Rhizosphere bacterial community is mainly determined by soil environmental factors, but the active bacterial diversity is mainly shaped by plant selection.

Background: The assembly of the rhizosphere community, even the diazotroph community, is mainly shaped by soil environmental factors (including soil climate and physiochemical characteristics) and plant selection. To better understand the driving forces on the active overall and nitrogen-fixing bacterial community compositions, we characterized the communities of tobacco rhizosphere soil collected from three sampling sites with a large geographic scale (> 600 km).

Results: The results indicate that the diversity and community composition of the overall bacterial and diazotroph communities are obviously differed according to the sampling sites. Still, no significant difference is found between the communities in rootzone and rhizosphere samples. Climate variables including mean annual precipitation (MAP) and mean annual temperature (MAT), soil physiochemical characteristics including available nitrogen (AN), available potassium (AK) and pH are main factors that affect the bacterial and diazotroph community structures in the three sampling sites. Furthermore, MAP and MAT, AN and available phosphorus (AP), total nitrogen (TN) and organic carbon (OC), AK and electrical conductivity (EC) showed similar effects, but pH showed independent effect on the composition of the overall bacteria and diazotroph communities. However, the alpha diversity indices of active overall and nitrogen-fixing bacteria in the rhizosphere are obviously higher than in the rootzone samples, and no significant differences are observed among different sampling sites. Proteobacteria is the predominant active phylum of all samples for overall and nitrogen-fixing bacteria. Escherichia-Shigella, Achromobacter, Streptomyces and Sphingomonas are the dominant active bacterial genera, and Bradyrhizobium, Skermanella and Extensimonas are dominant active nitrogen-fixing bacteria genera in rhizosphere. Furthermore, the high active abundance of Escherichia-Shigella but low abundance of Ralstonia in all three sampling sites indicate high root-knot nematode infection and low wilt disease endemic risk.

Conclusion: These results indicate that soil environmental factors contribute more to the tobacco rhizosphere bacterial community assemblage, but the rhizosphere contributes more to the diversity of active overall bacteria and nitrogen-fixing bacteria in the community. Our study provides novel knowledge for the assemble of rhizosphere bacterial and active bacteria communities across a large geographical scale.