Vertical changes in bacterial community composition down to a depth of 20 m on the degraded Loess Plateau in China

Abstract

Soil microbes are involved in the fundamental processes that underpin an ecosystem's function. However, little is known about the microbial communities that inhabit deep soil horizons, especially in degraded forest ecosystems. Here, we used high‐throughput sequencing to investigate the vertical distribution of soil bacterial communities to a depth of 20 m in Pinus tabulaeformis and Robinia pseudoacacia forests on the Loess Plateau, China. We found that bacterial richness declined from the topsoil to a depth of 2 m in P. tabulaeformis forests and declined from the topsoil to a depth of 1 m in R. pseudoacacia forests, and thereafter increased. The relative abundance of α‐Proteobacteria was higher in subsoils than in topsoils of P. tabulaeformis forests. It was highest at the depth of 20 or 14 m in both forest types, suggesting that α‐Proteobacteria can survive dry, alkaline, low‐nutrient environments. We also found a higher relative abundance of Acidobacteria in topsoils than in subsoils, indicating that Acidobacteria can grow well in nutrient‐rich environments. Our results suggest that soil bacterial communities respond to nutrient changes associated with soil depth and plant species. These findings revealed that soil microorganisms persisted in deep and carbon‐starved soils, especially for α‐Proteobacteria, which may be adapted to resource‐poor environments in deep soils.

Evidence for signatures of ancient microbial life in paleosols

“…We found high actinobacterial abundances in both, loess and paleosols, although Actinobacteria appeared to be especially predominant in weakly developed soil horizons. This is consistent with recent studies which report that Actinobacteria were abundant in semi-arid loess soils 32,53 and more abundant under alkaline soil conditions 53 , as in our study. Actinobacteria with high relative abundances were e.g.…”

“…3). This overall decline in alpha diversity and abundance is well known from studies of other soil profiles, which analyzed www.nature.com/scientificreports/ profiles up to 20 m deep [29][30][31][32] . It is primarily reflecting the decreasing microbial population size with increasing depth.…”

“…With the aim to identify characteristic taxa for specific soil horizons, we had a closer look at the OTUs that occurred distinctly in modern soils, loess and paleosols. In a recent study, microbial communities of a vertical loess profile until 20 m from the semi-arid loess plateau in China covered with Pinus or Robinia forests were analyzed, showing that Acidobacteria, Actinobacteria and Proteobacteria were the predominant phyla over all depths 32 . In our study, acidobacterial OTUs (especially belonging to GP6, GP7, GP16; Fig.…”

Deep Soil Layers of Drought-Exposed Forests Harbor Poorly Known Bacterial and Fungal Communities

“…Some groups of microorganisms appear predominantly in deeper soil horizons, such as the candidate phylum Dormibacteraeota (Brewer et al, 2019), or tree root host-specific mycorrhizal fungal strains, as shown in Eucalyptus forests at 4 m soil depth (Robin et al, 2019). Also, it appears that microbial diversity is not necessarily lower in subsoils than in topsoils, as recently has been found on the Loess Plateau in China in soils down to 20 m depth (Liu et al, 2020).…”

“…In general, living conditions for soil organisms and roots are harsher in deeper soil layers, where the soil density is higher, oxygen concentrations are lower, and carbon and nutrients are less available (Lennon, 2020). Carbon and nutrients typically are more available in the upper soil, mainly due to larger quantities of leaf and fine root litter and root exudates, as well as more biotic activity (Eldridge et al, 2016;Liu et al, 2020). Consequently, microbial biomass is usually one to two orders of magnitude lower in the subsoil than in the topsoil (Eilers et al, 2012;Spohn et al, 2016).…”