VARIATION OF SOIL POPULATIONS OF RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII AND THE OCCURRENCE OF INOCULANT RHIZOBIA IN NODULES OF SUBTERRANEAN CLOVER AFTER PASTURE RENOVATION IN NORTH-EASTERN VICTORIA

Rhizobium leguminosarum by. trifolii soil populations were measured by mean probable number at two sites where the pasture had been renovate d, resown with subterranean clover, the soil limed and the seed inocul ated with Rhizobium strain WU95. Rhizobia were isolated from nodules a nd nodule occupancy by WU95 was verified using apr gel immune diffusio n. Over three growing seasons, the populations of R. trifolii varied f rom ca 10(3) to 10(6) g-1 soil. These numbers were similar to another survey carried out 25 yr previously, and therefore suggest no change i n the overall soil population of R. trifolii with time. In unlimed soi l, the rhizobia populations fluctuated with season, with smallest numb ers following the summer-autumn months, but where the soil was limed t he survival of rhizobia was less seasonally dependent. Rhizobium strai n WU95 was recovered from ca 2.5 to 17.5% of the nodules sampled in th e year after inoculation, but in the second year after inoculation WU9 5 was not detected in any nodules. It is not known whether WU95 is les s competitive at forming nodules compared with the indigenous R. trifo lii strains; however, it is likely that the indigenous strains would h ave a numerical advantage compared with WU95.