Grassland soil types (e.g. Vertisols and Mollisols) tend to have deep A horizons, yet few studies have been done on microbial communities from deep soil. Tallgrass prairie studies have shown rooting depths up to 1 m can occur. Rooting depth can differ by plant origin, with native prairie species having deeper rooting depths than exotic species. Yet, soil depth and its effect on microbial biodiversity is often neglected in soil studies, with most sampling taken at 0-20 cm. It is poorly known how microbial community diversity and network structure change with depths. We sampled soil cores from 0-100 cm deep from tallgrass prairie plots containing native or exotic plant species, with and without summer irrigation (128 mm annually). We used network analysis and amplicon sequencing (16S and ITS) on soil from four depth increments (0-10, 10-30, 30-60, and 60-100 cm) to determine the impact of soil depth, plant origin, and summer irrigation on the microbial community. Our predictions were 1) microbial communities will change composition and diversity at various depths between native and exotic plots; 2) native plots will have greater connectedness and modules than exotic plots; 3) irrigation will increase the connectedness of both exotic and native plots’ networks.
Results/Conclusions
Microbial diversity measures showed a significant decrease at depths 30-100 cm (depth P<0.01). However, microbial communities still persisted up to 100 cm, showing the lower soil depths needed to be included to fully capture microbial communities. Microbial alpha diversity (Simpson’s and Chao1) was 15-20% higher in native than exotic plots, with a stronger response in fungal than bacterial communities. Microbial diversity measurements were not impacted by irrigation (P>0.05). Native plots had lower node counts, but significantly higher connectedness than exotic plots across all depths (P<0.01). Connectedness, linkages, increased with deeper soil depths up to 60 cm, with the greatest connectedness occurring in 30-60 cm, under native plots. Irrigation only impacted native networks, with greater connectedness under summer irrigation, but fewer nodes.
Our results suggest that the replacement of native plant communities by exotic species can alter plant-microbe interactions. The differences in microbial networks and diversity measurements observed amongst soil depths are important findings as they demonstrate the need to include greater soil depth in microbial models and microbial experimental design.