Sinorhizobium meliloti in Australian soils: population studies of the root-nodule bacteria for species of Medicago in soils of the Eyre Peninsula, South Australia

Abstract

Populations of Sinorhizobium meliloti(formerly Rhizobium meliloti — the root-nodule bacteria for Medicago) from soils at 32 sites on the Eyre Peninsula, South Australia, were enumerated and evaluated for nitrogen-fixing effectiveness in association with Medicago laciniata, M. littoralis, M. polymorpha, M. rugosa, M. tornata and M. truncatula. These symbiotic values were related to physical features of the environment, viz. soil type, soil reaction, mean annual rainfall, and the presence or absence and frequency of occurrence of sown and naturalised annual species of Medicago (medics). Large populations of S. meliloti (ranging from 270 to 460000 per gram of soil) were detected in 28 of the 29 soils where medics occurred. One or more species of medic were found in 29 of the 32 soils examined. The other 3 soils were relatively moist and acidic in reaction with a mean soil pH (CaCl 2 ) of 5.5. It could be inferred that the presence of populations of S. meliloti was dependent on the occurrence of species of Medicago. Other features of the environment had no impact on the size of S. melilotipopulations, except insofar as they influenced the occurrence of medics. All populations of S. meliloti were effective in nitrogen fixation for M. littoralis and M truncatula, the 2 species most commonly sown as legume components of the cereal–pasture farming systems of the Eyre Peninsula. Effectiveness of populations of S. meliloti for M. polymorpha, M. rugosa and M. tornata was significantly greater (P<0.05) in soils where M. polymorpha occurred than where it did not. Otherwise, the symbiotic capacity of the root-nodule bacteria was not influenced by soil type, soil pH, rainfall or the presence or absence of particular medics. There are many indications that medic productivity on the Eyre Peninsula is in decline with detrimental consequences for the nitrogen economy of the farming systems. The almost universal occurrence in soils of the Eyre Peninsula of large populations of S. meliloti that were effective for M. littoralis and M. truncatula indicated that, whatever the reasons for ‘medic decline’, the condition is not attributable to inadequacies of the naturally occurring populations of root-nodule bacteria.