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

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 alpha-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 alpha-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 alpha-Proteobacteria, which may be adapted to resource-poor environments in deep soils.