Contrasting biogeographic patterns of bacterial and archaeal diversity in the top- and subsoils of temperate grasslands

Biogeographic patterns and drivers of soil microbial diversity have been extensively studied in the past few decades. However, most research has focused on the topsoil while the subsoil is assumed to have similar microbial diversity patterns as the topsoil. Here we compare patterns and drivers of microbial alpha and beta diversity in and between the top- (0-10 cm) versus subsoils (30-50 cm) of temperate grasslands in Inner Mongolia of China along an aridity gradient covering a ~1500-km transect from arid to mesic ecosystems. Counter to the conventional assumption, we find contrasting biogeographic patterns of diversity and influencing factors for different bacterial and archaeal groups and between depths. While bacterial diversity remains constant or increases with increasing aridity in topsoil and decreases in subsoil, archaeal diversity decreases in topsoil and remains constant in subsoil. Microbial diversity in the topsoil is most strongly influenced by aboveground vegetation and contemporary climate, following the indirect effect of historical temperature anomaly since the Last Glacial Maximum (LGM), but is most strongly and directly influenced by historical temperature anomaly and soil pH in the subsoil. Moreover, the biogeographic patterns of top-subsoil community dissimilarities vary for different microbial groups and are overall influenced by soil fertility difference between depths for bacteria and contemporary climate for archaea. These findings suggest that diversity patterns observed in the topsoil may not be readily applied to the subsoil horizons. Overall, our study provides novel information for understanding and predicting soil microbial diversity patterns at depth.