Distinctive role of soil type and land use in driving bacterial communities and carbon cycling functions down soil profiles

Soil microbial communities are governed by soil types and altered by land use. However, it is still unclear how these two driving forces shape soil microbial communities and their carbon (C) cycling processes. We surveyed contrasting soils of natural forest and cropped vineyard under four soil types (brown Chromosol, brown Kurosol, red Kurosol, and red Calcarosol) at depths of 5–10 cm, 45–55 cm, and 90–100 cm over a 20,000-hectare region in New South Wales, Australia. The bacterial community and 35 C-related functional genes were investigated by 16S rRNA amplicon sequencing and high-throughput qPCR. Results show that land use governed the bacterial community distribution in the topsoil. Changes in topsoil bacterial community structure and functional genes were due to agricultural impacts on soil properties. In contrast, edaphic characters of soil types, such as pH and EC, showed significant impacts on the microbial community assembly in the subsoil. Topsoil organic C was decreased in the vineyard compared to the forest, and was correlated with an attenuated ratio between C fixation and decomposition gene abundances, which potentially accelerated C loss via imbalanced input and output. This study highlights i) the distinctive role of soil type and land use in structuring soil microbial community along with soil profiles, and ii) the shift in C-related genes was related to changes of soil organic C content caused by agricultural management.