UC Riverside

Soil microbial communities are vital to a wide range of ecosystems, but they are poorly understood. Advances in high-throughput amplicon sequencing have enabled a greater understanding of how communities differ from each other. Previous studies have both found important variations with depth within soil profiles and across geospatial location, but few studies have examined both depth and location simultaneously. This study utilizes the Critical Zone Observatory Network to assay a total of 18 sites at a continental scale, each sampled to approximately 100 cm in 10-cm increments. Each of the approximately 180 biologically distinct samples was assessed for a variety of pedological variables including pH, particle size distribution, and nutrient composition. In addition, 16S (prokaryotic) and fungal ITS1 amplicons were sequenced to examine microbial community composition. Although previous literature has seen a marked decrease in diversity with increasing depth for 16S communities, this study shows that such soil profiles form a distinct subset, and that in many sites, there is no significant decrease in diversity with depth (termed "gradient" and "uniform" profiles here, respectively). Futhermore, while site-to-site variations tend to be important drivers of community differences, there are also strong similarities between extremely geographically distant communities that share similarities in ecological setting. In particular, soil taxonomic order and dominant vegetation types show strong correspondences with similarities between communities, but the tendency towards distinction between such groupings is overshadowed by the gradient-uniform dichotomy. This study, therefore, suggests that future work in comparisons of microbial communities across geospatial gradients needs to account for the ecological setting of selected sites, in particular accounting for soil taxonomy and vegetation. Although there are hints that the gradient-uniform dichotomy is linked to changes in soil texture and the relative availability of carbon and nitrogen, further research is required to robustly explain this distinction.