Semi-arid soil bacterial communities are refined by altered plant selection pressure under conservation management practices

Plants' ability to select for, and refine, their microbiome over time is well known. In soil, the strength of plant selection pressure on microbial composition can result in distinct rhizosphere, and root-zone communities, but may be masked in soils with lower native diversity such as semi-arid soils. Semi-arid soils are also known for their high potential for wind erosion, which may be mitigated by the implementation of cover crops. These cover crops may also alter the microbial composition of the semi-arid soils, but the strength of this effect, and the resulting changes in the composition of the microbiome, are less well known. This study aimed to address whether the implementation of a single-species wheat cover crop in semi-arid agricultural soils would impact the bacterial composition of the cash crop. We investigated three tillage regimes-no-tillage with a winter wheat cover crop, no-tillage winter fallow, and conventional tillage winter fallow-to determine the effects of tillage removal, and cover crop implementation on the soil bacterial community present in the root zone of the cash crop. It was determined that the composition of the bacterial microbiome was similar between cover cropped and non-cover cropped treatments. The similarity of bacterial composition was likely due to the strong effect of environmental selection reducing overall biodiversity in these soils. However, the variability of bacterial community composition was strongly reduced for the cover cropped treatment compared to the two non-cover cropped treatments. The similarity of bacterial communities with cover cropping, without significant soil physicochemical changes, suggests that plant-selection pressure, rather than management-induced changes in soil physicochemical properties, drove the refinement (decreased dissimilarity compared to non-cover cropped treatments) of the bacterial community. The composition of this refined community under cover cropping was determined to contain an enrichment of several genera of interest including N cycling and potential plant-growth promoting bacteria. In summary, the introduction of cover cropping in the nutrient and microbial-poor soil of the semi-arid Southern High Plains strongly refined the bacterial community and selected for organisms with the potential to improve soil functions associated with greater agronomic sustainability.