Persistence of introduced strains of Rhizobium leguminosarum bv trifolii in acidic soils of north-eastern Victoria

A 5-year study was undertaken to establish if introduced rhizobia with higher tolerance to Al than the current inoculant Rhizobium can persist and continue nodulating subterranean clover (Trifolium subterraneum L.) in acidic soils. Two Rhizobium leguminosarum by trifolii strains were introduced as seed inoculants with subterranean clover at 2 acidic sites (PHCa 4.1 and PHCa 4.3), where lime and gypsum had been applied as soil amendments. Strain NA3001 was selected for its tolerance to high Al concentrations when grown on an agar medium and WU95, which is a widely used commercial inoculant strain, for its relatively poor tolerance to Al when grown on agar. Liming the soil increased its pH and reduced the concentration of extractable Al at both sites. In the year the subterranean clover was sown, strain WU95 had nodule occupancy of 20-49%, decreasing with time to 4-7% after 5 seasons (1991-95). The nodule occupancy of strain NA3001 was initially lower than strain WU95 (14-16%), but its occupancy did not vary with time (significant strain x time interactions, P<0.05). These data indicate that the acid-tolerant strain NA3001 has the potential to persist in these strongly acidic soils and, despite the presence of high background populations of naturalised rhizobia, to continue initiating nodulation. The use of soil amendments (lime and gypsum) to increase pH and reduce soluble Al concentrations did not affect the nodule occupancy of either NA3001 or WU95 with time, nor did it slow the rate of decline in nodule occupancy of WU95.