Responses of bacterial communities and their carbon dynamics to subsoil exposure on the Loess Plateau

Highlights: • Subsoil exposure had depth-specific effect on bacterial community and activities. • Both copiotrophic and oligotrophic groups increased in exposed vs. control soil. • Bacterial community composition altered at 60–100 cm in exposed vs. control soil. • Enzyme activities were greater at 20–60 and 60–100 cm in exposed vs. control soil. • The exposed subsoils had consistently lower K_c and Q₁₀ compared with the topsoil. Subsoil exposure due to factors including erosion and terracing, evidently decreases soil organic carbon storage and productivity, but the responses of bacterial communities and their carbon dynamics remain unclear. Soils from 0–20 cm, 20–60 cm and 60–100 cm were collected from three 100 cm profiles in bare land on the Loess Plateau, and incubated in buried pots for a year (July 2016 to July 2017) to simulate subsoil exposure, with ongoing monitoring of the microbial mineralization rate of soil organic carbon (K_c), using Li-Cor 8100. At the end of the incubation period, the exposed soil and the in situ control soil were sampled to investigate changes in bacterial community composition, as represented by 16S rRNA, and the activities of enzymes involved in soil carbon cycling. Both copiotrophic (Actinobacteria and Alphaproteobacteria) and oligotrophic (Thermoleophilia) groups were stimulated in the exposed vs. control soil at 20–60 and 60–100 cm. The exposed vs. control soil from 60 to 100 cm produced the greatest bacterial responses, such as greater diversity and altered keystone groups (Thermoleophilia vs. unidentified Acidobacteria). Enzyme activities were greater in the exposed vs. control soil at both 20–60 cm (β-D-xylosidase and cellobiohydrolase) and 60–100 cm (β-D-xylosidase and β-D-glucosidase). The exposed soil from 20–60 cm and 60–100 cm had lower K_c and Q₁₀ values than those at 0–20 cm. Our findings revealed the existence of bacterial depth-specific responses to subsoil exposure, and highlight the effect of anthropogenic soil redistribution on soil carbon flux and its potential responses to future climate change.