Soil acidity and Rhizobium: their effects on nodulation of subterranean clover on the slopes of southern New South Wales

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Abstract
Rhizobium trifolii and root nodulation were compared in acid and limed soil to investigate mechanisms by which current soil acidity on the slopes of southern New South Wales may reduce the growth of Trifolium subterraneum. Increasing soil pH increased both the rate at which Rhizobium trifolii colonized soil and the frequency of nodules/g root (NF). Thus, numbers of R. trifolii were greater in carbonate-treated soil throughout autumn and winter compared to unlimed soil. In limed soil R. trifolii were measured at 104-105/g soil as early as seedling germination, 40-200 x more than in unlimed soil. Despite this, by spring, the numbers of rhizobia in soil without lime reached large and even similar levels as occurred in limed soil. Subsequently, with the drying of soil in summer, the numbers of R. trifolii declined markedly in both untreated and limed soil. Maximum NF could not be achieved with addition of Ca without increase in pH. From the field observations and other research, a hypothetical model is proposed to explain how differences in the rates of change in rhizobial numbers due to acidity may determine nodulation (NF). Thus, it is suggested that, when there is a repetitive decline in numbers of R. trifolii to low levels in summer, their subsequent rate of increase in the following autumn influences nodule abundance. The production of DM in an acidic soil appeared to be limited by symbiotic N2 fixation since added N fertilizer removed lime response. The limitation to N2 fixation did not appear to be due to lower occupancy of nodules by Rhizobium strains with less potential for N2 fixation than those in limed soil. It is possible that the effect of acidity on NF influences DM, though sampling limitations made it difficult to generalize in this regard.