Distribution, abundance and symbiotic effectiveness of Rhizobium leguminosarum bv. trifolii from alkaline pasture soils in South Australia

The current dissatisfaction with low productivity of annual medic (Medicago spp.) pastures has highlighted the need to seek alternative legumes to provide efficient N-2 fixation in low rainfall, alkaline soil environments of southern Australia. Clover species adapted to these environments will have limited N-2 fixation if effective rhizobia are not present in sufficient quantities. A survey of 61 sites was conducted across South Australia to determine the size, distribution and effectiveness of Rhizobium leguminosarum by. trifolii (clover rhizobia) populations resident in these low rainfall, alkaline soil environments. Clover rhizobia were detected at 56 of the sites, with a median density of 230-920 rhizobia/g soil. Most rhizobial populations were poor in their capacity to fix nitrogen. Rhizobial populations from fields provided 11-89% and 10-85% of the shoot biomass of commercial reference strains when inoculated onto host legumes T. purpureum (purple clover) and T. resupinatum (persian clover), respectively. Rhizobial population size was correlated negatively to pH and the percentage of CaCO3 in the soil, and was significantly increased in the rhizospheres of naturalised clover, found at 17 sites. Management options for rhizobial populations to improve legume diversity and productivity are discussed in terms of rhizobial population dynamics and likely soil constraints to successful rhizobial colonisation.