The Bradyrhizobium-legume symbiosis is dominant in the shrubby ecosystem of the Karst region, Southwest China

• We investigated the diversity of rhizobium-legume symbiosis in the shrubby ecosystem of Karst landscape.
• The nodulation of potential N2-fixing plants in Karst landscape is largely different from that in non-Karst landscape.
• All of wild legumes except Callerya nitida associate with the rhizobia belonging to the genus Bradyrhizobium.
• Derris fordii and its associated rhizobia may have potential for application in restoration of degraded ecosystems.

Nodulation symbiosis is a key component of natural ecosystems, and the nitrogen-fixing ability of such symbioses can have a significant impact on the restoration of degraded ecosystems. Here, we investigated the diversity of legume-rhizobium symbioses, compared the N2-fixing efficiency of legume-rhizobium symbioses, and clarified the relationship of rhizobia, host species, and microhabitat. Eighteen potential N2-fixing species were surveyed, and only nine of them were found to be nodulated. Most of wild legumes were found to be associated with rhizobia belonging to the genus Bradyrhizobium, except for Callerya nitida, which formed associations with the genus Mesorhizobium. δ15N signatures of Derris fordii were significantly closer to zero than all the other plants tested, suggesting that the D. fordii-ISA2507 (Bradyrhizobium elkanii) symbiosis might have the highest N2-fixing efficiency of all the associations. D. fordii and its rhizobial symbionts may have potential for application in ecological restoration and agricultural production. Host species selection was the major factor, and there was no significant effect of slope position on nodulation-legume symbioses in our study area, in which altitudes ranged from 287 m to 521 m at the small scale. The Shannon diversity and the evenness of N2-fixing species (nodulated legumes) were significantly positively correlated (P < 0.05) with the Shannon and evenness across non-N2-fixing plants (non-nodulated plants), suggesting that the Bradyrhizobium-legume nodulation symbiosis plays an important role in maintaining plant community structure in this degraded ecosystem.