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).