The rhizobia nodulating shrubs for revegetation of arid lands:: Isolation of native strains and specificity of the plant-rhizobia interaction by cross inoculation tests

Medicago strasseri, M. citrina, Colutea arborescens, and Dorycnium pentaphyllum are legume shrubs potentially useful for the revegetation of semiarid Mediterranean ecosystems and for animal grazing. As nodulation with specific rhizobia and/or mycorrhizal fungi, may become essential for the settlement and growth of legumes, the objective was to ascertain the specificity/promiscuity of the plant-rhizobia interaction, by detecting the occurrence of infective rhizobia in the soils to be revegetated and by cross inoculation tests. Native rhizobia for the target species, were isolated from nondisturbed native areas. The four species were inoculated with four semiarid soils located in central Spain, that we intend to revegetate. Specific rhizobia for M. strasseri and M. citrina were detected in three of the four soils. The soil that did not show infective rhizobia had received sewage-sludge from a waste-water treatment plant, and levels of heavy metals were high. None of the 4 soils had specific rhizobia for C. arborescens and D. pentaphyllum. The cross inoculation tests revealed that M. strasseri and M. citrina belonged to the same symbiotic grouping as alfalfa. On the other hand C. arborescens and D. pentaphyllum only nodulated with their own specific native rhizobia, and the rhizobia from C. arborescens and D. pentaphyllum were not infective with any of the tested species. For M. strasseri the effectiveness of the native rhizobia was significantly higher than the effectiveness of the infective rhizobia from the soils to be revegetated. Consequently for M. strasseri, C. arborescens and D. pentaphyllum, inoculation with native strains (I. MsS1, I. CA and I. DP respectively) would be advisable.