Key drivers of uniformity or change with depth in soils across the US

Emma L. Aronson, University of California Riverside, Keshav Arogyaswamy, Genetics, Genomics, and Bioinformatics, University of California, Riverside, Riverside, CA, Jon K. Botthoff, Center for Conservation Biology, UC Riverside, Tess Brewer, University of Colorado, Boulder, Nicholas C. Dove, Biosciences, Oak Ridge National Laboratory, Hannah Freund and Mia Maltz, University of California, Riverside

Background/Question/Methods
Soil microbial communities perform a wide variety of ecological functions. The Critical Zone (CZ) is defined as the Earth’s upper, dynamic skin, situated between the highest treetops and the lowest groundwater. In other words, it is the zone of greatest heterogeneity and the highest concentration of life, encompassing most terrestrial life, soil, regolith, and the vegetation-atmosphere interface. We collaborated with a large group of CZ scientists to collect samples to depth across the United States to better understand how deep the influence of surface processes on deep soils and vice versa. Soil pits were excavated by hand or by auger at 20 sites within the United States (including two in Puerto Rico) representing the 10 former Critical Zone Observatories (CZOs). At most sites, soil pits were dug to at least 100 cm or refusal. Basic site characteristics were compiled, and samples were processed for nutrients, pH, and other environmental parameters. Samples were also sequenced using amplicon primers for 16S and ITS, to target bacteria and fungi, respectively, as well as metagenomic shotgun sequencing.
Results/Conclusions
Initial analysis of the amplicon sequences showed that while 16S communities of most soil profiles were fairly similar, some samples below 40 cm showed a strong divergence from this cluster. This occurred in only a subset of sites (5 out of 20). This divergence in composition of microbes coincided with a stronger decrease in diversity below 40 cm for the five sites, termed “gradient” sites, as opposed to “uniform” sites that do not vary significantly with depth. A comparison of fungal vs bacterial diversity showed a strong positive correlation (R=0.47, p<0.0001). This trend was robust across both gradient and uniform soil types, as well as across vegetation type and both gradient uniform soils. While previous studies have seen important changes in overall microbial communities with depth, this study suggests that at broader (continental) scales, microbial community composition is driven more by ecological setting than by variances of depth within soil profiles. Further research across a more focused set of soil profiles can provide important insights into the environmental factors that drive the distinction between gradient and uniform profiles. The importance of the gradient-uniform dichotomy, in particular, as well as soil taxonomic order, must be accounted for in studies that seek to examine how microbial communities are structured by depth.