, we observed an increase of alpha-diversity with time
since glacier retreat for all the taxonomic groups. For Eukaryota and
Mycota, alpha-diversity was significantly higher in communities
retrieved at surface layers with depth of 0-5 cm, compared to the
communities detected in the deeper layer of soil (Fig. 2; Table S3).
Furthermore, for Collembola we detected an interaction between depth and
time since glacier retreat. For this group, the alpha-diversity index
was close to one (mean: 1.19 ± 0.51; corresponding to richness
~ 0) at relatively young sites (< 30 years
after glacier retreat) and increased with time, but the increase was
faster in communities at 0-5 cm of depth. All results were highly
consistent when we repeated analyses using q = 0 (Table S3). Results
were very similar when we used soil carbon content as a predictor
variable instead of time since glacier retreat, as GLMMs showed a
significant increase in alpha-diversity with average organic carbon
content of the plot, even though the R 2 values
of these models were generally lower than theR 2 of models with age as independent variable
(Fig. S1; Table S4).