Impact of the energy crop Jatropha curcas L. on the composition of rhizobial populations nodulating cowpea (Vigna unguiculata L.) and acacia (Acacia seyal L.)

Jatropha curcas, a Euphorbiaceae species that produces many toxicants, is increasingly planted as an agrofuel plant in Senegal. The purpose of this study was to determine whether soil priming induced by J. curcas monoculture could alter the rhizobial populations that nodulate cowpea and acacia, two locally widespread legumes. Soil samples were transferred into a greenhouse from three fields previously cultivated with Jatropha for 1, 2, and 15 years, and the two trap legumes were grown in them. Control soil samples were also taken from adjacent Jatropha-fallow plots. Both legumes tended to develop fewer but larger nodules when grown in Jatropha soils. Nearly all the nifH sequences amplified from nodule DNA were affiliated to the Bradyrhizobium genus. Only sequences from Acacia seyal nodules grown in the most recent Jatropha plantation were related to the Mesorhizobium genus, which was much a more conventional finding on A. seyal than the unexpected Bradyrhizobium genus. Apart from this particular case, only minor differences were found in the respective compositions of Jatropha soil versus control soil rhizobial populations. Lastly, the structure of these rhizobial populations was systematically imbalanced owing to the overwhelming dominance of a very small number of nifH genotypes, some of which were identical across soil types or even sites. Despite these weak and sparse effects on rhizobial diversity, future investigations should focus on the characterization of the nitrogen-fixing abilities of the predominant rhizobial strains.