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.